Many biomechanical models of human movement are complex by the standar
ds of physical mechanics. but this paper presents two that are exceedi
ngly simple. The first helps to explain the transition from walking to
running by showing that a straightlegged style of walking would becom
e impossible above a certain speed. The second uses the force-velocity
properties of muscle to explain why high jumpers run up much more slo
wly than long jumpers. Simple models are particularly useful in identi
fying basic principles because the simpler the model. the easier it is
to discover which of its features gives rise to the observed effect.