Fig. 2. Contributions of individual muscle groups to the net vertical acceleration of the center of mass of the walking model. Muscle symbols used are: CDF, dorsiflexors of the contralateral limb; CGAS, contralateral gastrocnemius; CGMAX, contralateral gluteus maximus; CGMEDA, contrlateral anterior gluteus medius/minimus; CGMEDP, contralateral posterior gluteus medius/ minimus; CLIG, ligaments of contralateral limb; CSOL, soleus of contralateral limb; GAS, gastrocnemius; GMAX, medial and lateral portions of the gluteus maxumus; GMEDA, anterior gluteus medius/minimus; GMEDP, posterior gluteus medius/minimus; SOL, soleus; VAS, vasti. Only muscles that on the limb in contact with the ground contributed to the vertical acceleration of the center of mass. (From Anderson FC, Pandy MG. Individual muscle contributions to support in normal walking. Gait Posture 2003;17(2):159-69; with permission.)

medius, yet the anterior gluteus medius contributed very little to support during early stance. The reason for this is that the anterior gluteus medius possesses a moment arm at the hip that acts to flex the hip as well as abduct it. These two actions oppose one another and prevent the anterior gluteus medius from generating support in early stance no matter how large its force. As the hip extends during mid and late stance phase, the anterior gluteus medius moment arm falls close to zero. The muscle becomes more of a pure abductor and its action more closely resembles the actions of the posterior gluteus medius. The value of the study by Anderson and Pandy [38] is that this study estimated true muscles forces (N) for each muscle (Fig. 2), offering considerably more information then one can derive from electromyography (EMG) alone or from inverse dynamic analysis techniques.

Studies have been published that examine the specific forces encountered in walking, climbing stairs, skiing, and in routine daily activities [39-42]. Variance of forces rises from incongruence of the femoral head to the acetabulum and the hip muscles that control these motions. It is estimated that the hip endures forces ranging from one-third of the body weight with double leg support to five times the body weight during running [43,44]. The asymmetry between the femoral head and the acetabulum allocates weight to multiple areas. This incongruence is inherent to the hip and necessary for sustaining normal function [45].

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