The extensibility of the plantar fascia influences the windlass mechanism during human running

The arch of the human foot is unique among hominins as it is compliant at ground-contact but sufficiently stiff to enable push-off. These behaviours are partly facilitated by the ligamentous plantar fascia whose role is central to two mechanisms. The ideal windlass mechanism assumes that the plantar fascia has a nearly constant length to directly couple toe dorsiflexion with a change in arch shape. However, the plantar fascia also stretches and then shortens throughout gait as the arch-spring stores and releases elastic energy. We aimed to understand how the extensible plantar fascia could behave as an ideal windlass when it has been shown to strain throughout gait, potentially compromising the one-to-one coupling between toe arc length and arch length. We measured foot bone motion and plantar fascia elongation using high-speed x-ray during running. We discovered that toe plantarflexion delays plantar fascia stretching at foot-strike, which likely modifies the distribution of the load through other arch tissues. Through a pure windlass effect in propulsion, a quasi-isometric plantar fascia’s shortening is delayed to later in stance. The plantar fascia then shortens concurrently to the windlass mechanism, likely enhancing arch recoil at push-off.

Methods
High-speed biplanar videoradiography and markerless tracking algorithms measured the kinematic orientation and position of the bones (calcaneus, tibia, first metatarsal, first proximal phalanx and sesamoids). The provided data contains both filtered and unfiltered data, and is normalized to stance phase (101 points). All methods are explained in the manuscript.