Factors limiting maximal performance in humans

Theoretical best performance times (t _theor) in track running are calculated as follows. Maximal metabolic power (Ė _max) is a known function of maximal oxygen uptake (V̇O_2max), of maximal anaerobic capacity (AnS) and of effort duration to exhaustion (t _e): Ė _max=f (t _e). Metabolic power requirement (Ė _r) to cover the distance (d) in the performance time t _p is the product of the energy cost of locomotion per unit distance (C) and the speed: Ė _r=C×d/t _p. The time values for which Ė _max (t _e)=Ė _r (t _p), assumed to yield t _theor, can be obtained for any given subject and distance provided that V̇O_2max, AnS and C are known, and compared with actual best performances (t _act). For 15 min≥t _e≥100 s, the overall ratio t _act/t _theor was rather close to 1.0. To estimate the relative role of the different factors limiting V̇O_2max, several resistances to O₂ transport are identified, inversely proportional to: alveolar ventilation (R _V*), O₂ transport by the circulation (R _Q), O₂ diffusion from capillary blood to mitochondria (R _t), mitochondrial capacity (R _m). Observed changes of V̇O_2max are accompanied by measured changes of several resistances. The ratio of each resistance to the overall resistance can therefore be calculated by means of the O₂ conductance equation. In exercise with large muscle groups (two legs), R _Q is the major (75%) limiting factor downstream of the lung, its role being reduced to 50% during exercise with small muscle groups (one leg). R _t and R _m account for the remaining fractions. In normoxia R _V* is negligible; at high altitude it increases progressively, together with R _t and R _m, at the expense of R _Q.