Fig. 3.1 Static model of vector forces exerting on the hip. A is the force of abductor muscles, W is the body weight and l and d represent their arms. JRF is the Joint Reaction Force whch counterbalances the aforementioned forces on the femoral head
(3.1)
Thus, knowing the body weight of the individual and his abductor force, the resulting force (JRF) can be calculated as a vector force (Eq. 3.2):
(3.2)
It usually assumes a value of about 2.7-fold the body weight and the vector has a vertical inclination of about 18°.
Pauwel’s model can be easily applied to the clinical practice, since single leg stance position is clearly reproduced during slow gait or asking to the patient to maintain single leg stance. Trendelenburg sign is the most important clinical sign to evaluate abductor muscle force, since in case of abductor insufficiency, the patient cannot maintain a correct horizontal balance of the pelvis during single leg stance. Therefore, when this occurs, the weight is moved more closely to the rotational centre of the femoral head, providing a shortening of the body weight force arm (d) and consequently increasing the joint reaction force (Figs. 3.2 and 3.3).
Fig. 3.2 Abductor force (Ab) represents the reaction of gluteal muscles to single leg stance position
Fig. 3.3 When gluteal insufficiency occurs and abductor reaction (Ab) decreases, JRF increases, as W force is closer to rotational center of the femoral head (d decreses)
Therefore, the use of crutches during gait helps to decrease the loading on the hip (so the JRF) by 50%. In the same way, losing about 20% of the body weight can be comparable to the use of crutches.
3.6 Hip Function During Gait
Gait process modifies continuously the hip position, with a maximum extension during heel-off phase and maximum flexion during heel-strike phase. Muscles function varies during these phases [9]. In the heel-strike phase, hip extension begins through the activation of hamstring and greater gluteus muscles. Subsequently, at the beginning of the heel-off phase, the tensor of the fascia and medium and small gluteus provide complete extension with anterior propulsion of the body. Heel-off phase is characterized by the activation of iliac muscles, and in toe-off phase, the adductor longus is also involved and participates in providing load shift to the contralateral limb. In the swing phase, the group of muscles activated to allow the hip flexion and anterior propulsion of the limb are the iliopsoas, gracilis, femoral rectus and sartorius. Eventually, hamstrings and greater gluteus activate to prepare to the following stance phase [10].
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