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Months of training, recovery, and planning all build toward a single moment — the start line. You feel ready. Heart racing. Muscles primed. Breath shallow but steady. Yet inside your body, a complex physiological symphony is already unfolding — one that will determine how you perform, how long you can sustain it, and how deep you can go before the wheels come off.

Understanding what happens inside your body on race day doesn’t just satisfy curiosity — it gives you control. It connects the science of endurance with the sensations of competition: that first surge of adrenaline, the burn of lactate, the rhythm of breath, the razor-thin balance between power and collapse.

Let’s go beneath the skin and inside the systems that drive you from the start line to the finish.

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The Main Feature

Leg 1: The Start Line Surge — Adrenaline and Glycogen Ignition

As the gun goes off, your body instantly shifts into fight-or-flight mode. The sympathetic nervous system floods your bloodstream with adrenaline and noradrenaline, priming your muscles for immediate action.

• Heart Rate: Jumps to 70–90% of max within minutes.

• Blood Flow: Redirected from organs to working muscles.

• Glycogen Mobilisation: The liver and muscles release glucose to fuel immediate energy demand.

For the first few minutes, your body runs on stored muscle glycogen — the high-octane fuel that powers intensity. Each muscle contraction burns adenosine triphosphate (ATP), and glycogen is the fastest way to replenish it. Oxygen uptake lags behind, creating an oxygen deficit — that breathless feeling early in the race.

As oxygen delivery catches up, aerobic metabolism stabilises, and your body begins to burn more fat for fuel. But glycogen remains king for speed. Managing its use — through pacing and carbohydrate intake — is what determines how long you can sustain intensity.

T1: Mental Preparation

Leg 2: The Steady State — Lactate, Heat, and Hydration Balance

Once you settle into race rhythm, the real endurance test begins.

Lactate as Fuel, Not Fatigue

As intensity rises, muscles produce lactate, a byproduct of glucose metabolism. Contrary to old beliefs, lactate isn’t the cause of fatigue — it’s a secondary fuel. Your body recycles it through the Cori cycle, converting lactate back into glucose or oxidising it directly for energy.

Trained athletes develop a higher lactate threshold, meaning they can sustain higher intensities before lactate accumulation exceeds clearance. This threshold — often near 80–90% of VO₂ max — is the dividing line between sustainable effort and implosion.

Heat and Hydration

At the same time, your thermoregulation system goes into overdrive. As muscle contractions generate heat, core temperature can climb from 37°C to 39°C or higher. Sweating becomes your cooling mechanism — but it costs you sodium and plasma volume.

As dehydration sets in, blood viscosity increases, forcing your heart to work harder to maintain output. For every 1% of body weight lost through sweat, performance can drop by 2–3%. Electrolyte balance — particularly sodium — is critical for nerve conduction and muscle contraction (see Threshold #149 | Sodium and Hydration for strategy).

Hormonal Equilibrium

During this steady phase, cortisol levels rise moderately, helping mobilise fat stores and maintain blood glucose. But if intensity or heat stress pushes too far, cortisol surges — impairing recovery and cognition. Efficient pacing keeps hormones in balance and preserves clarity deep into the race.

Leg 3: The Final Push — Fatigue, Endorphins, and the Central Governor

In the closing stages, performance becomes a negotiation between the body and the brain. Glycogen stores dwindle, lactate levels peak, and the central governor — the brain’s protective mechanism — starts applying the brakes.

This “wall” isn’t absolute failure. It’s a perception of risk — your brain’s way of preserving homeostasis by reducing muscle recruitment. Elite endurance athletes train this system through both physical conditioning and psychological resilience (see Threshold #153 | The Central Governor).

As pain mounts, endorphins and dopamine flood your system, blunting discomfort and sharpening focus. These neurochemicals, combined with adrenaline spikes, create the “runner’s high” — the paradoxical euphoria of exhaustion.

Your stride tightens. Breathing grows heavy. Every signal says “slow down.” Yet with adrenaline, focus, and pacing aligned, you access your final reserves — the anaerobic kick fueled by the last glycogen fragments and sheer mental resolve.

In that moment, you’re no longer training adaptation. You’re expressing it.

Conclusion

Inside every great race lies a coordinated masterpiece of physiology — oxygen, energy, heat, hormones, and neural control working in harmony. The more you understand it, the more you can shape it.

When the next start gun fires, remember: you’re not just racing the clock — you’re orchestrating your biology.

Coaches Corner

Workout of the Week: Threshold + Fatigue Simulation

Goal: Replicate late-race fatigue and train the body’s energy systems to perform under strain.

Session (90 min):

• Warm-Up: 15 min Zone 2 jog or spin + 3 x 30s strides.

• Main Set:

  • 3 x 10 min at threshold (Zone 4), 3-min recovery between.

  • After final rep: 20 min steady Zone 3 effort.

• Cool Down: 10 min easy jog, deep nasal breathing.

Why it works: Mimics glycogen depletion and trains the body to sustain aerobic power while lactate levels rise — exactly the conditions faced in the final stretch of a race.