Abstract
Examination of how the ankle and midtarsal joints modulate stiffness in response to increased force demand will aid understanding of overall limb function and inform the development of bio-inspired assistive and robotic devices. The purpose of this study is to identify how ankle and midtarsal joint quasi-stiffness are affected by added body mass during over-ground walking. Healthy participants walked barefoot over-ground at 1.25 m/s wearing a weighted vest with 0%, 15% and 30% additional body mass. The effect of added mass was investigated on ankle and midtarsal joint range of motion (ROM), peak moment and quasi-stiffness. Joint quasi-stiffness was broken into two phases, dorsiflexion (DF) and plantarflexion (PF), representing approximately linear regions of their moment-angle curve. Added mass significantly increased ankle joint quasi-stiffness in DF (p < 0.001) and PF (p < 0.001), as well as midtarsal joint quasi-stiffness in DF (p < 0.006) and PF (p < 0.001). Notably, the midtarsal joint quasi-stiffness during DF was ~2.5 times higher than that of the ankle joint. The increase in midtarsal quasi-stiffness when walking with added mass could not be explained by the windlass mechanism, as the ROM of the metatarsophalangeal joints was not correlated with midtarsal joint quasi-stiffness (r = −0.142, p = 0.540). The likely source for the quasi-stiffness modulation may be from active foot muscles, however, future research is needed to confirm which anatomical structures (passive or active) contribute to the overall joint quasi-stiffness across locomotor tasks.
Highlights
Humans dynamically alter the stiffness of their limbs in response to environmental and functional demands
Leg stiffness is modulated during locomotion to control multiple important biomechanical factors including: center of mass excursion when stepping between surfaces of differing compliance (Ferris, Liang & Farley, 1999), resistance to perturbation when walking on uneven terrain (Voloshina & Ferris, 2015) and changing stride frequency (Farley & González, 1996)
The moment produced at the ankle and midtarsal joints increased with added mass, but their range of motion (ROM) did not increase correspondingly, which is consistent with the overall trend of a stiffer joint and in agreement with our hypothesis
Summary
Humans dynamically alter the stiffness of their limbs in response to environmental and functional demands. Leg stiffness is modulated during locomotion to control multiple important biomechanical factors including: center of mass excursion when stepping between surfaces of differing compliance (Ferris, Liang & Farley, 1999), resistance to perturbation when walking on uneven terrain (Voloshina & Ferris, 2015) and changing stride frequency (Farley & González, 1996). Understanding the stiffness behavior of individual joints (i.e., the hip, knee, ankle and foot) may enable better understanding of overall locomotor function (Günther & Blickhan, 2002). Ankle and midtarsal joint quasistiffness during walking with added mass. Understanding foot and ankle behavior during walking may be important for identifying gait pathology, human performance or improving prosthetic and assistive device design
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