The Science of Race Week: What Really Happens to Your Body Before the Big Day

The Calm Before the Storm

Every year, the same conversation. An athlete emails me on Wednesday before their marathon: "Henri, my leg feels weird. Could it be a stress fracture?" They ran 18 weeks without a single suspicious twinge, and suddenly, five days from the start line, the body starts sending strange signals.

It's not a coincidence. It's physiology, and it's worth understanding instead of fearing.

Race week is not a week of passive rest. It's a window where very specific biological processes occur — some working in your favor, others simply the nervous system's noise as it adjusts to reduced stimulus. Telling one from the other is, in my experience, one of the least-trained and most valuable skills a runner can have.

Supercompensation: the "why" behind the taper

The principle behind tapering has a name: supercompensation. The basic idea is that hard training creates fatigue and mild physiological damage; the recovery that follows doesn't just return the body to baseline — it pushes it slightly above baseline, provided rest is sufficient and the prior stimulus was appropriate.

This isn't just muscle theory. There's evidence that exercise regulates deep metabolic pathways, including AMPK/SIRT1 signaling and BDNF (brain-derived neurotrophic factor) production, which affects both glucose metabolism and cognitive and recovery function [PMID:40765038]. BDNF in particular plays a dual role: it supports neuronal plasticity and has been consistently associated with regular aerobic exercise, even at moderate doses [PMID:25455510].

What does this mean in practical terms? That tapering isn't "doing nothing" — it's reducing total volume while maintaining some intensity, so those signaling pathways stay active without continuing to accumulate damage. Classic taper studies (Mujika & Padilla, 2003, and later work) show volume reductions of 40-60% over 7-14 days, while maintaining intensity, as the formula that best preserves performance while allowing structural recovery.

What I tell my athletes: the taper doesn't erase months of work. It reveals it.

Glycogen replenishment: the bank account that actually matters

Muscle and liver glycogen are the primary fuel for moderate-to-high intensity efforts, and fully replenishing them takes longer than most people assume — generally 48 to 72 hours with adequate carbohydrate intake, not the "48 hours before" that many people consider sufficient.

Research on glucose metabolism during exercise, including studies combining fasting protocols with training, confirms that glycogen-regulating pathways (via AMPK/SIRT1) respond differently depending on prior metabolic state [PMID:40765038]. This has a direct implication for race week: it is not the time to experiment with fasting, ketosis, or caloric restriction "to feel light." It's the time to fill the tank in a predictable way.

Practical loading protocol

• 7-4 days out: keep carbs at normal levels (no need to load yet), progressively reduce training volume.
• 3-1 days out: increase carbohydrates to 8-10 g/kg of body weight, prioritize easy-to-digest sources (rice, pasta, bread, fruit), reduce fiber and fat in the final 48 hours to minimize GI distress.
• The night before: a familiar dinner, nothing new. This is not the moment for the trendy new restaurant.

Phantom aches: when the nervous system gets anxious

This is what generates the most questions every season. Aches that appear out of nowhere in the final week — a knee that "pinches," a calf that "feels tight," a general fatigue that doesn't match the reduced training volume.

The most likely explanation isn't structural — it's a combination of neurological and psychological factors. When you drastically reduce training volume, the central nervous system — which had been busy processing constant signals of effort, muscle fatigue, and adaptation — suddenly has free "bandwidth." That attention often gets redirected toward bodily sensations that would normally be ignored because they were masked by real fatigue.

On top of that, there's a real component of pre-competition anxiety. The autonomic nervous system, in a state of anticipation, can generate somatic sensations (mild muscle tension, changes in pain perception) that don't reflect tissue damage. This isn't "being crazy" or "psychosomatic" in a dismissive sense — it's a documented neurophysiological phenomenon in endurance athlete populations.

My golden rule with athletes: if the ache hasn't changed how you run over the last 10 days of normal training, it's probably not new — you're just noticing it more now. If the pain is sharp, localized, and worsens with every stride, that's a different conversation and warrants evaluation.

Sleep disruption: the night before isn't the one that counts

Almost nobody sleeps well the night before a big race. Nerves, unfamiliar alarm times, unfamiliar hotel beds. And here's the data point that reassures my athletes most: the night before competition matters far less than you think, because your body arrives at the race carrying the accumulated sleep of the whole week, not just one night.

What actually matters is sleep over the preceding 5-7 days. That's the window where much of the physiological recovery associated with tapering gets consolidated, including tissue repair processes and hormonal regulation. BDNF, mentioned earlier as a regulator of neural plasticity, is also influenced by sleep quality and quantity — another reason to protect rest in the days leading up to the race, not just the final night [PMID:25455510].

Practical advice: prioritize sleeping well on Tuesday, Wednesday, and Thursday before a Saturday or Sunday race. If Friday night is rough, don't panic — biologically, you've already done the important work.

The heart under prolonged stress: a reminder for masters

As a USATF Masters athlete myself, there's a finding I think is especially relevant for runners over 40 preparing for long distances. Recent research on the cardiac effects of prolonged endurance exercise shows that both younger and older athletes experience transient alterations in cardiac biomarkers and ventricular function after prolonged efforts, though recovery patterns may differ with age [PMID:40667749].

This isn't cause for alarm, but it is cause for respect. Race week is not the time to add unnecessary cardiovascular stress — no "test" workouts at max intensity, no doubling up on sessions to "feel secure." The heart, like the rest of the system, benefits from the same supercompensation logic: less stimulus, occasional intensity maintained, more recovery.

Biomarkers: the future (not yet applicable for most of us)

It's worth mentioning that the science of exercise monitoring is advancing quickly. Recent literature on exercise biomarkers explores how blood and physiological markers may predict fatigue, recovery, and injury risk with more precision than traditional metrics [PMID:41922043]. For the recreational runner, this isn't yet part of the weekly protocol — but it's an interesting direction for the future of data-based coaching, as opposed to intuition-based coaching.

For now, your best biomarker is still simple: how are you sleeping, how are you eating, how do you feel when you wake up? Those three questions, answered honestly, say more than most watches do.

Practical summary for your race week

1. Trust the taper. Reducing volume while keeping occasional intensity is what allows real supercompensation to happen.
2. Load carbs with intention, not panic. 8-10 g/kg over the last 2-3 days, with familiar foods.
3. Don't give every new ache more weight than it deserves. Ask yourself if it changed how you ran over the last 10 days. If not, it's probably nervous system noise, not a new injury.
4. Sleep well from Tuesday through Thursday, not just the night before. That's the window that actually matters.
5. If you're a masters athlete, respect your heart. No unnecessary effort tests in the final week.

Race week doesn't decide your performance — months of training already did. But it can ruin it if you fill it with anxiety, last-minute experiments, or decisions driven by fear instead of evidence. Trust the process, trust the data, and save that nervous energy for the start line.

References

• Wang X, Shi J. Different Fasting Methods Combined With Running Exercise Regulate Glucose Metabolism via AMPK/SIRT1/BDNF Pathway in Mice. Compr Physiol. 2025. [PMID:40765038]
• Szuhany KL, Bugatti M, Otto MW. A meta-analytic review of the effects of exercise on brain-derived neurotrophic factor. J Psychiatr Res. 2015. [PMID:25455510]
• Frandsen J, Aaroe M, et al. Cardiac Effects of Prolonged Endurance Exercise in Young and Older Athletes. Scand J Med Sci Sports. 2025. [PMID:40667749]
• Siebers M, Bizjak DA. Exercise biomarkers. Adv Clin Chem. 2026. [PMID:41922043]