Human gait is a very complicated, coordinated series of movements. Walking is divided into two main phases. The stance phase is the weight bearing portion of each gait cycle. It is initiated by heel strike and ends with toe off of the same foot. Swing phase is initiated with toe off and ends with heel strike.
The two periods when both feet are on the ground, are called initial double support and terminal double support. Initial double support occurs from heel strike of support limb to toe-off of the opposite limb. Terminal double support occurs from opposite limb heel strike to support limb toe-off. Single limb support is identical to the period of swing of the opposite limb.
In normal gait the foot strikes the ground at the beginning of the contact phase in a supinated position of approximately 2°. The foot moves through 5 1/2 - 6° of pronation (passing through neutral position) to a position of approximately 3 1/2 - 4° pronated (allowing the foot to function as a mobile adaptor, adjusting to variances in terrain). At 3 1/2 - 4° pronation, the beginning of the mid-stance phase occurs. The foot begins to re-supinate and passes through neutral position, where the foot begins the propulsive phase continuing in supination through toe-off.
As a result of the foot supinating during the mid-stance propulsive periods, the foot is converted from a “mobile adaptor” (which it is during the contact period) to a “rigid lever” as the midtarsal joint locks in supination. By having the foot function as a “rigid lever’ (as a result of a locked midtarsal joint) during the time immediately preceding toe-off, the weight of the body is propelled more efficiently. If the foot over-pronates during mid-stance and propulsion, and is in a pronated position at toe-off, the foot acts more as a “mobile adaptor” (i.e. a relatively “loose bag of bones”), rather than a “rigid lever” in these latter stages. It would, therefore, take more muscle energy to propel the weight of the body off of such a platform and the foot remains more mobile (a “loose bag”) than it should be for propulsion.
Certain types of foot pathologies cause abnormal pronation during propulsion and a pronated position at the end of propulsion. As a result, there is significant foot and leg fatigue secondary to overuse of muscles. In addition to this, abnormal pronation during the propulsive period causes hypermobility (an unstable state of joints), that in turn produces joint subluxations. These distortions of the joints lead to trauma and damage of the joints and soft tissue. For example, abnormal shearing forces between the bones and skin of the forefoot produce skin calluses and corns.
The following is a simplified description of the foot anatomy and motions involved in a complete normal singlestance phase of gait. This period is from heel strike to toe-off. The stance phase consists of 60% of normal gait cycle. The stance phase is divided into three sub-phases: Contact, Mid-stance and Propulsion. The end of one sub-phase denotes the beginning of the next sub-phase. The four sub-phases are:
The heel hits the ground slightly lateral of center. The calcaneus is inverted about 2°. At this point the foot aids in shock absorption and accepts leg rotation from above. The calcaneus begins to pronate at heel strike and continues until about 22% of the stance phase when a position of almost 4° of pronation is reached (total pronation =almost 6°). Forefoot loading terminates contact phase.
Midstance begins with forefoot loading. Motion at the subtalar joint is continuous supination from 22% to 100% of the stance phase. The end of midstance is heel lift of the support limb. This occurs at about 50% of the stance. Stability of the limb is required at this point and to achieve this, the foot must be in a position to lock the midtarsal joint. Near the end of mid-stance, at about 55% of stance phase the subtalar joint should be in neutral position (which means the midtarsal joint is locked).
This is the final 50% of the stance phase. The foot continues to supinate (for a total of 6°) and attains about a 2° supinated position. The midtarsal joint is locked and maximum forefoot loading takes place at about 75-80% of the stance phase. Toe off is at a 2° supinated position. The leg moves into swing phase.
The swing phase consists of 40% of normal gait cycle and occurs from toe-off to heel strike. During this phase the foot remains supinated. Supination shortens the foot, which helps it to clear the ground. Supination also minimizes the energy expenditure necessary for ground clearance as the non weight-bearing limb passes the weight-bearing limb. Supination stabilizes the bony architecture of the foot thus preparing it for heel strike, when the foot must absorb the shock of striking the ground.