Limits to human locomotor performance: phylogenetic origins and comparative perspectives

Studies of human exercise physiology have been conducted from a largely ahi storical perspective. This approach usefully elucidates proximate limits to locomotor performance, but ignores potential sources of biomechanical and physiological variation that derive from adaptation to ancestral environmen ts. Phylogenetic reconstruction suggests that multiple hominoid lineages, i ncluding that leading to Homo sapiens, evolved in African highlands at alti tudes of 1000-2000 in. The evolution of human locomotor physiology therefor e occurred under conditions of hypobaric hypoxia. In contrast to present-da y humans running on treadmills or exercising in otherwise rectilinear traje ctories, ancestral patterns of hominid locomotion probably involved intermi ttent knuckle-walking over variable terrain, occasional bouts of arborealit y and an evolving capacity for bipedalism. All such factors represent poten tial axes of locomotor variation at present unstudied in extant hominoid ta xa. As with humans, hummingbirds evolved in mid-montane contexts but pose a n extreme contrast with respect to body size, locomotor mode and metabolic capacity. Substantial biomechanical and physiological challenges are associ ated with flight in hypobaria. Nonetheless, hummingbird lineages demonstrat e a progressive invasion of higher elevations and a remarkable tolerance to hypoxia during hovering. Upregulation of aerobic capacity and parallel res istance to hypoxia may represent coupled evolutionary adaptations to flight under high-altitude conditions.