Neuromechanical adaptations of foot function when hopping on a damped surface

Abstract

To preserve motion, humans must adopt actuator-like dynamics to replace energy that is dissipated during contact with damped surfaces. Our ankle plantar flexors are credited as the primary source of work generation. Our feet and their intrinsic foot muscles also appear to be an important source of generative work, but their contributions to restoring energy to the body remain unclear. Here, we test the hypothesis that our feet help to replace work dissipated by a damped surface through controlled activation of the intrinsic foot muscles. We used custom-built platforms to provide both elastic and damped surfaces and asked participants to perform a bilateral hopping protocol on each. We recorded foot motion and ground reaction forces, alongside muscle activation, using intramuscular electromyography from flexor digitorum brevis, abductor hallucis, soleus, and tibialis anterior. Hopping in the Damped condition resulted in significantly greater positive work and contact-phase muscle activation compared with the Elastic condition. The foot contributed 25% of the positive work performed about the ankle, highlighting the importance of the foot when humans adapt to different surfaces.NEW & NOTEWORTHY Adaptable foot mechanics play an important role in how we adjust to elastic surfaces. However, natural substrates are rarely perfectly elastic and dissipate energy. Here, we highlight the important role of the foot and intrinsic foot muscles in contributing to replacing dissipated work on damped surfaces and uncover an important energy-saving mechanism that may be exploited by the designers of footwear and other wearable devices.

Keywords: foot biomechanics; intrinsic foot muscles; longitudinal arch; multisegment foot models; quasi-stiffness.

Graphical abstract

Figure 1.

Platform mounting configurations. The right platform was placed atop a force plate for both conditions. Surface damping was altered by engaging and disengaging the shock absorbers located in parallel with the compression springs. The lower panel shows the platform in its damped setting with the compression springs and shock absorbers engaged. For the Elastic condition, the damper was disengaged by winding the locking collar counter-clockwise and turning the damper clockwise into threaded base plate. The locking collar was retained in both conditions to seat the compression spring in place.

Figure 2.

Medial view of experimental set-up on right foot. Medial view of right foot showing IOR foot marker locations and definition of Cal-Met angle. Cal-Met, metatarsal segment with respect to the calcaneus; IOR, Istituto Ortopedico Rizzoli.

Figure 3.

Group means ± SD data (n = 13) for the change in angle at the ankle (A) and midfoot (Cal-Met; B) from foot contact (FC) to toe-off (TO) for both the Damped (solid line) and Elastic (dotted line) conditions. Black bars and asterisk indicate significantly different net ankle plantar flexion and midfoot recoil, respectively. Group data for changes in ankle and midfoot angle vs. joint moment for the ankle (C) and midfoot (D) for the same Damped and Elastic conditions. Boxes highlight the significantly greater positive work performed in the Damped vs. Elastic condition performed immediately prior to take off. Positive change in angle indicates ankle dorsiflexion and midfoot compression, respectively.

Figure 4.

Group mean ensembles ± SD (shaded area) for normalized root mean square (RMS) EMG signal amplitude for flexor digitorum brevis (FDB; A), abductor hallucis (AH; B), soleus (Sol; C), and tibialis anterior (TA; D) for the damped (solid line) and elastic (dotted line) surface conditions. Ensembles are presented for a single hop cycle, i.e., from toe-off (TO) to toe-off. Contact with the surface (FC) is indicated by vertical dashed lines to highlight changes in duty factor between conditions (Elastic = dotted line, Damped = solid line). For each muscle, data are normalized for each subject to the peak amplitude recorded on a locked surface.