Human hopping on damped surfaces: strategies for adjusting leg mechanics

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Abstract
Fast–moving legged animals bounce along the ground with spring–like legs and agilely traverse variable terrain. Previous research has shown that hopping and running humans maintain the same bouncing movement of the body's centre of mass on a range of elastic surfaces by adjusting their spring–like legs to exactly offset changes in surface stiffness. This study investigated human hopping on damped surfaces that dissipated up to 72% of the hopper's mechanical energy. On these surfaces, the legs did not act like pure springs. Leg muscles performed up to 24–fold more net work to replace the energy lost by the damped surface. However, considering the leg and surface together, the combination appeared to behave like a constant stiffness spring on all damped surfaces. By conserving the mechanics of the leg–surface combination regardless of surface damping, hoppers also conserved centre–of–mass motions. Thus, the normal bouncing movements of the centre of mass in hopping are not always a direct result of spring–like leg behaviour. Conserving the trajectory of the centre of mass by maintaining spring–like mechanics of the leg–surface combination may be an important control strategy for fast–legged locomotion on variable terrain.

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