The Windlass Mechanism in the Foot
A windlass refers to a centuries-old mechanism used for shifting heavy objects. The windlass apparatus is a lifting or pulling device consisting of a rope or belt winding around a horizontally placed drum that rotates by motor or by turning a crank.
Hicks Windlass Effect:
John Hicks (1954) was the first to describe this mechanism in the foot.
During the gait cycle, the foot plays two major mechanical functions:
– It acts as a shock absorber (during initial contact)
– Helps in propulsion (during propulsion)
According to Hicks, the bones and ligaments of the foot are arranged in a triangular arch form.
Starting posteriorly, the calcaneus, the midtarsal joint and then the first metatarso-phalangeal joint (the big toe joint) form the triangular arch. The plantar aponeurosis or plantar fascia stretching from the calcaneus (heel bone) to the proximal phalanges of the toes forms the base of this triangle.
In the foot, the hallux or big toe is the crank that winds the plantar aponeurosis (the rope) around the metatarsal heads (the rotating cylinder).When the toes flex this winds the plantar fascia around the heads of the metatarsal bones, making it taught. This stretching of the plantar aponeurosis brings the heel bone closer to the metatarsal heads, thus raising the arch of the foot. The more the toes flex, the greater would be the tension in the aponeurosis and consequently the higher would be the arch of the foot.
As the foot touches the ground, the toes are flexed and the arch is high. This position is ideal for absorbing the shock of the whole body weight shifting to the foot through the tibia. When on the ground, the toes straighten out, relaxing the plantar aponeurosis. The arch of the foot flattens under the vertical load of the body, thus dispersing the weight in an organized fashion. Further flattening of the arch is restricted by the stretching of the plantar aponeurosis which, by virtue of its tensile strength, maintains the longitudinal arch of the foot even during the midstance (weight bearing position) phase of the gait cycle.
The windlass effect is also seen during the gait cycle just after the midstance as the foot prepares to lift off the ground. During this push-off phase the heel is lifted upwards, which bends the toes. The bending of the toes leads to winding of the plantar fascia around the metatarsal heads, as explained above. The fascia is tightened, lifting the heel further upwards and compressing all the joints in the foot. This transforms the foot from a loosely packed bag of bones to a rigid lever, an efficient means to go successfully through the subsequent propulsion phase.
Free movement of the toes is essential for effective windlass function in the foot. Toe flexion is imperative for both shock absorption and propulsion of the foot the during gait cycle. This is a key point to remember during footwear selection. The footwear should allow for maximum toe mobility as well as supporting the plantar fascia to maintain the longitudinal arch of the foot.
The absence of the windlass mechanism has been proven to play a role in many foot and lower limb injuries.