The Geometry of Superior Performance - Nick Mark
Nick Mark
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The Geometry of Superior Performance<br>Cube–Square Scaling and Comparative VO₂max Across Species
Nick Mark<br>May 27, 2026
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VO₂max - the maximum rate of oxygen consumption during peak exercise - is one of the most informative numbers in physiology. Among athletes, physiologists, and intensivists alike, it functions as a kind of summary statistic for the entire oxygen delivery cascade: how well the lungs extract oxygen, how efficiently hemoglobin carries it, how powerfully the heart pumps it, how densely the capillaries deliver it, and how effectively mitochondria consume it. Each step is potentially rate-limiting, and VO₂max tells you how well the entire oxygen cascade functions. VO₂max also happens to be one of the strongest independent predictors of all-cause mortality.<br>VO₂max is best understood not as a single trait, but as the product of a chain, as described by the Fick equation:<br>VO₂max = Cardiac Output × Arteriovenous O₂ Difference<br>Or, expanding the Fick equation more fully: pulmonary ventilation × extraction efficiency × hemoglobin concentration × cardiac output (stroke volume × heart rate) × capillary density × mitochondrial oxidative capacity. Each of these steps can be a bottleneck. In the short term, training, and in the long term evolution, can widen any these bottlenecks.
Humans: Exceptional, But Not Maximal
Humans are remarkable endurance animals. Elite distance runners can sustain 75–85% of VO₂max for the duration of a marathon, a feat of metabolic efficiency that almost no other mammal can match over comparable time scales. Our capacity for thermoregulation via eccrine sweating, combined with upright bipedal locomotion that dissociates respiration from stride frequency, likely gave early Homo a decisive advantage in persistence hunting: the ability to run prey to exhaustion across open savanna in midday heat. That hypothesis (which I’ve explored in a previous post) may also bear on why human brains are so large relative to body size.<br>In terms of raw VO₂max numbers, elite performance by sport looks roughly like this:
Elite rowers can have a a VO₂max of 75 ml/kg/min and the highest absolute VO2max of up to 7 liters/min ; however we typically normalize for body weight and express VO₂max in ml/kg/min , which tends to slightly penalize heavier athletes like rowers. (As we’ll see this also has big implications across species…) Elite swimmers’ VO₂max can be 80 ml/kg/min, and distance runners’ up to 85 ml/kg/min. Cyclists and cross country skiers are the highest recorded with values as high as 90 and 96 ml/kg/min, respectively. Cross-country skiers hold the human record for good reason: their sport demands simultaneous maximal engagement of upper and lower body musculature, maximizing the venous return and cardiac output that constrain peak oxygen delivery.<br>Human athletic achievement is impressive. Elite marathoners sustain ~75–85% VO₂ max for hours. Humans can outrun horses in extreme heat and over distance. But evolution has optimized humans for endurance efficiency rather than for maximum oxygen flux. How do we compare to other animals and what can we learn about physiology from their adaptations?
Sled Dogs: The Mammalian Ultra-Marathoners
Every March, teams of sled dogs depart Willow, Alaska on the Iditarod: a 938 mile race to Nome, run in legs totaling roughly 16 hours of effort per day, over 8–11 days (the current record is 7 days, 14 hours). The dogs run in temperatures that can drop below −40°C, consuming 10,000–12,000 kcal per day. They do not develop the rhabdomyolysis that would destroy a human athlete attempting equivalent work.<br>Published VO₂max values for sled dogs reach 198 ml/kg/min. Unpublished measurements on Iditarod-trained sled dogs report up to 200–240 ml/kg/min — roughly 2.5 to 3 times the best human values.<br>How?<br>The sled dog’s adaptations hit nearly every node in the cascade simultaneously. Cardiac output is enormous, driven by a massive stroke volume. Red blood cell mass is high at baseline, and splenic contraction during exercise releases a stored erythrocyte reserve, acutely boosting oxygen-carrying capacity in a way that amounts to endogenous blood transfusion. Metabolic flexibility is extraordinary: sled dogs oxidize fat at rates that would be impossible in humans, shifting to near-complete fat dependence within the first day of sustained effort and maintaining that state for days without the glycogen depletion that floors human performance (”hitting the wall”).<br>Their rhabdomyolysis resistance is worth its own note: Dogs running the Iditarod accumulate muscle damage markers that would indicate severe injury in a human, yet somehow recover between legs and finish the race. The mechanisms (enhanced heat shock protein expression, differences in membrane repair kinetics, local anti-inflammatory adaptations in type I fibers) are not fully elucidated, but may someday lead to clinically...