Plantar pressure mapping is one of the few performance assessment technologies that offers insight into the ability of how the body to adapt and interact with the environment and its constraints, as well as the overall complexity of human movement.
For optimal function, the foot must accomplish the dual role of mobile adaptor and rigid lever and be adaptable enough to absorb and dampen shock and rigid enough for a stable and powerful push-off.
Three main mechanisms are fundamental and serve these purposes:
The arch of the human foot is a unique structure as it is compliant at ground contact but sufficiently stiff to enable push-off, and the ligamentous plantar fascia partly facilitates these complex behaviors.
The windlass mechanism is a mechanical model describing how plantar fascia is supporting the foot during weight-bearing activities.
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 stretches and shortens throughout gait as the arch-spring stores and releases elastic energy.
For the longitudinal arch to lengthen and lower during stance and absorb loading forces as elastic strain energy, an intricate interaction of micromovements occurs in a series of tiny joints in the foot.
In a later stage during stance, the passive elastic recoil of the plantar aponeurosis contributes to positive work generation for push-off, with the help of the windlass mechanism, which effectively stiffens the longitudinal arch during toe extension.
The plantar aponeurosis and the windlass mechanism are considered the key contributors to foot stiffness during human gait. It is proposed that extension of the toes in mid- to late-stance creates increased tension in the plantar aponeurosis, resulting in shortening of the longitudinal arch via flexion and adduction of the metatarsals in combination with supination of the rearfoot. These alterations in bony alignment act to stiffen the foot and transform it from a compliant attenuator to a rigid lever, allowing ankle plantar flexor torque to be efficiently transmitted to the ground.
The calcaneocuboid locking mechanism is the mechanism by which the tendon of the peroneus longus muscle travels behind the lateral malleolus of the ankle, then underfoot, around the cuboid to insert into the lateral aspect of the base of the first metatarsal and adjacent to the first cuneiform.
Contraction of the peroneus longus allows for plantar flexion of the first ray, eversion of the cuboid, and locking of the lateral column of the foot; this minimizes the muscular strain required to maintain the foot in supination (the locked position for propulsion).
In addition, the soleus muscle maintains supination during propulsion by plantar flexing and inverting the rearfoot via the subtalar joint. This is assisted by the peroneus brevis and tertius, which also dorsiflex and evert the lateral column, helping keep it locked.
By precisely measuring the pressure distribution and force transmission on the plantar aspect of the foot with pressure insoles, the above mechanisms responsible for optimal shock absorption and propulsion can be identified and evaluated.
Combining timing analysis with the force-time curves together with pressure distribution can support practitioners in assessing how an athlete’s foot interacts with the ground in each moment of the stance phase. By looking at areas of higher versus lower pressures, it is possible to understand which segments are more mobile, adaptable, and which ones are stiffer and rigid, thus understanding how the plantar fascia and longitudinal arch work to store and release elastic energy.