Limits to Maximal Performance

Body size fundamentally affects maximal locomotor performance in mammals. Comparisons of performances of different-sized animals yield different results if made using relative, rather than absolute scales. Absolute speed may be a reasonable way to evaluate the locomotor performance of an animal that must escape predators in real time. However, comparisons of metabolic power in animals of different size can only be made meaningfully on a mass-specific basis. Numerous factors associated with the mechanics, energetics, and storage of elastic energy during locomotion change with body size, which results in allometric relationships that make the energetic cost of locomotion (alpha Mb-0.3) more expensive for small mammals than for large mammals. Small mammals have lower enzymatic capacities for anaerobic glycolysis (alpha Mb0.15) and higher specific aerobic capacities (alpha Mb-0.13) than large mammals. However, the energetic cost of transport increases more than aerobic power as mammals get smaller. The higher ratio of cost to available power in small mammals may explain why they run more slowly than large mammals, as a rule. Maximum aerobic capacity is allometrically related to body size. Limits to VO2max can be imposed by mitochondrial oxidative capacity, as in goats, or by the O2 transport system, as in humans and horses. No single step in the O2 transport system can limit the flux of O2 by itself; however, in an average non-athletic species of mammal, any of the steps in the system might appear to be the weakest link. In highly aerobic athletic species, and possibly elite athletic individuals of other species (e.g. humans), the malleable elements of the O2 transport system may develop to the point that their O2 transport capacities approach that of the least malleable element in the system, the lung. VO2max is very high in such individuals, and appears to be limited by simultaneous failure of all components of the O2 transport system.