to knee flexion and the beginning of the initial swing phase. Swing phase Initial swing is the response of both stretch reflexes and concentric movements to the propulsion movements of the body. Hip flexion and knee flexion occur, beginning the return of the limb to the starting position and setting up for another foot strike. The initial swing ends at midswing when the limb is again directly underneath the trunk, pelvis, and hip with the knee joint flexed and hip flexion continuing. Terminal swing then begins as hip flexion continues to the point of activation of the stretch reflex of the hip extensors. The knee begins to extend slightly as it swings to the anterior portion of the body. The foot then makes contact with the ground with a foot strike, completing the running cycle of one side of the lower extremity. Each limb of the lower extremity works opposite to the other. When one side is in toe-off/propulsion, the other hand is in the swing/recovery phase preparing for footstrike. Following toe-off and the beginning of the initial swing of one side, there is a flight phase where neither extremity is in contact with the ground due to the opposite side finishing terminal swing. As the footstrike of the one hand occurs, the initial swing continues. The opposing limbs meet with one in midstance and midswing, beginning the propulsion and terminal swing phases. Upper extremity function The upper extremity function serves mainly in providing balance in conjunction with the opposing side of the lower extremity. The movement of each leg is paired with the opposite arm, which serves to counterbalance the body, particularly during the stance phase. The arms move most effectively (as seen in elite athletes) with the elbow joint at approximately 90 degrees or less, the hands swinging from the hips up to mid-chest level with the opposite leg, the Humerus moving from being parallel with the trunk to approximately 45 degrees shoulder extension (never passing the trunk in flexion) and with as little movement in the transverse plane as possible. The trunk also rotates in conjunction with arm swing. It mainly serves as a balance point from which the limbs are anchored. Thus trunk motion should remain mostly stable with little motion except for slight rotation, as excessive movement would contribute to transverse motion and wasted energy. Footstrike debate Recent research into various forms of running has