The interaction of surface stiffness and hopping frequency on the metabolic cost and biomechanics of human hopping.
Journal Article
Overview
abstract
The use of in-series passive-elastic devices, such as surfaces and shoes, can reduce the metabolic cost of bouncing gaits, such as running and hopping. These reductions in metabolic cost may arise from increased leg stiffness and ground contact time with less stiff surfaces or footwear. We examined how changes in surface stiffness influence the metabolic cost and biomechanics of hopping across a range of frequencies. As changes in hopping frequency result in changes in leg stiffness and ground contact time, we hypothesized that the metabolic cost associated with hopping on elastic surfaces would be modulated by hopping frequency. Sixteen participants hopped in place on both feet at 2.4, 2.6, and 2.8 Hz on surfaces with different linear stiffness (rigid ∼35,000 kN/m, and 0.9, 0.7, and 0.5 kN/m/kg) using a custom-built adjustable elastic springy platform while we collected metabolic, ground-reaction force, and kinematic data. Hopping on elastic surfaces reduced net metabolic power (W/kg) by 17.6-44.7% compared with a rigid surface; however, the surface stiffness that minimized net metabolic power was stiffer at faster hopping frequencies. We also found that leg stiffness and ground contact time increased as surface stiffness decreased, and this effect was amplified at faster hopping frequencies. These biomechanical changes were not consistently associated with net metabolic power changes. Our data suggest that surface energy return was most closely timed with leg extension on stiffer surfaces at faster hopping frequencies. Thus, tuning in-series passive-elastic devices to a user's movement frequency may reduce the metabolic cost of bouncing gaits.NEW & NOTEWORTHY Metabolic cost reductions for stationary hopping on elastic compared with rigid surfaces are frequency dependent. We found that the surface stiffness that minimized metabolic cost was stiffer at faster hopping frequencies. Leg stiffness and contact time increased as surface stiffness decreased, and this effect was amplified at faster hopping frequencies. These findings suggest that the stiffness of passive-elastic devices, like elastic surfaces and footwear, should be tuned to movement frequency to minimize metabolic cost.
