With every new holiday season comes the focus on this mythical barrier: 2 hours for the marathon in the media, and perhaps 3 hours in the immediate future for us amateur runners. Will Santa Claus leave this official record under the tree? Perhaps. But for you, coaches and athletes living the reality of the field, the real question lies elsewhere.

Beyond the dreams of the elites, what is the priceless gift you could offer yourself? Not another gadget, but the key to progressing without breaking yourself, nor sacrificing your personal life to hours of useless training.

The true gift is efficiency. It is the method that allows you to respect your physical integrity while maximizing your potential. For 40 years, the field has been my laboratory. Thanks to advanced ambulatory devices, we have dissected the reality of effort to distinguish marketing hype from what actually makes you run faster.

This week, I offer you an unprecedented confrontation: the data synthesis of the very latest international science (Grivas 2025) versus our own field discoveries.

The Table of Truth: Where to Gain Your Minutes?

I have calculated the real weight of each factor for you. For the Elite (Sub-2h), gains are marginal and cumulative. For YOU (3h00 goal), the levers are totally different.

Look at the “Self-Pacing” row: that is where your gift lies.

Performance Factor

What the Numbers Tell Us (And What Salespeople Forget)

The “ideal” Sub-2h model sells us a stable thermal machine that claws back seconds thanks to technology (1.5 to 3 min for shoes).^[1]

But for you, reality is disruptive.
For a 3-hour runner, physiological uncertainty is greater than 5%. Buying shoes to theoretically gain 1.5% is a statistical illusion. On the other hand, Self-Pacing (your ability to manage yourself) carries heavy weight. Poor pace management can cost you 15 to 20 minutes (the famous wall at the 30th km), or 10% of your total time!

This is where your margin for progress lies: much more than in the sole, it is in your brain and your sensations.

My Gift in Your Sleigh: 2 Self-Pace Sessions!

For Christmas, I am not offering you fragile carbon foam, but autonomy. I am slipping two Self-Pace sessions into your training plan.

This is the best way to test and develop your “effort governance” competence (a concept I detail in my book Courir à la sensation: Available here on Amazon).

The Challenge of the Week: The “Rabbit” Test
The best way to validate your Self-Pace skill is to repeat the “Rabbit” session:

· The Goal: Run a given distance by adjusting your speed purely by sensation, without looking at the watch, and see if you are capable of reproducing the performance without excessive cardiac drift.

· The Challenge: Try to chain at least 3 “Rabbit” sessions every two days.

It is radical. You will see that fatigue does not lie, but your brain knows how to manage it if you listen to it.

Speaking of which, next week I will give you the surprising results of Arnaud, who took up the crazy challenge of completing 8 Rabbits in 7 days. His data shakes up our certainties about recovery and load!

Example of a session on a 10 km:
Alternating medium and hard zones around critical speed.
For a runner with a critical speed of 4 min/km (15 km/h) and a marathon record at 90% of critical speed (3h06min).

· Protocol: Start by doing the same thing on 2 x 5 km to begin with, interspersed with 1 km at an easy pace.

Happy holidays and happy intelligent preparation!

Véronique Billat

References:
Grivas, G. V. (2025). Toward a record-eligible sub-2-hour marathon... European Journal of Applied Physiology.^[1]
Billat, V. L. et al. (2001). Physical and training characteristics of top-class marathon runners. Medicine & Science in Sports & Exercise.^[2]
Pycke, J. R., & Billat, V. (2022). Marathon performance, oscillations... Int. J. Environ. Res. Public Health.^[4]
Billat, V., Vitiello, D., Palacin, F., Correa, M., & Pycke, J. R. (2020). Race Analysis of the World’s Best Female and Male Marathon Runners. International Journal of Environmental Research and Public Health, 17(4).^[3]

Read more

Key Idea

AI can be your best ally—or your worst advisor. It’s only as good as its underlying assumptions. Your brain acts as an extended sensor-controller (ECGM) for homeostasis: integrating temperature, O₂, heart rate, metabolism, muscle recruitment, nutrition, and more. To perform sustainably, you must listen to this central signal and use AI that respects variability—not those that force a “constant pace.”

What Our Field Study Shows

· Protocol: Two marathoners (~3h10) ran with sensors: VO₂/gas exchange, EEG, stride metrics, GPS speed, and heart rate.

· Early warning: Around 15 km, EEG reached levels comparable to above the ventilatory threshold, while heart rate was still moderate—the brain said “too fast.”

· Wall predicted: Determined to hold a target pace, the runner ignored the warning and “hit the wall” around 26 km. The speed profile became asymmetric with many slow kilometers at the end: mean speed < median.

· Skewness (asymmetry): Measures how symmetric distribution is. Well-paced → skewness ≈ 0 (mean ≈ median). Very slow finish → negative skewness = poor pacing.

https://publications.billatraining.com/publications/2019/detecting_the_marathon_asymmetry_with_a_statistical_signature.pdf

What AI Adds (When Used Wisely)

· VAE: Compresses all the time series (VO₂, HR, speed, breathing, stride...) into a usable latent signature. A Variational Autoencoder (VAE) is a neural network that compresses complex data (images, signals, etc.) and can generate new examples. Unlike a classic autoencoder, the VAE learns to represent data as probabilistic distributions (not just a single vector), allowing for original and varied samples from the latent space.

o Encoder: Turns input into a distribution (mean and standard deviation).

o Decoder: Uses a sample from this distribution to reconstruct or generate data.

o VAEs are tools for exploring, compressing, and generating complex data, used in AI for images, sound, medicine, and more.

·

Lyapunov analysis: On this signature, a drop in the Lyapunov exponent signals loss of variability/adaptability (the system becomes rigid). This signal drops before speed falls—a predictive alert and intervention window for the coach.

o The Lyapunov exponent measures the stability of a dynamic system—its sensitivity to small changes at the start.

§ If positive, the system is unstable: small differences grow quickly (chaotic behavior).

§ If negative, the system is stable: differences fade over time.

§ If zero, the system is bounded but neither stable nor unstable.

o In physiology (like marathon running), a drop in the Lyapunov exponent means the body’s variability and adaptability are decreasing, often preceding performance loss or “the wall.”

· Practical conclusion: The brain warns, AI confirms and makes it actionable. Useful AI doesn’t lock in a “magic pace,” but detects early drift and suggests micro-adjustments.

Why “Constant Pace” Can Be Dangerous

Popular algorithms, fed by online myths, often prescribe a strictly constant pace. For non-elites, this overreaches early, destroys variability, and results in negative skewness and a lower average speed. In contrast, small, controlled oscillations protect variability and improve average speed over 42.195 km.

Mini Coach Checklist (Brain + AI)

1. Listen early: If EEG/RPE/stride says “too hard” while HR looks fine, trust your brain.

2. Track variability: Monitor local variability (speed/stride) and VAE signature; a Lyapunov drop = yellow flag—plan a micro-recovery before collapse.

3. Aim for a symmetric histogram: Mean ≈ median; if the left tail grows, intervene.

4. Planned oscillations: ±5–10 s/km around your individual “comfortably hard” zone.

5. Personalize: Calibrate from tests (VO₂max, oxygen-deficit tolerance, RPE) and let AI adapt to the person, not the internet.

https://publications.billatraining.com/

The Audacity to Learn from Error (AZR Spirit)

Like the AZR model, which improves by failing and adjusting without external labels, athletes and coaches benefit from experimenting, analyzing, and correcting. Mistakes become data that strengthens strategy.

Takeaways

Term

Definition

EEG

Brain electrical activity, indicator of effort/mental fatigue

VO₂

Oxygen consumption, cardio-respiratory performance

ECGM

Model of the brain integrating physiological signals

Skewness

Asymmetry of the speed distribution

VAE

Compresses multidimensional data into a latent signal

Lyapunov

Measures system variability/stability

AZR

AI that learns by error and self-training

Meta-description:
Before the wall: EEG, VO₂, and AI detect loss of variability. Listen to your brain, not just your watch: “constant pace” gives way to intelligent pacing.

Summary:
Your brain acts as an extended central governor: it warns you well before your pace collapses. By combining EEG, VO₂, and AI that measures variability (Lyapunov exponent), you can anticipate the wall and win with small rhythm oscillations.

SEO slug:
before-the-wall-brain-ai-variability-marathon-pacing

Keywords:
marathon, pacing, variability, EEG, VO₂, Lyapunov, VAE, homeostasis, central governor, training, coach

At the end of this blog, you’ll find sample training sessions for young people aged 9 to 18, tailored to their age group.

-40% VO2max in Young People Over 10 Years!

Recent studies show a dramatic drop in aerobic capacity (VO2max) among youth over the past decade. This decline is compounded by findings from MIT showing that young people who rely 100% on AI for learning use 50% less brain activity, resulting in an inability to write assignments independently.

We are losing both our minds and our legs—and calling it progress!

Fortunately, there is hope: the French government has encouraged PE teachers to conduct endurance tests (like the Luc Léger test from 1984), 30m sprints, and long jumps. But the real innovation comes from the RABIT (Running Advisor Billat Training) test, introduced in 2019, which brings hope for a new era of human endurance from a young age. This approach, championed by forward-thinking PE teachers, is more natural and respects children’s physiology and their love for spontaneous, intermittent running speeds.

The Science: Why Sensation-Based Training Matters

Advances in exercise physiology urge us to reconsider training and learning principles. The traditional model of three energy systems (anaerobic alactic, anaerobic lactic, aerobic) is now seen as too simplistic. Instead, a hybrid approach is favored, where all systems cooperate—much like a hybrid car that uses both electric and fuel engines as needed.

Example: Two Billatraining Sessions for Children

1. Power Session (Strength × Speed)

· Duration: 30 minutes

· Warm-up: 5–10 minutes

· Main set: Run at an “easy pace” for 20 minutes, with a 10-second sprint every 2 minutes

· Cool-down: 5–10 minutes

2. Effort Management Session

· Duration: 30 minutes

· Warm-up: 5–10 minutes

· Main set: 2 × 10-minute sequences, each with:

o 4 min easy pace

o 2 min moderate pace

o 1 min strong pace

o 3 min recovery

· Cool-down: 5–10 minutes

Cooperative Metabolic Systems

This approach considers the cooperation of the three energy systems: anaerobic alactic (phosphocreatine), anaerobic lactic (lactate fermentation), and aerobic (oxidation of carbs and fats in mitochondria). Phosphocreatine, regenerated during low-intensity contractions, acts as an energy buffer, smoothing ATP fluctuations and preventing power drops. For example, at the start of a race, a rapid acceleration depletes phosphocreatine, giving the “thermal engine” (aerobic system) time to ramp up. Throughout activity, slightly slowing down before acidosis sets in allows for optimal oxygen use to replenish phosphocreatine and even re-oxidize lactate for extra fuel.

The Double Advantage

This hybrid approach, especially for races lasting 5 minutes to 3 hours, allows muscles to generate higher power than at constant intensity while resisting fatigue better. The muscle no longer operates in a single contraction-relaxation mode.

Training Consequences: Short, Global, Individualized Sessions

This leads to training methods that develop the hybrid system in all its complexity. Instead of separating strength/speed and endurance sessions, workouts are short (about 30 minutes), global, and tailored to the activity or goal. For running, the aims are:

· Maximal power or specific muscle strength (energy transfer at each foot strike)

· Recovery between efforts (cardiac and pulmonary adaptation)

· Repetition of effort (acid tolerance)

· Effort management (perception of workload)

This method requires ongoing work so athletes can finely perceive and adjust their effort levels, aligning as closely as possible with target performance zones.

Assessing Energy Resources: Vameval, Luc Léger, and RABIT

To personalize development, students are often given physiological tests. The Luc Léger and Vameval tests are common for measuring maximal aerobic speed (MAS), but the RABIT test is less widespread. The RABIT test emphasizes self-paced effort based on sensory cues, making it accessible to all students—including those overweight or less active—since paces are set by individual sensations.

School Adaptation of the RABIT Test

· 5 min easy pace, then 6 sec sprint

· 3 min moderate pace

· 2 min strong pace

· 20 sec sprint

· 3 min moderate pace
Each phase is separated by 1 min recovery.

Instructions for Each Phase:

· Find your cruising pace for 5 min; adjust as needed, focus on sensation, start/stop on whistle

· Sprint as fast as possible for 6 sec

· Find your moderate pace for 3 min; adjust as needed, focus on sensation

· Find your strong pace for 2 min; adjust as needed, focus on sensation

· Sprint as fast as possible for 20 sec

· Find your moderate pace for 3 min; adjust as needed, focus on sensation

After the test, students record data (distance, heart rate), which is then analyzed in class.

Comparative Results: Performance, Efficiency, and Self-Management

Studies show that the RABIT test enables students to reach a maximal speed about 8% higher than with the Vameval, with a lower heart rate and 11% greater energy efficiency (meters per heartbeat). The RABIT also fosters self-management: students learn to vary their pace, recognize their limits, and optimize their resources.

Unlike the Luc Léger test, which pushes to exhaustion, the RABIT puts the student at the center, encouraging action based on personal capacity. Data collected (distance, heart rate, sensations) are visualized as an “energy radar,” allowing for personalized sessions to develop strength, effort management, recovery, and repetition.

Educational Perspectives: Autonomy, Enjoyment, and Lasting Engagement

By promoting self-awareness and self-management, the RABIT test fits perfectly into a modern educational approach. It develops motor and methodological skills, individualizes teaching content, and boosts motivation. Over time, each student gains autonomy, learns to perceive the effects of effort, and commits to physical activity for life.

In summary: The RABIT test is an innovative, effective alternative to classic tests, promoting performance, energy efficiency, and autonomy in young athletes. It is a powerful lever for transforming engagement and enjoyment in physical activity throughout life.

RABIT vs. Luc Léger: Comparison Table

Criteria

RABIT Test

Luc Léger Test

Test Type

Progressive protocol with variable intensity phases, including short sprints and hard efforts

Shuttle run with back-and-forths and pace set by beeps

Required Engagement

Real engagement with high-intensity phases (10s sprint, 30s at very hard pace)

Maximal effort to exhaustion

For Kids/Teens

Yes, phases can be adjusted, but real engagement is needed

Less suitable, tiring, requires coordination for turns

Duration

About 30–40 min with effort and recovery phases

Short and intense, often <15 min

Main Measure

Complete energy profile: strength, acid resistance, cardiac efficiency, effort perception

MAS and VO2max estimate

Maximal Effort?

Yes, but split into phases with recovery

Yes, continuous maximal effort to exhaustion

Practicality

Adaptable to individual, needs equipment (heart rate monitor)

Easy to organize in groups, standardized

Main Goal

Optimized, personalized training prescription

Quick estimate of aerobic capacity

Comfort/Safety

Demands real engagement but with recovery phases, respects sensations

Can be intense, may cause excessive fatigue or stress

Three Sample Billatraining Sessions by Age

9–11 years (Elementary)

Goal: Learn to manage effort, vary paces, develop agility and enjoyment.

· Warm-up: 8 min easy pace (brisk walk or jog, can talk)

· Main set: 3 × (4 min easy + 2 min moderate + 1 min strong pace), 1 min walk between blocks

· Fun sprint: 10s sprint every 3 min

· Cool-down: 5 min walk or jog
Tip: Ask kids to describe their feeling (easy, medium, hard) after each block.

12–14 years (Middle School)

Goal: Build endurance, ability to change pace, and self-management.

· Warm-up: 10 min easy pace

· Main set: 2 × (6 min moderate + 2 min strong + 1 min very hard), 2 min walk or slow jog between blocks

· Technical sprint: 15s sprint uphill or on varied terrain, 2 times per session

· Cool-down: 5 min walk
Tip: Use the talk test to check intensity (can you talk while running?).

15–18 years (High School)

Goal: Optimize power, recovery, acid tolerance, prep for competition.

· Warm-up: 10 min easy + 3 × 30s progressive accelerations

· Main set: 3 × (5 min moderate + 2 min strong + 1 min very hard), 2 min active recovery (slow jog or brisk walk) between blocks

· Acid sprint: 20s all-out sprint at the end of each block

· Cool-down: 8 min walk or very slow jog
Tip: Note heart rate and feeling after each sprint to track progress.

Common points for all sessions:

· Alternating intensities (easy, moderate, strong, very hard)

· Short sprints for speed and agility

· Regular recovery to learn effort management

· Expressing feelings to develop autonomy

These sessions promote rapid progress, effort management, and enjoyment of running, while respecting each age group’s abilities.

The marathon is often seen as a fierce battle against fatigue, where every runner tries to manage their energy to avoid the dreaded “wall.” Traditionally, it’s believed that to improve performance, you need to vary your pace—speeding up at times, slowing down at others—to better recycle your metabolic resources. Yet, a surprising idea is emerging running at a constant—but optimal—pace could be the key to breaking your marathon record, without overloading your body with unnecessary fatigue.

Constant Pace vs. Varying Pace: What the Numbers Say

Let’s look at an example based on a precise physiological model:

· Hard pace (VD) for a runner: 18 km/h

· Easy zone: below 55% VD (9.9 km/h)

· Medium zone: between 55% and 80% VD (up to 14.4 km/h)

If the runner maintains a constant pace at 80% of their hard pace (14.4 km/h) throughout the race, they would finish the marathon in about 2 hours and 56 minutes.

Scenarios simulating controlled pace variation (always below hard pace, between 55% and 80%) show that the overall average speed is actually slightly lower than 80% VD, which slightly increases the total time compared to the optimal constant pace. Even aiming for a fraction of the “hard” pace (RPE 16-17 on the Borg scale of 6 to 20), strictly maintaining this speed without fatigue is unrealistic (unless you’re a robot runner!).

Why Vary Your Pace?

While the maximum constant speed gives the best mechanical average, varying your pace serves an essential function: it helps manage metabolism, specifically lactate recycling—a major source of muscle fatigue.

Running uniformly near the maximal threshold can quickly lead to lactate accumulation that’s hard to eliminate. On the other hand, modulating your pace within the medium zone allows you to alternate between effort phases and active recovery, reducing the feeling of exhaustion while maintaining strong mechanical momentum.

The Key: A Compromise Between Mechanics and Metabolism

· Constant speed near threshold is the ideal mechanical goal for the best time performance.

· Varying speed helps preserve performance over time by optimizing bioenergetic balance.

So, the optimal strategy isn’t just to run fast, but to run efficiently—using the right effort zone and maintaining a constant “action” (a physical concept combining speed and cumulative distance).

Rethinking Pacing with Physics and Biology

Marathon performance isn’t just about endurance or mental strength. It combines dynamic management of the motor and metabolic systems, integrated into a concept called “action”—the product of speed and distance, which must remain stable.

The challenge is to run fast enough to break your record, while avoiding abrupt fluctuations that would waste energy and cause early slowing. This science opens new perspectives for smarter pacing, supported by computerized modeling and artificial intelligence.

Conclusion

Breaking your marathon record by running at a constant pace is possible, as long as that pace is well-calibrated to be as high as possible without exceeding the critical metabolic zone. This approach, combined with a solid understanding of underlying physiological mechanisms, offers a promising horizon for sustainably improving performance while minimizing fatigue.

Trying to run faster by multiplying pace changes without strategy risks increasing fatigue and compromising performance. It’s better to aim for the perfect balance between mechanical strength and lactate management, allowing you to go faster, longer, without breaking down.

To Go Further

Discover how stride entropy reveals essential motor signals for runners, or how artificial intelligence can simulate your ideal marathon strategy in real time. We’ll explore this next week and beyond, sharing insights from our research on real-time marathon exploration using complete biomechanical, neurophysiological, and cardiorespiratory measurements in the field.

Key Session: Sensation-Based Fartlek for Marathon Pace Variation

Goal: Get your body and mind used to alternating paces in the medium zone, listening to your sensations, to optimize lactate recycling and fatigue management for the marathon.

“Sensory Marathon Fartlek” Session

Warm-up

· 20 minutes at total respiratory ease (you should be able to talk effortlessly, legs feel light).

Main block (60 minutes)

· Alternate every 5 to 7 minutes between two sensations:

o “Energetic but controlled” pace (upper medium zone): You feel “active,” your stride is dynamic, you could hold this rhythm for a long time but you feel you’re working (deep breathing, you can speak in short sentences but not hold a full conversation).

o “Relaxed but engaged” pace (lower medium zone): You feel like you’re “actively recovering,” your stride stays smooth, you focus on keeping your arms and shoulders relaxed, you could speed up at any moment, but you stay in a solid comfort zone.

· Golden rule: Never feel “blocked” or “burning” in your legs. You should always feel you could accelerate, but choose to stay in control.

· Tip: If you feel you’re “accumulating” (heavy legs, short breath), extend the relaxed phase.

Finish with 10 minutes

· Return to a very easy pace, total relaxation, you should finish the session feeling like you “still have energy.”

Sensory cues to remember:

· “Energetic but controlled” = upper medium zone, you feel the effort but remain master of your stride.

· “Relaxed but engaged” = lower/median medium zone, you recover without switching off.

· Never enter the “hard zone” (no burning legs, no short breath, no tension).

· You should finish the session feeling you’ve worked, but not exhausted.

Why this session?

· It trains you to sense and manage the micro-variations in pace needed on marathon day to recycle lactate and maintain constant mechanical momentum.

· It develops your ability to listen to your sensations, anticipate fatigue, and stay in the “green zone” of sustainable performance.

Next week, I’ll share the simulation framework for these 10 possible pacing strategies to help you outperform constant pace running.

Great session—send me your Garmin or other brand files!

Yes, you can reach VO₂max during a marathon. No, that doesn’t mean marathon pace is vVO₂max. In reality, vVO₂max is the lowest speed in an incremental test (ramps or stages) at which your oxygen uptake plateaus. In real racing—because of VO₂ kinetics, the slow component, and variable pacing—several speeds can elicit VO₂max if the effort lasts long enough.

Five key points

· vVO₂max ≠ marathon pace: it’s a lab marker, not a pacing dogma.

· VO₂max is reached over 42.195 km… but briefly (a few percent of race time).

· On the 3000 m, athletes spend about half the race at VO₂max; in the marathon, only ~4–5%.

· Marathon performance is better explained by the fraction of vVO₂max (mean race speed as % of vVO₂max) than by time at VO₂max.

· Pace variation (waves) preserves force, spring/rebound, and running economy → higher average speed.

Where vVO₂max comes from (and why it “locks” the debate)

Definition, lab-to-field. vVO₂max is the lowest speed in an incremental test (stages or ramp) at which VO₂ reaches a plateau. It’s essential to calibrate training, but contextual: it assumes a linear, continuous increase in workload.

In real racing, VO₂ also depends on time: it needs time to rise, then it drifts upward (slow component, rising energetic cost). As a result: - you can hit VO₂max at speeds below vVO₂max if you sustain the effort long enough; - you can maintain VO₂max for a while with variable pace (above and below threshold); - the “constant-speed” paradigm is reductionist for the marathon.

What the data say (100 m → marathon)

· VO₂max was reached in all race distances studied (100 m to marathon).

· The fraction of race time at VO₂max is highest in the 3000 m (~51%) and lowest in the marathon (~4–5%).

· In the 3000 m, the more time at VO₂max, the better the performance (r = 0.86). In the marathon, it’s the fraction of vVO₂max (mean speed as % of vVO₂max) that best predicts the result (r² ≈ 0.81).

Historical note. As early as 1976, Michael Maron observed VO₂max attainment in the marathon (intermittent “Douglas bag” sampling from a convertible car). Modern portable systems (breath-by-breath) let us record continuously in real races.

Why varying pace often raises your average

· Varying pace prevents recruiting the same motor pattern at the same load all the time → slowing loss of force.

· Short, reasoned surges followed by a return below threshold “re-activate” the system without breaking the overall perceived effort, which should remain moderate (RPE 13–14).

· In running, the energy cost per km is nearly constant; thus, fine pace oscillations help optimize the overall cost and the spring/rebound mechanics that drive running economy.

Running economy. The energy cost to run at a given speed. The lower it is, the faster you can go at the same VO₂. The spring/rebound (leg stiffness/stride) contributes strongly.

Power vs. speed side note: on cycling flats, external (aero) power rises steeply with speed (~v³). In running, the cost per km is almost constant, and metabolic power rises almost linearly with speed—hence the interest in careful pace modulation.

Q.E.D.: widen your power reserve

Your useful reservoir for the marathon lies between your marathon pace (MP) and your maximal ~10 s speed. The wider this reserve, the more you can create pace waves that break monotony, preserve force, and increase your average speed by the finish.

Two “marathon waves” sessions (Billatraining)

Shared principles: keep a moderate perceived effort (RPE 13–14) by adjusting cadence (think trotter horse) while oscillating around MP. Warm up 15–20′ progressive + 4–6 strides. Sip fluids whenever a block exceeds 10′.

1) 1 km Waves (marathon-specific)

· Block 1 (6 km): alternate 1 km at MP + 3–4% then 1 km at MP – 3–4% (three out-and-backs).

· Recovery: 3′ very easy.

· Block 2 (6 km): repeat waves but at +5% / –3% (more punch on the surge, kinder on the float).

· Cool down: 10–15′ easy. Goal: learn to surge briefly without drifting perceived effort; maintain spring → economy.

2) RABIT on M (controlled micro-sprints)

· 30′ at MP – 2% with a 10 s technical sprint every 3′ (flying start, gradually increase amplitude, never all-out, focus on foot quality).

· 10′ easy.

· 20′ at MP, with a 10 s surge every 4′.

· 10–15′ easy. Goal: maintain very-short speed (10 s) within the specific session to widen the power reserve without inflating aerobic load.

Frequency: 1×/week, alternating (Week A → 1 km Waves; Week B → RABIT on M).

Short on time? Here’s the magic workout that takes less than an hour (shower not included! 😉😂)

The science in brief

· VO₂ kinetics and the slow component explain how you can reach VO₂max at speeds below vVO₂max if you go long enough.

· In the dataset, the 3000 m packs ~half the time at VO₂max and this fraction best explains performance. In the marathon, it’s the % of vVO₂max (relative intensity) that separates the best performers—and that percentage benefits from pace variation.

· Continuous field measurements with portable breath-by-breath analyzers make these observations ecological (real terrain & competition) and free us from the constraints of treadmill protocols (which also alter gait patterns).

Take‑home messages

· vVO₂max = test marker, not a pacing rule.

· You can reach VO₂max in the marathon, but only briefly (~4–5% of race time—still several minutes in absolute terms; e.g., ~9′ in a 3 h race).

· To perform: combine pace variability with a power reserve between MP and very-short speed (10 s), while keeping a moderate perceived effort.

Further reading

Molinari C.A., Edwards J., Billat V. Maximal Time Spent at VO₂max from Sprint to the Marathon. IJERPH, 2020.

Figures (conceptual)

Figure 1 — 1 km Waves (MP ± %)

1 km Waves — concept (MP ± %)

Figure 2 — RABIT on M (10‑s spikes)

https://publications.billatraining.com/

Fall marathon season is rolling in—perfect timing to revisit two limiting beliefs:

• Marathon performance is purely about endurance, and

• The “wall” is inevitable because of glycogen depletion.

This week, we’re cracking those dogmas to free you from the myth of the inescapable wall.

Have you ever felt that moment when your stride seems to stick to the pavement? The “wall” isn’t just about glycogen. It’s first and foremost the moment when you no longer have enough useful strength to rebound quickly, cleanly, and almost “for free.” What if running economy were, at its core, a question of available power?

Running is fleeing and predating (and that changes everything)

Human running evolved to escape and pursue. It alternates propulsion and suspension: in the air, you apply no force to the ground; on contact, everything happens in a few hundredths of a second. The secret? Being able to push off fast enough to take off and maintain momentum (impulse ≈ Δp, where p = m·v). When impulse fades, mechanics seize up—and energy cost climbs.

Yes, running “costs” more than walking

A simple order of magnitude: ~1 kcal/km when running vs ~0.5 kcal/km when walking. Why? Because of the suspension phase and the need for an effective rebound. If the leg–tendon unit isn’t “strung like a bow” at landing, energy doesn’t transfer smoothly: you land heavy, lose spring, and must re-inject more energy at each step.

Under fatigue: the trap of “I’ll just increase cadence”

We tell ourselves: v = step frequency × stride length, I’ll spin the legs faster. Except that moving the lower limbs is costly (rotational energy). The backward sweep is energy-hungry, and propulsion is the most force-dependent phase. If strength drops (glycogen low), ground contact time lengthens, elastic return falls, and energy cost can spike (> 20%). In short: economy comes from power, not from fiddling with cadence.

“Useful” variability beats the myth of a perfectly steady pace

The best marathons are not perfectly flat pace lines—they show smart oscillations in speed. In our analyses, high performers allow small, asymmetric fluctuations that help them fine-tune energy systems, recover, and avoid cardio/VO₂ drift. (See the study on pace oscillations and performance.) [Billat et al., “Marathon performance & oscillations.”]

Humans can self-regulate acceleration (for a long time)

Another key point: we can maintain self-paced accelerations (gentle, moderate, hard) up to exhaustion thanks to small stochastic corrections around an average value. Modeled with an Ornstein–Uhlenbeck process, these accelerations remain remarkably stable despite fatigue. This is strong scientific support for acceleration-based training by feel, rather than rigid, square-wave intervals.
[Billat et al., “Humans are able to self-paced constant running acceleration until exhaustion”; “Long-term constant acceleration sustained via stochastic short-term corrections.”]

What this changes for training (spoiler: strength is specific)

No need to bet everything on heavy squats. Marathon-useful strength is specific: fast, elastic, with contraction angles and speeds close to running (force–velocity and tension–length). Prefer building elastic-functional strength and the ability to accelerate cleanly (short vertical time + tonic propulsion) to protect rebound when glycogen dips.

Experimental evidence (and not only in humans!)

In older mice, a short acceleration-based training model outperformed a long endurance protocol: higher max speed, longer time to exhaustion, and better mitochondrial/enzymatic markers (CS, LDH, CK). Practical takeaway: short, well-measured accelerations can revive both the aerobic–anaerobic “powerhouse” and muscular “grip.”
[Billat et al., “A new model of short acceleration-based training.”]
To calibrate effort precisely, a low-acceleration ramp protocol (0° slope) proved optimal to elicit true VO₂max in animals.
[Ayachi et al., “Validation of a Ramp Running Protocol.”]

Signature Session — HIIT by Acceleration (not square waves)

Why? Because F = m·a. Training acceleration means training impulse (force × ground-contact time), the thing that makes your stride lift off—without breaking your mechanics.

Format (40–50′)

• Warm-up 20–25′ + 2–3 progressive strides.

• Block A: 8 × 15–20″ progressive acceleration (from tonic → strong), 40–60″ easy jog recovery.

• Block B: 6 × 10″ on a gentle hill (focus on vertical rebound), 50–60″ recovery.

• Option: 4–6 technical tendon strides (80–100 m).

• Cool down 10–15′.

Technical pointers

• Short contact (“hot touch”).

• Tonic vertical push (rebound, no dragging).

• Active arms (sync the impulse).

• Shortened backward sweep (don’t let the leg trail).

Race Day: why strength gets you past the wall

• A reserve of power protects your rebound when glycogen falls.

• Contact time stays short, so economy stabilizes.

• Useful variability (micro-adjustments in cadence/stride length) prevents mechanical saturation.

Teaser — What’s next

How to subtly vary speed throughout the marathon to alternate energy systems, avoid saturation (cadence, amplitude), and optimize economy via controlled cadence changes—a “black” that isn’t monochrome, in the spirit of Pierre Soulages’ Outrenoirs.

References (selection)

1. Billat V. et al. — Marathon performance & oscillations (analysis of rhythm oscillations and performance).

2. Billat V. et al. — Humans are able to self-paced constant running acceleration until exhaustion.

3. Billat V. et al. — Long-term constant acceleration can be sustained freely in running via stochastic short-term corrections.

4. Niel R., Billat V. et al. — A new model of short acceleration-based training.

5. Ayachi M., Billat V. et al. — Validation of a Ramp Running Protocol.

see all publications:

https://publications.billatraining.com/

However, physiology tells a different story: well-structured training involving heavy and explosive strength work, training close to the vVO₂max threshold and controlled pace variability yields greater, more reliable and sustainable improvements in energy cost and sustainable speed. As for ‘4%', it is often lost in biological variability and measurement uncertainty. This post puts the numbers in perspective — and the runner at the centre.

Why is the '4%' a practical (and scientific)

The famous figure comes from heterogeneous laboratory protocols and limited samples. Repeat the measurement on another day, change your pace slightly or switch from the treadmill to the track and you can easily get a variation of ±2–4% just from variability and measurement noise. Recent literature emphasises this point: to determine whether a shoe 'gives you an advantage', you need to conduct multiple tests, counterbalance the conditions and accept that the benefit is not universal. Certain body types (such as those with pronated feet) derive less benefit from the plates, while for others, the benefit is real and sometimes clear, but conditional on speed, technique and dynamic stability. In short, there is no general recipe, only individual responses.

What training really does — and to a much greater extent

— is that the most powerful tool is in your legs and in your training plan. Reviews and meta-analyses show that adding heavy strength training (≥80% 1RM) and/or explosive/plyometric training to endurance training lowers economy (cost) by 2–8%, depending on the protocols. It also improves vVO₂max and increases performance over distances of 1.5–10 km. The longer the cycle (8–16 weeks), the more pronounced the effect. In short, gains often exceed 4%, with lasting benefits.

These gains can be mechanically explained by better neuromuscular coordination, more efficient muscle-tendon stiffness and delayed recruitment of less economical fibres. The impact is not limited to the laboratory, however; it can also be seen on the stopwatch.

The shoes are not 'fake' — but they won't run for you!

Certain geometries (e.g. curved plate, pronounced rocker and resilient foams) redistribute the work, placing less strain on the ankle and more on the knee, which can sometimes result in better mechanical efficiency of the ankle. This may or may not result in a reduction in cost. In addition, shifting loads has trade-offs: depending on the model and your profile, stress on the knee and Achilles tendon can increase, and the leg return phase can become more vigorous, increasing the eccentric demand on the hamstrings if you are not ready. Hence the golden rule: gradual use, eccentric strengthening and an individualised approach.

The runner-centred method: the energy radar.

Rather than buying into a promise, measure your profile and let the shoe adapt to it. Here is a simple energy radar to follow:

Aerobic power and speed reserves (easy → hard → sprint).

Cadence and contact time (spatio-temporal stability).

Controlled variability of pace (governance).

Dynamic stability (directional/rotational).

Mechanical tolerance (leg, ankle, Achilles and knee).

Specific endurance (cost at target pace).

From there, we pair foam (return/relaxation), rocker/apex, stack height, stiffness and plate shape (flat vs. curved) according to your needs, not the other way around. Then we train the areas that need improvement (heavy/eccentric strength, technical work close to vVO₂max and speed oscillations) to consolidate the gains made.

What the literature data says (order of magnitude to remember).

Shoes (group): reported modest and heterogeneous average O₂-cost reduction (not universal; depends on model, speed and profile).

Pronators: reduced benefit with plates.

Weight: the rule of thumb is that there is an energy cost of approximately 1% per 100 g added or removed.

Training: heavy/explosive strength + intervals close to vVO₂max → 2–8% cost reduction (according to studies), vVO₂max and times increase — a lasting and replicable effect.

Test instructions: Test at home (ground > treadmill): 2 × 10 minutes at marathon pace with a 3-minute recovery. Compare heart rate (HR), rating of perceived exertion (RPE) and contact time between the two repetitions.

Conclusion:

The '4%' sales pitch is an unreliable average. Physiology is more generous: if you train intelligently, you can achieve more than 4% (an 11% increase in 'easy' speed, 'average' and 'hard' speeds, and even 'very hard' speed in 8 weeks with BillaTraining), with less risk and more control. The shoe is a tool; you are the engine. 😉

References (selection, for the curious reader):

Barnes, 2014; Denadai, 2016; Blagrove, 2018; Lanos-Llagos, 2024; Mayoralas, 2017; Montero, 2015; Rønnestad, 2013; Van Hooren, 2024–2025; Miyazaki, 2024; Seo, 2025; Toshiya, 2025; Werkhausen, 2024; Brueggemann, 2025; Carranza, 2025.

For more details, see my updated bibliographic research on the issue of carbon shoes from 2024–25:

1) Why 'energy cost' really governs performance

In trail running, as on the road, performance is not just a question of VO₂ max; it is the energy cost of movement (ECM)—the energy required per kilogram per kilometre—that converts VO₂ into actual speed. This is why the simplistic 'equivalence' of 1,000 metres of elevation gain equalling 10 kilometres on flat ground is not universal. Depending on the slope, pace, technique and equipment (i.e. shoes), the ECM varies greatly, as does the resulting speed. Recent studies linking biomechanics and running economy remind us that this cost depends on multiple factors (technique, anthropometry, muscle architecture, etc.) and inter-individual variability — a key point in understanding heterogeneous responses to modern shoes.

2) The mythical threshold of ~3.8 kJ·kg⁻¹·km⁻¹ and the rush for carbon plates

In the quest for a sub-2-hour marathon, a CED of around 3.8 kJ·kg⁻¹·km⁻¹ has been identified as a prerequisite for achieving record speeds. This goal has given rise to a wave of 'super shoes' (highly resilient foam + plate—often carbon—curved into a high sole). The first Nike Vaporfly 4% was released on 20 July 2017, after prototypes emerged as early as 2016. The regulatory change by World Athletics then set the sole height at 40 mm and required a commercially available model (2020 rules).

From a scientific perspective, pioneering research demonstrated reductions in O₂ under controlled conditions, but without identifying a single, universal mechanism, which necessitated more detailed analysis. (PubMed)

3) 'Effective'... but for whom and under what conditions?

The literature from 2024–2025 (including your contributions) provides a more nuanced picture:

The curvature of the plate and the 'teeter-totter effect'

Beyond just 'longitudinal bending stiffness' (LBS), geometry matters. A protocol on an instrumented track shows that a curved plate can reduce mechanical work and improve mechanical efficiency at the ankle (less plantar torque during much of the stance phase), which is consistent with more economical muscle contraction.

This study highlights segmental energy transfer (shank to foot) and explains how curvature enhances the 'rocker' function, shortens the lever arm and moderates unfavourable angular velocities.

Stiffness (LBS) does not explain everything.

A 2025 meta-analysis examined LBS and energy return from foam and found heterogeneous effects and modest effect sizes, which depend on the runner's profile and the shoe model. In short, there is no single '+4%' button.

Biomechanical mediators vary from person to person.

A 2025 review led by Van Hooren emphasises that no mediator (e.g. kinematics, morphology, muscle architecture, comfort filter) stands out as universal in explaining RE gains through shoes. It also insists on the need for multiple trials per condition, as biological and instrumental variability can mask or mimic a 'gain'.

Pronators experience less or no gain.

A dedicated study shows that people with pronated feet derive less benefit in terms of energy return (RE) from a carbon plate than people with "normal" feet, possibly due to reduced ankle and toe range of motion (ROM) and a lower capacity to store and restore energy from the plate.

Kinematic/kinetic analysis of the foot-leg

Comprehensive 3D measurements (2025) show modulations in angles and moments at the foot and leg joints under the carbon plate. This confirms that the mechanical action shifts (less at the metatarsophalangeal joint and more elsewhere, depending on the model and gait). However, these redistributions do not systematically result in better RE for everyone.

Stability and fatigue:

At the same time, changes in dynamic stability are observed (stiff insoles) as well as the effects of fatigue on the shape and efficiency of the plate. In other words, what 'works' well at a moderate pace may be different at the end of a marathon or on a downhill trail.

Treadmill vs. terrain / road vs. trail

Comparisons on uphill/downhill trails show responses that are specific to the slope. Extrapolating the results from a treadmill to terrain is difficult, and many contradictory conclusions are explained by heterogeneous protocols.

4) Mechanics first, physiology second: a mismatch

As physiologists with 35 years' experience, and as former athletes and coaches, we believe that shoes have been designed with mechanics in mind, such as increasing the useful lever arm, storing and restoring energy, and stiffening 'costly' areas (MTP). In real life, it is often the runner who adapts to the shoe, not the other way around, except for a few champions who have custom-made shoes (geometry, foam, rigidity and "governance" of pace). Recent developments precisely argue for individualisation: selecting the right combination of foam, plate, rocker and stiffness for a given energy and biomechanical profile, rather than "one plate for all".

5) Why the '+4%' is often a measurement error

On small samples, a single test per condition is enough to 'create' a +/- 2–4% variability (day-to-day, VO₂ calibration, cardio drift, micro-pacing), hence the methodological warning to repeat the tests, counterbalance the orders and respect the equilibrium times. The 2024–25 reviews emphasise this point.

Add to this the fact that certain populations (pronators and those with specific technical adaptations) respond differently or negatively.

6) A new approach: shoes dictated by the runner's 'energy radar'

Rather than imposing mechanics, we should measure the individual's 'energy radar' (power, cadence, speed variability, power reserve, eccentric tolerance, MTU elasticity, MTP efficiency, directional stability, etc.). Based on this radar, we choose the geometry (stack/rocker/apex position), stiffness (flat vs. curved plate) and foam (return/relaxation time). Recent data supports this individualisation (clustered responses to stack heights, LBS, etc.).

This is exactly the spirit of Billatraining: working at vVO₂max and using speed oscillations (e.g. 30”/30”) to maximise time spent at VO₂max and improve pace control and economy within realistic constraints, before validating the 'shoe' tool for the athlete (PubMed, publications.billatraining.com). (PubMed, publications.billatraining.com).

7) Injuries and 'stiff recovery': watch out for hamstring injuries.

MAX/plate technologies can shift loads, relieving the ankle and Achilles tendon via the rocker. However, this increases moments at the knee and sometimes puts stress on the Achilles tendon, depending on the position of the fulcrum. There is also an increased risk of injury (observations over 12 months: knee and Achilles tendon). In certain settings, the 'snap' of the return can accelerate leg return and increase proximal eccentric demand (hamstrings) at the end of the cycle, especially if the technique is not prepared for this.

In short, without progressive adaptation and eccentric strengthening, the promise of 'economy' can result in days off, which cancels out any theoretical gain.

What your added papers say (in brief):

Miyazaki (2024, J Biomech): The curved plate improves mechanical efficiency at the ankle (MEC increases, MEE decreases) by modulating the angular velocities of the shank and foot, and shortening the lever arm. This is a plausible way to improve RE, but direct metabolic evidence is still needed.

Stephen 2025 (review/meta): LBS + energy return → inconsistent results dependent on subject/model; optimisation is not a simple case of 'stiffer = better'.

Van Hooren (2025): no single mediator, high variability and the need for multiple trials.

Toshiya (2025): pronators → reduced RE gain with plate via limited ROM and reduced storage-restitution.

Seo (2025): foot-leg kinematics and kinetics are modified by the plate (reallocation of forces); there is no guarantee of metabolic gain in all subjects.

Fukuchi (2024, trial): specific responses to uphill/downhill; caution regarding extrapolation from treadmill to terrain.

Gao, Xu and Yang (2025): stability and fatigue interact with fitness and stiffness, and the effects may be attenuated or reversed depending on fatigue status and pace.

Brüggemann (2025, pre-print) and 12-month cohort: 'MAX' increases loads and the risk of knee and possibly Achilles injuries compared to other technologies. The tool strongly influences the injury profile regardless of age or volume.

Let’s admit it: the equivalence “1000 m of vertical gain = 10 km flat” is a mountain Dahu (a myth). It’s told to novice mountain runners to haze them, and a few apprentice “scientific types” repeat it solemnly, wielding the mechanical talisman below. Except… mountains don’t read rulebooks.

The often-invoked mechanical talisman:

Why it’s a Dahu (myth)

• The energetic cost is neither linear nor symmetric: it depends on slope i. Running downhill at −10% to −20% can cost less than flat, whereas steep uphills blow up the cost per meter.

• The movement pattern is not unique: we run, we hike, we use poles. A “pure running on flat” model is off-base.

• The terrain is not standardized: snow, mud, scree… the surface changes the cost.

Actual speed depends on a simple ratio: available power over cost per meter:

When slope i changes, C(i) changes; therefore, for the same power, you don’t get the same speed. So we replace the back-of-the-envelope rule with a falsifiable measurement: integrate the GPX trace, compute the slope-dependent cost, and convert total energy into a flat-equivalent distance (EFD).

Consequence for rankings

Forcing the Dahu into a “1000 points” ceiling pegged to the marathon world record ends up ranking trail races as if they were marathons in disguise. What you mostly grade is the scoring system’s error: EFD computed from GPX systematically diverges from “10 km/1000 m”, sometimes massively.

Take-home

• The “1000 m D+ = 10 km flat” calculation is a Dahu legend: easy to tell, wrong in practice.

• The mountain imposes real bioenergetics: C(i) varies with i, with surface, and with locomotion mode.

• To compare and rank, measure energy along the actual trace (GPX) and derive EFD—not a mechanical shortcut.

Therefore, the energetic cost of positive and negative slopes—including snow (cf. UTMB 2025)—changes the flat-equivalent distance (EFD), reduces speed for the same trail finishing time, and thus lowers the true flat-equivalent distance. Otherwise, the marathon world record would be… shattered.

Demonstration (with equations and numbers): why the correction “1000 m ascent = 10 km flat” is mechanically tempting but bioenergetically wrong

Realizable speed depends on energy cost per meter, which itself is a function of slope (positive and negative).

1) Mechanical reminder (insufficient)

The classic argument compares gravitational potential to kinetic energy:

Even though this suggests a numerical “equivalence” (since g ≈ 9.81), it ignores locomotor physiology and the dependence of energetic cost on slope and muscle contraction type (concentric/eccentric).

2) Energy cost of running as a function of slope

The mass-specific energy cost per meter of running (level: 3.6 J·kg⁻¹·m⁻¹) varies nonlinearly with slope i (decimal; e.g., +0.10 = +10%):

The curve has a minimum around −17% (moderate downhill), and rises steeply for large magnitudes of slope—both uphill and downhill (eccentric braking).

3) Realizable speed and “Equivalent Flat Distance” (EFD)

For a given athlete, sustainable speed on slope i relates to the sustainable fraction of VO₂max and to the cost:

On a GPX trace, integrate segment energy to obtain EFD:

4) 2025 application (men’s winners) — Sierre‑Zinal & UTMB

Public profiles and cautious average slopes for uphill/downhill segments provide a lower bound on the divergence from the “10 km/1000 m” rule.

The “10 km/1000 m” rule overestimates EFD by +22% (Sierre‑Zinal) and +34% (UTMB) compared with the bioenergetic calculation. Conditions like snow/mud would further increase actual EFD.

• “1000 m = 10 km” rule: 31 + 2.2×10 = 53.0 km → overestimation ~25%.

UTMB 2025 (shortened + snow): three illustrative scenarios:

Naïve comparison ignores (i) the downhill cost minimum around −10% to −20% and (ii) the surface penalty (snow) when present.

Abbreviations & notation

• D+, D−: positive/negative elevation gain (m).

• i: slope in decimal (e.g., +0.10 = +10%).

• C_r(i): mass-specific cost per meter at slope i [J·kg⁻¹·m⁻¹].

• EFD: Equivalent Flat Distance — flat distance with identical total energetic cost.

• VO2max (\(\dot{V}O_{2}^{max}\)): maximal oxygen uptake (rate).

• f: sustainable fraction of VO2max (depends on duration/pacing).

• GPX: track file (distance/elevation/time points).

• UTMB: Ultra‑Trail du Mont-Blanc; ITRA: International Trail Running Association.

Summary

Comparing a trail to a standardized flat race via “1000 m D+ = 10 km” is physiologically unfounded. Running cost depends on slope (up/down) and terrain conditions, so average speed and points derived from a naïve flat‑equivalent distance are biased. EFD from integrating C_r(i) along the trace provides a robust basis for evaluation.

It's a free, open blog with a serious side to it, perfect for my Sundays! You can give a gift to show you care, but it must be something you like, even if it's free (doom).

Friday, August 29th, the spectacle will be total: in the morning, the CCC sets off from Courmayeur (101 km and 6,050 m of elevation gain), then, in the evening, comes the main UTMB loop (174 km, 10,000 m of elevation gain, through France, Italy, and Switzerland).
More than 3,000 runners will be at the start, and the story is well known: nearly a third will drop out before reaching Chamonix. Why? Not for a lack of training, but often due to poor effort management.

My message is simple: you can avoid dropping out. How? By following your natural pace.

Can You Really Trust Your Sensations?

The answer is: yes... and no.
For over 15 years, at our Billatraining lab at the University of Évry, we’ve studied how the body regulates exertion. After exploring classical approaches (constant power, constant heart rate, constant lactate), we tested a new hypothesis:

👉 The athlete adjusts their ascent speed based on inspired oxygen pressure (PIO₂), which decreases with altitude.

So, we took the lab into the real world—not just on treadmills but on the slopes of Mont-Blanc and elsewhere, sometimes sacrificing sleep to recharge batteries under a refuge table. Physiology is not just for the gym!

Result: Whether you're an experienced mountaineer or a trail runner, as soon as you have a sufficient VO₂max (≈ vVO₂max ≥ 15 km/h), you automatically adjust your climbing speed with remarkable precision. The big difference?

· Experienced mountaineers use half as much oxygen per meter climbed compared to novices.

The Brain: Your Ally... and Your Safety Net

Our marathon studies have shown that the brain detects early signals of fatigue (as soon as 20 km!): rising energy costs, decreasing speed variability.

👉 This is when you need to listen to yourself.
The danger isn’t lack of willpower but forcing yourself against internal signals by following an overly rigid plan.

Simple Recipe for a Successful UTMB

· ✔ Train by feel: at least 6 months before the race, practice running without relying on a watch.

· ✔ Lighten up on tracking: leave the watch in the locker room. Just keep your phone for safety.

· ✔ Refocus: on climbs, try the “sentence rule”: if you can speak a full sentence clearly, you’re in the right zone.

· ✔ Manage your fuel: prep a simple, mildly sweet drink to avoid hypoglycemia without ruining your stomach.

Hypoglycemia and Perceived Effort (RPE): Beware the Trap

· ⚠ The brain overestimates effort → RPE artificially climbs.

· ⚠ Concentration drops, cognitive fatigue rises.

· ⚠ Hormones (cortisol, adrenaline) amplify perceived effort.

· ⚠ Result: you feel like you’re maxed out… even when you still have fuel in the tank.

👉 Don’t rely solely on RPE during hypoglycemia; it’s a safety net during this extreme race in a merciless mountain setting – don’t forget!

Conclusion: Run With the Intelligence of the Body

The UTMB is not just about physical strength. It’s a dance between the brain, muscles, altitude, and sugar.

Listen to your sensations, respect your limits, accept slowing down, and you’ll discover that the pleasure of running around Mont-Blanc comes less from the stopwatch and more from the clarity of a well-managed effort.

And remember: your sensations never lie about your overall fatigue state, even if your heart rate is low (altitude promotes this) or your lactate level (“the legendary ‘lactate paradox’”).

If you feel that the effort is very hard but you're barely moving forward, it’s probably time to have that homemade sports drink—or the one you’re used to (never experiment with new food or drinks on race day).

Practical Recipe for a 500 mL Soft Flask

· 1 sugar cube no. 4 (≈ 5g sucrose)

· Add pure glucose to reach a 2:1 glucose/fructose ratio

· Add a pinch of salt (≈ 0.5g)

Hawaii. Kona. The Ironman World Championship.
The ocean sparkles. The heat crushes. Three giants stand on the start line:

- Léon Marchand – World record swimmer, master of the water.
- Tadej Pogačar – Two-time Tour de France champion, cycling phenomenon.
- Eliud Kipchoge – The fastest marathoner ever.

Same starting gun. Same brutal course. And for this experiment: the same VO₂max — 85 ml·min⁻¹·kg⁻¹.

Same Engine, Different Gears

In their own sport, each can tap 80% of that VO₂max at peak efficiency. Outside their comfort zone? Efficiency drops to 60% and energy costs skyrocket.

Translation:
- Marchand cuts through water like a torpedo… but on a bike, he’s just another rookie.
- Pogačar flies on two wheels… but running burns through his oxygen budget.
- Kipchoge runs like the wind… but in the ocean, drag is his worst enemy.

Base Scenario (80% / 60% efficiency)

The True Cost of a Kilometer

- Swimming: Oxygen-hungry.
- Cycling: ~20× cheaper than swimming in energy cost.
- Running: ~4× more expensive than cycling.

The final result is shaped as much by where you’re moving as by how fast.

Who Wins?

In most scenarios, Pogačar’s insane bike split carries him to victory — even with slower swim and run times. Kipchoge often takes second thanks to his legendary marathon pace, while Marchand dominates the swim but can’t recover the time lost on land.

But the real winner? The one who spends the least energy per meter. Big engines are great… but low fuel consumption wins the race.

Technology: The Silent Competitor

- Swim: High-performance wetsuits slash drag.
- Bike: Aero frames, disc wheels, wind tunnel positions.
- Run: Carbon plate shoes for elastic energy return.

Even pacing strategies — like Sub2h marathon pacer formations — are tech-optimized.

Optimized Scenario (Tech boost + Equal Costs)

Final Take

Sometimes, the all-rounder beats the legend. And in Kona, the ocean decides first.

This hypothetical match shows that the winner isn't necessarily the one with the most powerful engine, but the one who can convert every watt into useful speed while minimising energy losses. So, working on energy costs is really important, and technology plays a big part in that. Even though around 71% of the planet is covered in water, our bodies are actually adapted to being on land. When you compare this to other options, water is pretty energy-intensive, air is manageable, and the ground provides elasticity. The Ironman shows that the winner isn't just the person with the highest VO2max, but also the one who pays the least for each meter they travel.

The role of tech

Aerodynamics, hydrodynamics, and the return of elastic energy through carbon-plated shoes are all really important for performance. For the Sub2h marathon project, even the positioning of the pacemakers was optimised in a wind tunnel. We can picture the role of tech in cycling: streamlined frames, lenticular wheels, extreme positions in time trials.

When it comes to swimming, high-performance wetsuits can help you swim faster by reducing drag. When you're running, carbon plate shoes help you to get the most out of every stride.

And most importantly, make sure you're getting the most out of your strength and muscle power to avoid hamstring cramps or worse, a pulled muscle. With carbon shoes, you can save time and avoid getting injured for weeks at a time with a VO2max that can only handle a break from training for three weeks at most!

When it comes to cycling, you'll need to put in the hours to earn the thousands of dollars needed for a machine that battles friction and gravity.

You could spend your time training!

It's always the same story: should you lose your life trying to win it?

Anyway, the general rule in sport is to save time by training at all levels of intensity so that you're ready to put on those shoes or get on that bike of yours.

Fancy a swim? Here's some advice: start young, very young! And with a good coach who doesn't just make you swim laps, especially if you're aiming to win an Ironman in a real aquatic environment with waves and maybe even (again?) fish.

The good news is that if you're a pretty good triathlete, you'll beat those sports titans!

Why two flavors of HIIT feel so different

Not all HIIT is created equal. Some sessions live around 90% vVO₂max (2–4 min repeats, equal recoveries). Others attack ~100% vVO₂max (30–60 s bouts with active recovery). They load the system differently—central (heart) vs peripheral (muscle).

90% vVO₂max: power with physiological stability

At ~90% vVO₂max you stay in the severe domain (above lactate threshold yet below all-out). HR rises but stays submaximal, preserving ventricular filling and keeping stroke volume high; arterial oxygen saturation remains ~98%, maintaining a strong a–vO₂ difference. Translation: big cardiac output without destabilizing oxygenation.

· Central: strong volume loading → diastolic function and ejection improve.

· Peripheral: well-oxygenated muscle → mitochondrial and capillary adaptations.

· Exercise-induced hypoxemia risk: low, ventilation-perfusion stays matched.

100% vVO₂max: maximal cardio-respiratory stimulus

Short-interval formats (30/30, 60/60) drive VO₂ to peak quickly. HR hits near-max, shortening filling time; stroke volume can dip and cardiac output relies more on rate. Ventilation may outstrip perfusion briefly; some athletes show mild desaturation (exercise-induced hypoxemia). Still, these sessions are excellent to elevate VO₂max and cardiac power.

· Central: rapid, high-intensity stimulus → pushes VO₂max upward.

· Peripheral: fast-twitch recruitment and buffer training.

· Hypoxemia risk: higher in sensitive athletes → monitor HR–speed for a deflection/plateau.

Fick in one line

VO₂ = Q × (CaO₂ − CvO₂). In trained athletes, the bottleneck is often Q (cardiac output). Long HIIT (~90%) grows how long you can live near this ceiling; short HIIT (~100%) raises the ceiling itself.

How to train (3 scenarios)

1. Goal = ↑ VO₂max & cardiac power → emphasize short HIIT (30/30, 60/60) at ~100% vVO₂max with controlled reps and active recovery.

2. Goal = ↑ severe‑domain tolerance (time at high VO₂) → emphasize long HIIT (2–4 min) at ~90% vVO₂max; keep oxygenation high and avoid early central fatigue.

3. Goal = Health + Performance → mix both weekly: 2 hard days (one ~100%, one ~90%) + 1 easy day (RPE ≈ 10).

Coach’s Field Notes

• RPE 15–17 = useful severe domain; if breathing fragments too early, ease off slightly.

• Cardiac cost (beats per meter): if it rises faster than speed falls, you’re paying too much.

• Women, marathon: a 1000 m all‑out after 10 km at marathon pace is a sensitive checkpoint.

Go deeper (original sources)

Classic papers and notes:

• Billat (1994) — Tlim at vVO₂max (reproducibility)

• Faina et al. (1996) — Anaerobic contribution at minimal intensity eliciting VO₂max

• Lepretre et al. (2004) — Exercise intensity, VO₂max & cardiac output

• Lepretre et al. (2004) — Heart‑rate deflection to defend stroke volume

• Billat Training Publications (portal)

Beyond their incredible engine, a Tour de France rider literally devours energy (up to 8,000 kcal in a single mountain stage)… and leaves behind a significant carbon footprint.

Food surplus: about ± 95,000 extra kcal over three weeks ≈ 2.4 t CO₂e
Flight surplus: ± 30,000 km of air travel per year, i.e., 4.5 t CO₂e

And yet, if you convert all of that to the actual mechanical energy produced on the bike (84 cumulative hours of racing), each human kilowatt-hour ends up costing 500,000 times more than the one you pay your electric utility for!

2. How much is a champion’s watt “worth”?

The power gap between the best male and female road cyclists is roughly 1 W/kg:

Aerobic power (70 kg):
- Elite man: 7.6 W/kg → 530 W
- Elite woman: 6.1 W/kg → 425 W
- Difference: + 105 W

Over the 83 h 39 min of the 2024 Tour:
E = P × t = 105 W × 83.65 h ≈ 8.8 kWh

The winner’s prize (€500,000) values each “red” kilowatt-hour at €57,000/kWh — that’s 500,000 times the average residential rate.

3. The carbon cost of the plate

Over three weeks of racing:

- Total energy requirement: 145,000 kcal
- Basal metabolism: 50,000 kcal
- Extra due to the Tour: 95,000 kcal

With an estimated 2.5 kg CO₂e for every 1,000 kcal produced and transported:
95 × 2.5 = 240 kg CO₂e just to fuel the performance.

4. The hidden kilometers

Adding up all travel over a season:

- Flights: 30,000 km × 0.15 kg/km = 4.5 t CO₂e
- Bus/cars: 10,000 km × 0.18 kg/km = 1.8 t CO₂e
- Total: 6.3 t CO₂e

5. And what about “pink” energy?

For the 2024 Tour de France Femmes:

- 5.9 W/kg (≈ 413 W) for 24.6 h → 10.2 kWh
- Final prize: $50,000
- Each “pink” kWh is worth $5,000 — ten times less than the Yellow Jersey.
- Extra calories consumed: 60,000 kcal (~ 150 kg CO₂e).

And what if you earned your own watt?

Good news: you don’t need a podium or a publicity caravan to gain 1 W/kg by fall.

Here’s my favorite workout:

30/30 session
- 10 min progressive warm-up
- 5 × 30 s accelerations to open up the legs
- Main block: 2 × 10 repetitions:
- 30 s at 100% vVO₂max
- 30 s active recovery at 50%
- 5 min recovery
- 10 min cool-down

2 sessions per week for 8 weeks = +1 W/kg guaranteed

One watt can change a life

A simple +3.5 mL/kg/min VO₂max (≈ 1 W/kg), even after age 50, reduces mortality by 17%.

In economic terms:
2024 Value of Statistical Life = $13.7 M
13.7 M × 17% ≈ 2.3 M
The health value of 1 W/kg: $2.3 million.

Conclusion

Producing your own electricity by pedaling? Bad plan.
Human energy is precious… but very inefficient as a source of power.

On the other hand, investing in your watt/kg means:
- 17% lower mortality
- +10 extra years of life
- A health capital that no stock market can match.

So let the Tour make you dream: your power meter will be better for it… and so will your mitochondria!

Gains from watts earned through training

(30 s at vVO₂max / 30 s at 50% vVO₂max for 15 min running, and 15 × 1 min at PMA / 1 min at 60% PMA on the bike):

- Electricity gain: equivalent to 8.8 kWh over a full Tour – a human energy output that would cost 500,000 times more than your utility bill.
- Dollar gain: $2.3 million “statistical value” for each watt/kg gained through improved VO₂max.
- Life gain: +10 years of life and –17% mortality after 50, just by improving your watt/kg.

Illustration: CO₂ Emissions of a Pro Cyclist

And, just out of curiosity, what sort of Maximal Aerobic Power (MAP) would that take?

Harvest season is about to start early—though this time we’re not talking about Montmartre’s wine. We’re “taxing” Pogačar by handing him a 1990‑style Parisian icon: the 23 kg Vélib’. Meanwhile, you’ll need at least the bare‑minimum MAP we’re about to crunch with our very own Geppetto. You’ve got 48 hours to train and place that last‑minute order for the Colnago (lucky you, it’s actually in stock!).

Or maybe you’d rather watch the Tour de France finale, then hustle back in time to help with the grape harvest?

Just so we’re clear: Pogačar will be grinding up Rue Lepic’s 7 % grade on a clunky rental bike that feels like it’s rolling on coarse sandpaper. Sounds fair, right?

I do have a brilliant plan for the less‑motivated among you: click “buy now” (delivery in two days!) on the champ’s bike—an 18‑grand ticket without the Slovene legs. Then graciously hand over your flea‑market Vélib’ (we’re talking Porte de Saint‑Ouen vibes, not my dog’s fleas).

So, can you hit 386 watts of MAP at 66 kg in just two days? Sure—if you follow BillaTraining’s nuclear program [https://www.billatraining.com/en/accueil-2/](https://www.billatraining.com/en/accueil-2/) and already push 385 watts today.

In other words, this is straight‑up fiction. As my students love to say: “Not even in your dreams!”

Yes, our dogs can save us from mediocrity—they can let us feel the rush of wind on our faces as we accelerate beyond our limits!

Running nearly 30 km/h over 1000 meters, even breaking the world record pace for that distance (27.48 km/h), is absolutely possible—with the right dog. A well-trained German Shorthaired Pointer is ideal. As for me, I’m a big fan of female German Shepherds—and while I may not break any records, AI let me imagine it anyway:

This week, I had two major emotional experiences:

1. Watching Krypto the Superdog—Will he survive the movie?

2. And the very next day: Will my university validate our new diploma on the contribution of science and sport to animal (and human) well-being?

Great news: Starting this fall, we’ll welcome you into this innovative program, which aims to elevate human training to match the intelligence and sensitivity of our dogs. A new kind of training—animal, intuitive, sensory.

But before that? We need to measure, verify, calibrate our training sessions—with sensors (already invented!) and smart algorithms that factor in the human-canine duo.

Here’s a perfect example brought to life:

Running with Your Dog: A Turbo Wasted? Analysis of a Canicross Test

What the Chart Shows: Four Synchronized Curves

- Red: Speed (km/h)

- Green: Cadence (steps/min)

- Orange: Heart Rate (bpm)

- Blue: Oxygen Consumption (VO₂, in mL/min/kg)

What We Learn from the Graph

We observe two back-to-back tests:

- Without the dog: Caro runs at 19 km/h, then 15 km/h

- With the dog: Caro reaches 25 km/h, then drops to 19.5 km/h

VO₂ max without the dog: 65 mL/min/kg

VO₂ with the dog: plateaus at 60 mL/min/kg

She only taps into 92% of her energy potential when running with her dog.

Cadence reserve underutilized:

- Without the dog: cadence increased by +19%

- With the dog: only +4%

Her step frequency was “held back,” limiting the physiological benefit of the speed boost.

Heart rate rose with the dog, but didn’t reach maximal levels—even though speed was higher.

Conclusion: Untapped Potential

The key takeaway: “Caro doesn’t fully engage her energy systems when assisted by the dog.” Despite running faster (25 vs. 19 km/h), her body didn’t respond with maximal effort—VO₂, cadence, and heart rate all stayed below peak levels.

Physiological Insight

The dog offers mechanical assistance, but the human must adapt—their stride, step frequency, and cardiovascular commitment—to turn that help into real physiological gain. If we simply “ride the pull” without adjusting technique, we waste some of the benefit.

Lessons for Canicross Athletes

To get the most out of the dog-human partnership:

- Improve coordination (stride + cadence)

- Dare to push into higher speed zones

- Use progressive tests (like the 30s “TD” test) to quantify your untapped reserves

This graph reveals a counterintuitive truth: Running faster with a dog doesn’t always mean you’re working harder. Sometimes it means you’re not engaging enough—or not synchronized. Canicross isn’t just about being pulled. It’s about running together, with rhythm and intent.

The Objective: Assessing the Dog’s Effect on Performance

- Caro runs:

- Without dog: 19 → 15 km/h → VO₂ max = 65

- With dog: 25 → 19.5 km/h → VO₂ capped at 60

- Result: Only 92% of energy potential used

- Cadence gains muted (+4% vs +19%)

- Heart rate: elevated but sub-maximal

- Interpretation: Mechanical aid not turned into full physiological commitment

My recommendation to Caro (and soon offered on our BillaTraining SAS platform):

➡️ Focused coordination training

➡️ Overspeed cadence work—with sessions on a gentle downhill to train high-speed power output

Meanwhile, I’m off to do some scent tracking (with meat bits!) with Una, my German Shepherd, who’s taking it easy after a stress fracture—caused by chasing down two canicross superdogs!

Every dog has its specialty—and every sport has its flavor. Scent work is delicious fun!

Because in the end, motivation is everything—and you’ll need plenty to keep up with your dog, even if they’re pulling you!

What is torque?

Torque (T) is defined as:

T = F × L

F = force applied on the pedal (in N or kgF).
L = crank length (in meters).

This torque, multiplied by cadence (angular velocity), determines final power:

P = T × ω
where ω is angular velocity in rad/s.

Force–velocity: the cyclist’s keystone

The force–velocity (or torque–cadence) profile describes a cyclist’s ability to produce torque at various cadences. Each cyclist has a specific signature: some are strong in force (high torque at low cadence), others are strong in velocity (high cadence with lower torque).

F₀: theoretical maximal force extrapolated at 0 rpm (theoretical isometric maximum force).
fMax: extrapolated cadence at which force would be zero (indicator of maximum potential velocity).

In practice, the actual maximal cadence (~110–120 rpm for sprinters) is always lower than fMax.

When age steps in

With age, maximal dynamic force (MDF) and the extrapolated force (F₀) decrease (~20–25% by age 58). Champions thus shift from a force-oriented style to a velocity-oriented style.

Shift of the maximal power point

Maximal power (Pmax) always lies at a compromise between force and velocity.
With age, F₀ declines more than maximal cadence.
The peak power point therefore shifts downward (less force), rather than leftward (slower cadence).

Thus:
- The optimal cadence for power remains relatively stable.
- Older cyclists preserve their ability to spin the legs to compensate for lost force.

The inspiring centenarian example

The case of centenarian Robert Marchand, described by Billat et al. (2017), is emblematic.
At age 101, he covered 24.25 km in one hour; at 103, he improved his record to 26.92 km (+11%).
How? Thanks to polarized training (80% low intensity, 20% high intensity) and focusing on cadence: he increased his maximal pedaling cadence from 69 to 90 rpm (+30%).

This proves that even after age 100, it is possible to offset force loss by improving velocity and technical efficiency.

The “young seniors” of the Étape du Tour

In 2025, the average age of participants in the Étape du Tour cyclosportive is 45 years: a generation of young seniors already facing age-related muscular adaptations.

These cyclists should anticipate this transition:
- Preserve F₀ through strength training.
- Develop cadence to maintain power despite the gradual force decline.

Choosing crank length and gear ratio

Example: gear ratio 54 x 11 (~10.3 m per revolution).

- Longer crank (172–175 mm) → more torque but lower maximal real cadence.
- Shorter crank (165–170 mm) → higher cadence, favors velocity.

From our models:
✅ A longer crank increases energy per pedal stroke.
✅ Higher cadence reduces instantaneous muscular effort.
✅ With age, favoring smoother cadence and moderate torque becomes essential.

Going “beyond” VO2max

What matters is not just oxygen uptake, but how this capacity is turned into watts, speed, and above all mechanical efficiency.

The force–velocity test allows:
- Identifying whether a cyclist is strong in force or velocity.
- Personalizing crank length and gear choices.
- Predicting the optimal pedaling strategy depending on age and muscle profile.

Summary and conclusion

VO2max ≠ final destiny: the force–velocity profile is decisive.
F₀ declines with age, requiring compensation via cadence and mechanical optimization.
Personalization of gear ratio and crank length is crucial.
Robert Marchand proves we can improve even after 100.
Young seniors of the Étape du Tour are already in this anticipation phase.
“On the pedals,” it all comes down to each cyclist’s unique force–velocity balance.

Beyond VO2max, the true battlefield is torque, cadence, and crank choice. It all comes down to “pedaling.”

Sarcopenia and loss of strength with age

Sarcopenia is a progressive and generalized loss of skeletal muscle mass, accompanied by decreased muscle strength and physical function, which appears with aging. It increases the risk of falls, fractures, dependence, and mortality.

Maximal isometric force (Fmax iso) declines gradually:
- Up to 30 years: stable or slightly increasing strength.
- From 30–35 years: start of a slow decline (~1% per year).
- After 60 years: more rapid decline (~1.5–3% per year).

Compared to 25 years old:
- At 50: ~15–20% loss.
- At 70: ~30–40% loss.
- At 80: up to 50% or more loss.

This loss is often more pronounced than muscle mass loss alone, due to qualitative changes and dynapenia.

Dynapenia: strength loss independent of mass

Dynapenia refers specifically to age-related loss of muscle strength not fully explained by muscle mass loss.

It results mainly from neuromuscular alterations:
- Reduction in the number and quality of motor neurons.
- Degeneration of neuromuscular junctions.
- Lower recruitment and voluntary activation of muscle fibers.
- Qualitative shift in fiber type (type II fibers).

Thus, an elderly person can maintain reasonable muscle volume yet lose much of their capacity to produce rapid or explosive force, increasing fall risk and reducing functional performance.

Summary:
- Sarcopenia = loss of muscle mass (quantity).
- Dynapenia = loss of muscle strength (function), even without major mass loss.

His VO₂max, instead of decreasing, increases with each decade he travels backward. At 90 years old, he breaks his own records. At 70, he smashes the masters' records. And at 25, he finally reaches 90 ml/kg/min, symbolically joining Pogačar.

That's what a “Benjamin Button” scenario could have revealed to us.

He would remain not only the absolute champion but also the eternal champion of his age category. In the end, it’s not just a victory over others but a victory over time and finitude. That’s the real “strange story” of eternal cycling.

Estimating VO₂max from a 6-minute maximal cycling test

Here is a reference table to help you find your physiological age on the VO₂max vs. age curve!

Formula to estimate relative VO₂max (ml/min/kg) from cycling power per kg:

VO₂max (ml/min/kg) ≈ (Power in W/kg × 1000) ÷ 70

It is considered that the global mechanical efficiency is about 70 W per liter of oxygen consumed. This approximation is conservative and commonly used to avoid overestimating VO₂max.

Example: at 6 W/kg, VO₂max ≈ (6 × 1000) / 70 ≈ 86 ml/min/kg.

Pogačar’s climb records

Remember, he rode before the climb, and the ascents last more than 30 minutes, so he is at about 90% of his Maximal Aerobic Power (power at VO₂max plateau onset).

You know that I draw a lot of inspiration from watching horses and dogs—creatures that, like children, are far from fond of running at a constant speed. Yesterday, in my role as deputy mayor of my village of 1,200 inhabitants, I witnessed their fantastic endurance during a 2‑hour‑30 show in blistering heat: the little ones (3–5 years), the middles (6–8 years) and the big kids (9–10 years) followed one another in intense dances and songs. They gave everything for their teachers, dressed in costumes that traced the march of time, from dinosaur dances to those of the future ("The Spook" by KSHMR:

Not a hint of breathlessness: the kids performed a musical‑comedy‑style interval session while, in the shade, the parents sweated into their espressos and grew more tired fanning themselves than the children did hopping about.

So I hereby open the “Kid Session for the Mums and Dads,” inspired by the BillaTraining “by‑feel” method: depending on your training level and the temperature where you train, choose your session, ladies and gentlemen.

If your VO₂max is above 40 ml·min⁻¹·kg⁻¹, you can copy the 9–10‑year‑olds’ session. I can guarantee these sessions are effective—we tested them with fourth‑graders (CM1) and compared them with a classic endurance cycle during the school cross‑country race. For everyone else, cut the session time in half (≈ 15 minutes).

1. Children’s Physiological Particularities & Their Session Implications

2. Typical 30‑Minute Session

• Warm‑up: 8 min in the easy zone (green) + 4 motor‑skill games (30 s each*)
• Main block – repeat 3×:
– 5 min easy (comfortable conversation)
– 5 min moderate (broken phrases)
– 1 min hard (no talking)
– 1 min walk or balance game
• Finish: 3 × 5 s sprints on a gentle downhill (≤ 5 % grade for over‑speed) – 55 s walk between sprints.

Fit adults may repeat this session provided the temperature and humidity do not fall into the red zone of the table below.

3. “By‑Feel” Code vs. Temperature & Humidity

Green: all BT components—Easy, Moderate, Hard, Sprints (over‑speed). Normal hydration.
Blue: shorten the Hard phase to 30–40 s, lengthen recoveries. Keep Easy and Moderate; limit Sprints to 2 reps.
Red: cancel or move the session. If it must take place: Easy zone only with slow games; hydrate + mist every 5 min.

4. Zone ↔ Physiological Objective

5. Safety & Individualisation

• No self‑imposed or group competition during the “somewhat hard” phase; priority to how you feel.
• Use the “talk / blow” rule to gauge intensity without a heart‑rate monitor.

Conclusion

High temperatures are an opportunity to take inspiration from children’s playground running—the type of spontaneous interval play that inspired the BillaTraining method. It’s the perfect chance to awaken the child inside us!

In a future blog post I’ll provide illustrated sessions for milder weather, from which you can expect to gain about 10 % VO₂max in 12 weeks with two specific BT sessions per week.
Source (FR): https://asepscience.fr/et-si-nous-nous-entrainions-comme-un-enfant-sous-ces-fortes-chaleurs/