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.