resulting in some energy conservation. But recent studies testing this third determinant of gait have reported varied results. It was found out that stance-phase knee flexion did not contribute to the reduction in vertical trajectory of COM. Furthermore, Gard and Childress (1997) indicated that maximum COM is reached at mid-stance when knee is slightly flexed, depicting minor reduction of the maximum height of the COM by a few millimeters. Foot and ankle motions: Saunders et al. showed relationship between angular displacement and motions of foot, ankle and knee. This results in two intersecting arcs of rotation at the foot during stance phase at heel contact and heel rise. At heel contact the COM reaches its lowest point of downward displacement when the foot is dorsiflexed and the knee joint fully extended in order for the extremity to be at its maximum length. The ankle rockers at heel strike and mid-stance leads to decrease COM displacement through the shortening of the leg. Studies by Kerrigan et al. (2001) and Gard & Childress (1997) have showed the major role played by heel rise in reducing the COM vertical displacement. Knee motion: The motion of the knee are related to those of the ankle and foot motions and results in the reduction of COM vertical displacement. Therefore, an immobile knee or ankle could lead to increases in COM displacement and energy cost. Lateral pelvic displacement: In this key gait feature, the displacement of the COM is realized by the lateral shift of the pelvis or by relative adduction of the hip. Correction of disproportionate lateral displacement of the pelvis is mediated by the effect of tibiofemoral angle, and relative adduction of the hip, which results in reduction in vertical COM displacement. It is clear that these kinematic features play a critical role in ensuring efficiency in normal gait. But there may be the need for further extensive testing or validation of each of the key determinants of gait. Abnormal gaits Abnormal gait is a result of one or more of these tracts being disturbed. This can happen developmentally or as the result of neurodegeneration. The most prominent example of gait irregularities due to developmental problems comes from studies of children on the autism spectrum. They have decreased muscle coordination, thus resulting in abnormalities in gait. Some of this is associated with decreased muscle tone, also known as hypotonia, which is also common in