This plays an important role in facilitating hip joint flexion in frame 2. Reduced rotational inertia accompanies the hip flexion. Table 1 provides the relationship between lower extremity joints, actions, and the type of contraction for particular muscle groups recruited during the preparation phase from frame 1 to 3. A force couple made up of the abdominal and hip extensor muscle groups counteract the ability of the hip flexor and back extensor muscle groups from moving the pelvis into anterior tilt during the explosive hip flexion maneuver in frame 2. A subsequent control mechanism, provided by eccentric contraction of the gluteal and hamstring muscle groups, limits the degree to which hip flexion occurs up to floor contact in frame 3.
Angular momentum continues without additional segmentation at the hip joint.
Additional hip flexion would place the pelvis beyond the base of support in frame 3, create a lower angle of projection in frame 4, and result in decreased vertical displacement of the total center of gravity. The knee, ankle, subtalar, and metatarsophalangeal joints remain stabilized throughout the preparation phase until immediately prior to floor contact. Ankle dorsiflexion is necessary at this point in order for the toes to clear the floor and facilitate the optimum position for a quick plantar flexion maneuver in frame 3. Dorsiflexion places the plantar flexor muscle group on a quick stretch resulting in increased force during plantar flexion. Slight knee joint flexion is essential immediately prior to floor contact, through concentric contraction of the hamstring muscle group and gastrocnemius, but subsequent contraction of the quadriceps muscle group negatively accelerates knee joint flexion.