*2.2. Gait Cycle*

The body segments are referenced to the anatomical position and are described as occurring in three planes (Figure 2a). The body is divided into left and right halves by the sagittal plane. The frontal plane divides the body into anterior (front) and posterior (rear) portions. The body is divided into superior (upper) and inferior (lower) portions of the transverse plane. Joint motions during walking in the sagittal reference plane are shown in Figure 2b. The motion of the hip and knee in the sagittal plane are referred to as flexion and extension. Ankle movement in the sagittal plane is referred to as dorsiflexion and plantar flexion. Motions in the frontal plane are abduction and adduction, and motions in the transverse plane are internal and external rotations. Translating the center of mass of the human body is the primary task of walking. The sagittal plane is the parallel plane to the progression of walking. The left and right step during walking is a single gait cycle, which is a pattern of repeatable movements.

**Figure 2.** One gait cycle of human walking: (**a**) description of the hip and knee local coordinate system that are referenced to the anatomical position in three reference planes; (**b**) description of the phase difference motion of the hip and knee in the sagittal plane [17].

One gait cycle of human walking consists of two consecutive steps. A step is a period from right heel contact (RHC) to left heel contact (LHC). Two symmetric steps define a typical gait cycle containing single support (SS) and double support (DS) phase. During walking, the leg moves in a repeatable motion giving each person a unique walk. The leg moves by the multiple degrees of angular movements of the hip and knee joints to achieve a periodic pattern that enables the forward movement of the body [18,19]. Figure 2b shows that there is a time difference between the two peak swing motions of the hip and knee in the sagittal direction. It is assumed that phase differences exist in all the three-dimensional reference planes, which can characterize a unique gait cycle pattern.
