The basic mechanics of human locomotion are associated with vaulting over stiff legs in walking and
rebounding on compliant legs in running. However, while rebounding legs well explain the stance
dynamics of running, stiff legs cannot reproduce that of walking. With a simple bipedal spring–mass
model, we show that not stiff but compliant legs are essential to obtain the basic walking mechanics;
incorporating the double support as an essential part of the walking motion, the model reproduces the
characteristic stance dynamics that result in the observed small vertical oscillation of the body and
the observed out-of-phase changes in forward kinetic and gravitational potential energies. Exploring the
parameter space of this model, we further show that it not only combines the basic dynamics of walking
and running in one mechanical system, but also reveals these gaits to be just two out of the many solutions
to legged locomotion offered by compliant leg behaviour and accessed by energy or speed.