Abstract
Mechanically, the most economical gait for slow bipedal locomotion requires walking as an ‘inverted pendulum’, with: I, an impulsive, energy-dissipating leg compression at the beginning of stance; II, a stiff-limbed vault; and III, an impulsive, powering push-off at the end of stance. The characteristic ‘M’-shaped vertical ground reaction forces of walking in humans reflect this impulse–vault–impulse strategy. Humans achieve this gait by dissipating energy during the heel-to-sole transition in early stance, approximately stiff-limbed, flat-footed vaulting over midstance and ankle plantarflexion (powering the toes down) in late stance. Here, we show that the ‘M’-shaped walking ground reaction force profile does not require the plantigrade human foot or heel–sole–toe stance; it is maintained in tip–toe and high-heel walking as well as in ostriches. However, the unusual, stiff, human foot structure—with ground-contacting heel behind ankle and toes in front—enables both mechanically economical inverted pendular walking and physiologically economical muscle loading, by producing extreme changes in mechanical advantage between muscles and ground reaction forces. With a human foot, and heel–sole–toe strategy during stance, the shin muscles that dissipate energy, or calf muscles that power the push-off, need not be loaded at all—largely avoiding the ‘cost of muscle force’—during the passive vaulting phase.
Highlights
The morphology and action of the human foot with— during walking—a grounded ‘heel’ behind a relatively distal ankle joint loaded early in stance, and ‘toes’ pushing off at the end of stance is very unusual outside the hominoidea [1,2,3,4]
To account for the stiff plantigrade human foot, with heel behind ankle and toe in front, and the heel–sole–toe walking strategy, requires an appreciation of both the energetically optimal powering/ support strategy of vaulting, impulsive inverted pendular walking and the cost of muscle force—which may be reduced by simple mechanisms that alter the mechanical advantage [22] between the muscle and the ground reaction force
Human walking is achieved with relatively high muscle mechanical advantages, allowing ground reaction forces to be supported with relatively small muscle forces [27,28]
Summary
The morphology and action of the human foot with— during walking—a grounded ‘heel’ behind a relatively distal ankle joint loaded early in stance, and ‘toes’ pushing off at the end of stance (i.e. a the heel–sole–toe stance or ‘plantigrade’ foot) is very unusual outside the hominoidea (apes including humans) [1,2,3,4]. Both main calf muscles increase in activity to generate positive mechanical work during late stance [18] that greatly contributes to the vertical ground reaction force, accelerating the centre of mass upwards in this period [19,20] Such accounts are persuasive in that they demonstrate the mechanism by which humans usually achieve walking broadly consistent with theoretically economical gaits. To account for the stiff plantigrade human foot (as distinct from other, more compliant, ape feet), with heel behind ankle and toe in front, and the heel–sole–toe walking strategy, requires an appreciation of both the energetically optimal powering/ support strategy of vaulting, impulsive inverted pendular walking and the cost of muscle force—which may be reduced by simple mechanisms that alter the mechanical advantage [22] between the muscle and the ground reaction force. Human walking is achieved with relatively high muscle mechanical advantages (compared with running), allowing ground reaction forces to be supported with relatively small muscle forces [27,28]
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