Contact is the cushioning phase of the gait cycle. At this time the knee flexes immediately prior to the foot hitting the ground and the foot pronates or rolls in. This causes the foot and leg to act together like a car’s shock absorbers. The foot needs to be quite mobile at this time to allow for any unevenness in the terrain. The beginning of the contact phase of gait is signaled by the instance of heel strike. The end of the contact period is signaled by the instant the forefoot makes contact. This event is called foot flat and it marks the beginning of the mid stance phase of stance. The contact period lasts for about 25% of the total time the foot is in contact with the ground.
By now the foot should have stopped pronating. Mid stance is the time when the foot and leg provide a stable platform for the body weight to pass over. If the foot is still pronating at this time there is too much movement and instability. During mid stance the other foot is in swing phase and so all the body weight is born on the stance limb alone. This means that mid stance is a time when lower limb is particularly susceptible to injury. Mid stance is also the longest phase of the stance period and it lasts approximately 50% of the total stance period. Because mid stance is the time the entire body weight is passing through one foot only, this has a flattening effect on the arch of the foot. The foot itself has specifically designed elements engineered to resist excessive arch flattening at this time. These are called the auto support structures.
Learn more about The Auto Support Structures
Propulsion is the final stage of the stance phase of gait. Propulsion begins immediately as the heel lifts. As the big toe dorsi flexes the windlass mechanism comes into play, tightening the plantarfascia and helping to raise the arch of the foot. This mechanism is very important since it allows the foot to become an efficient lever. The foot should be supinated during propulsion allowing the bones of the mid foot to brace against each other and producing a rigid structure capable of propelling the body weight forwards. If the windlass mechanism is either delayed, as with an abnormally pronating foot, or impeded by poorly designed foot wear, the foot will not function properly during the late mid stance and propulsive phases of gait. The result of this functional failure is an increased risk of musculo-skeletal injury.
Learn more about the Windlass Mechanism
