Muscle–tendon units about the ankle joint generate a burst of positive power during the step-to-step transition in human walking, termed ankle push-off, but there is no scientific consensus on its functional role. A central question embodied in the biomechanics literature is: does ankle push-off primarily contribute to leg swing, or to center of mass (COM) acceleration? This question has been debated in various forms for decades. However, it actually presents a false dichotomy, as these two possibilities are not mutually exclusive. If we ask either question independently, the answer is the same: yes! (1) Does ankle push-off primarily contribute to leg swing acceleration? Yes. (2) Does ankle push-off primarily contribute to COM acceleration? Yes. Here, we summarize the historical debate, then synthesize the seemingly polarized perspectives and demonstrate that both descriptions are valid. The principal means by which ankle push-off affects COM mechanics is by a localized action that increases the speed and kinetic energy of the trailing push-off limb. Because the limb is included in body COM computations, this localized segmental acceleration also accelerates the COM, and most of the segmental energy change also appears as COM energy change. Interpretation of ankle mechanics should abandon an either/or contrast of leg swing versus COM acceleration. Instead, ankle push-off should be interpreted in light of both mutually consistent effects. This unified perspective informs our fundamental understanding of the role of ankle push-off, and has important implications for the design of clinical interventions (e.g. prostheses, orthoses) intended to restore locomotor function to individuals with disabilities.
The authors declare no competing or financial interests.
Conceptualization: K.Z.; Methodology: K.Z., P.A.; Formal analysis and investigation: K.Z., P.A.; Writing - original draft preparation: K.Z., P.A.; Writing - review and editing: K.Z., P.A.; Funding acquisition: K.Z., P.A.
This work was supported in part by the National Institutes of Health [K12HD073945 to K.E.Z.]; and by institutional support from the University of Wisconsin-Madison [to P.G.A.] and from Vanderbilt University [to K.E.Z.]. Deposited in PMC for release after 12 months.
- © 2016. Published by The Company of Biologists Ltd