Arch-rivals? The roles of the windlass and arch-spring mechanisms in running

Abstract

The human foot is an integral part of locomotion due to its broad range of function. Its versatility derives from mechanisms that originated from cadaveric or static experiments, and were extrapolated to locomotion. Two such mechanisms are the windlass mechanism (Hicks, 1954), where the dorsiflexion of the metatarsal-phalangeal joints shortens and raises the arch through plantar fascia tension, and the arch-spring (Ker et al., 1987), where the arch-spanning ligaments store elastic energy in foot compression and return it in recoil. The clinical and research communities dogmatically accept that the windlass mechanism stiffens the foot for propulsion, which this work tested in vivo in a dynamic compression apparatus. Unexpectedly, the windlass mechanism made the arch more compliant, absorbing more energy when the windlass mechanism was engaged, compared to when it was not. This result suggests that the windlass mechanism’s function is to passively reduce arch apparent stiffness by changing its shape. The windlass mechanism’s ability to manipulate arch shape during running could be influenced by a straining plantar fascia however, as the plantar fascia is an important contributor to the arch-spring. Measured using high-speed biplanar x-ray imaging, there is a period just after the onset of heel-lift where the plantar fascia demonstrates pure windlass behaviour. The stretched plantar fascia is then released, and the arch rising effect of the windlass mechanism is enhanced by arch- spring recoil. The purpose of the recoiling arch has previously been suggested to propel the body’s centre-of-mass forward and upward, despite the relatively small magnitude of arch rise. Using a simulated arch without recoil, I identify that the arch-spring does not contribute directly to lifting the centre-of-mass. Instead, the recoiling arch increases the time available for the ankle to plantarflex and provides a mechanical advantage to the ankle plantar flexor muscles. Overall, this work contributes to an improved understanding of healthy foot function, which can underlie advancements in treatments for pathology and injury; footwear design to enhance performance; and, biomimetic robots and prostheses such that they more accurately represent foot function.