Research Article: The foot and ankle structures reveal emergent properties analogous to passive springs during human walking

Date Published: June 7, 2019

Publisher: Public Library of Science

Author(s): Erica A. Hedrick, Steven J. Stanhope, Kota Z. Takahashi, John Leicester Williams.


An objective understanding of human foot and ankle function can drive innovations of bio-inspired wearable devices. Specifically, knowledge regarding how mechanical force and work are produced within the human foot-ankle structures can help determine what type of materials or components are required to engineer devices. In this study, we characterized the combined functions of the foot and ankle structures during walking by synthesizing the total force, displacement, and work profiles from structures distal to the shank. Eleven healthy adults walked at four scaled speeds. We quantified the ground reaction force and center-of-pressure displacement in the shank’s coordinate system during stance phase and the total mechanical work done by these structures. This comprehensive analysis revealed emergent properties of foot-ankle structures that are analogous to passive springs: these structures compressed and recoiled along the longitudinal axis of the shank, and performed near zero or negative net mechanical work across a range of walking speeds. Moreover, the subject-to-subject variability in peak force, total displacement, and work were well explained by three simple factors: body height, mass, and walking speed. We created a regression-based model of stance phase mechanics that can inform the design and customization of wearable devices that may have biomimetic or non-biomimetic structures.

Partial Text

An objective understanding of human foot and ankle function can help identify the mechanisms that underlie healthy locomotion, and drive innovations and development of bioinspired wearable devices, such as prostheses and exoskeletons. For example, knowledge of the mechanical forces and work in the human foot and ankle are informative for understanding how the anatomical structures change the body’s energy states. Muscles in the lower limb such as the ankle plantarflexors do work through active muscles contractions [1–7]. Elastic structures such as the plantar fascia [8–11] and Achilles tendon [6, 7, 10, 12–16] can store and return mechanical energy [17]. Examining the work production of biological structures can determine what material characteristics (e.g., elastic or viscous) or components (e.g., battery-powered actuators) are needed for device designs to emulate natural structures and/or functions.

The purpose of this study was to quantify the total force, displacement, and work of all structures distal to the shank in normal walking. Quantifying the force and displacement aids in determining how the biological foot and ankle structures react to the dynamic loads, as well as determining the mechanical work performed by these structures. The distal to shank peak forces, total displacement, and work profiles can be used to create a data-driven model to help customize wearable foot-ankle devices. Our hypothesis was partially supported in that the subject’s speed, body height and mass did predict individual variability of peak force, total displacement and work. However, not all of the variables predicted force, displacement, and work.