Gait describes the patterns of limb movements during locomotion. When using gait to describe human walking, gait can be divided into two phases: stance phase and swing phase. As an example, consider the gait cycle to begin with the left heel striking the ground. This initial phase is known as the stance phase. Upon heel strike, both feet are on the ground, a condition known as double support. Weight is then transferred from the right foot to the left foot. When all the weight is transferred to the left foot, the right foot lifts off the ground, beginning the single support portion of the stance phase. The body pivots about the ankle joint like an inverted pendulum while the right leg swings past the left to a position in from of the body. The right heel contacts the ground, and double support is resumed. When all the weight is transferred to the right leg, the stance phase ends. Note that the left foot is constantly on the ground during the stance phase. The swing phase begins as the left foot lifts off the ground. It swings forward as the body pivots about the right ankle like an inverted pendulum. The body moves forward as the leg swings into position for the next step. Heel strike of the left foot completes the gait cycle and begins a new one.
In normal human gait, the stance phase accounts for about 60% of the gait cycle leaving about 40% for the swing phase. Double support occurs twice in the gait cycle, and each accounts for about 10–12% of the total cycle. Gait abnormalities can cause these percentages to shift. Observing gait can be useful in diagnosing a disease state. Many gait abnormalities result from abnormalities in the brain, muscle weakness, lack of proprioception (the ability to sense one’s own body position and movement), or a combination of these.
Gait analysis can be performed visually by a clinician, or using sophisticated camera systems to capture three-dimensional movement of each body segment. When camera systems are used, inverse dynamics can be used to evaluate forces within the joints that would be required to produce the observed movement. The calculations take into account body segment mass, moment of inertia, position, velocity, acceleration, angle, angular velocity, and angular acceleration at each moment in time. Camera systems can locate markers with an error of less than 1 mm. Force plates embedded in the walking surface are used to measure ground reaction forces. These include forces and moments in the x, y, and z directions.
Gait analysis is often conducted for several purposes:
