The windlass mechanism is a key foot function required for efficient walking and running. It tightens the plantar fascia by shortening the distance between calcaneus and metatarsal heads, thus elevating the medial arch.
Jacks test is an easy clinical test to perform with children and helps therapists explain the mechanics of the foot. It can also help therapists diagnose if there is a lack of windlass activation, which may be a factor in some pathologies.
What is the windlass mechanism?
The windlass mechanism uses the plantar fascia to create a structure similar to that of a winch on a sailboat (hence the name). This structure, when functioning properly, is responsible for enhancing foot stability and energy efficient gait.
During the gait cycle, as the foot pronates on heel strike, the plantar fascia winds around the first metatarsophalangeal joint (MPJ), tightening like a cable around a pulley system (hence the name). This action increases medial arch height and passively resupinates the feet by pulling the metatarsal heads closer to the calcaneus. This action is important as it helps provide a rigid base for push off.
When the foot transitions from mid to late stance, the toes dorsiflex, activating the windlass mechanism to re-supinate the feet and prepare them for push off. This is an important function as it helps the foot to become a rigid lever and reduces energy expenditure during the propulsive phase of walking.
When the windlass mechanism is not functioning properly, it can lead to poor foot posture, increased energy expenditure during walking, and decreased efficiency of gait. This is often due to lack of movement in the big toe joint or big toe rigidity. Conditions such as hallux limitus/rigidus, functional hallux limitus, and severe pes planovalgus restrict the motion at the first MPJ and interfere with the function of the windlass mechanism.
What causes the windlass to fail?
In a healthy foot with an effective windlass, the plantar fascia tightens as the toes extend towards the shin (dorsiflex). This shortens the distance between the calcaneus and metatarsophalangeal joints and raises the arch of the foot, compacting the small bones in the foot to form a strong rigid lever for push off. When the toes plantarflex back down during the propulsive phase, the arch collapses and the windlass loosens. This gives the foot a more mobile lever arm that allows for a smooth and balanced gait.
Occasionally, the windlass may not function as intended. One of the most common reasons for this is that there is weakness in the hips that causes the foot to excessively pronate at mid-stance. When the arch flattens out too much, the windlass loses its ability to support the foot and can become a source of pain.
What can I do to help the windlass work?
Another common cause of windlass failure is a lack of hallux dorsiflexion (big toe dorsiflexion). This is the primary motion needed to activate the windlass mechanism. When this fails, the plantar fascia is no longer tightened and the arch height can’t rise.
Research has shown that biomechanical socks can increase the force needed for hallux dorsiflexion and thus the windlass mechanism. This can help reduce tensile stress on the plantar fascia and possibly delay arch fatigue.