The Effects of Surface Stiffness on Human Hopping Frequency Preference and the Underlying Neuromuscular Function of the Foot and Ankle.

Typically, humans tune their lower limb mechanics to preserve center of mass motion when hopping or running on surfaces with different stiffnesses. However, much of our understanding of this interaction is based on frequency-constrained hopping and not preferred behavior, which may also be influenced by the stiffness of the surface underfoot. Therefore, we tested if preferred hopping frequency was different from a previously assumed value of 2.2 Hz and if preference was affected by a less-stiff surface. To help explain any observed trends, we quantified foot and ankle mechanics and muscle activations for frequencies ±20% of preferred. We used custom-built platforms to provide both an elastic and locked (inelastic) surface and asked participants to hop bilaterally in place on each. We measured multi-segment foot and ankle kinematics and ground reaction forces, alongside electromyography (EMG) of flexor digitorum brevis, abductor hallucis, soleus, and tibialis anterior. There was no significant difference between mean preferred hopping frequency and 2.2 Hz, for either surface. There was also no difference in mechanics between preferred frequency and 2.2 Hz conditions. However, there were effects of surface, frequency, and surface-by-frequency interactions on foot and ankle kinematics, kinetics, and EMG. Frequency preference appears to be partially driven by an effort to maximize energy stored and returned in the surface while trading off the costs of active muscular work and the cost associated with producing force. Frequency affects hopping mechanics differently on stiff vs. elastic surfaces.