24/03/2026
The heart or the muscle — what actually limits your endurance? 🫀🦵
This is one of the most debated questions in exercise physiology. And a 2021 review by van der Zwaard, Brocherie & Jaspers gives us the most complete answer yet — by going all the way down to the muscle fibre level.
Here's the honest answer: it's both, but they operate at different levels.
The heart sets the ceiling.
Classic one-legged exercise studies show the muscle can consume far more oxygen per unit mass when the heart isn't the bottleneck. Extra red blood cells and hyperoxic breathing both raise VO₂max — confirming that cardiac output is the primary limiter of absolute VO₂max.
The cardiovascular argument is strong.
But the muscle determines how close to that ceiling you race.
And this is where it gets interesting. Skeletal muscle characteristics alone explained 67% of the variance in performance VO₂ in elite cyclists. Mitochondrial oxidative capacity accounts for ~90% of VO₂max during cycling. Capillarisation + lactate threshold explained 92% of time-to-fatigue variance. In elite athlete groups — where cardiac outputs are similarly high — peripheral muscle factors become the decisive differentiator.
The key muscle-level players:
→ Type I fibre proportion — more efficient at endurance frequencies, directly linked to economy
→ Mitochondrial density — scales proportionally with VO₂max across cardiac patients to pro cyclists
→ Capillarisation — up to 9 capillaries per fibre in elite cyclists vs 3–4 in untrained. The bridge between cardiac delivery and mitochondrial uptake
→ Myoglobin — the underappreciated O₂ transporter inside the fibre, ~50% higher in Type I vs Type II
The practical synthesis: train your heart AND your muscle. Polarised training does both — long low-intensity work drives peripheral adaptations, targeted high-intensity blocks protect and develop cardiac function. Neither system operates in isolation.
Understanding which is your limiting factor is where physiology testing becomes genuinely useful.
Data + insight = success
🔗 athletelab.uk
