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

Abstract

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.

Keywords: EMG; bouncing; midfoot; plantar intrinsic muscles; surface compliance.

FIGURE 1

A schematic of one of the platforms, the motion capture markers on the foot, and the EMG electrode locations. The platform surface could be locked or made elastic by the spring‐dampers located underneath (green blocks).

FIGURE 2

(A, B) Group mean (± SD) quasi‐stiffness of the ankle (A) and midfoot (B) during the loading phase of ground contact for all combinations of surface stiffness and hopping frequency. (C, D) positive mechanical work done at the ankle (C) and midfoot (D) joints during the ground contact phase of hopping. *indicates a significant main effect of frequency; †significant effect of surface at that frequency.

FIGURE 3

Group mean (± SD) normalized EMG envelopes from preferred, low, and high frequency conditions, plotted against normalized hop cycle time from toe‐off (TO) to toe‐off. The top row (A–C) are for the Locked condition and the bottom row (D–F) the elastic condition. From left‐to‐right, the data are for FDB (A, D), AH (B, E), and Sol (C, F) muscles. Vertical lines indicate the respective average instances of foot contact (FC) for each frequency condition (solid line: preferred; dashed line: high; dotted line: low).