Hip extensors

The gluteal muscles, in their action as a hip extensor, assist in posterior rotation of the sacrum and pelvis, because their origin on the posterior margin of the ilium and the raphe overlying the sacrotuberous ligament give them the mechanical ability to produce further posterior rotation of the upper portion of the pelvis. 

In the lower limbs the quadriceps femoris muscles are most severely affected and undergo progressive atrophy, particularly of the vastus medialis and lateralis, with relative sparing of the rectus femoris (Figure 9.2). As in the upper limbs the weakness is often more severe on the nondominant side. Weakness of the ankle dorsiflexors is also common but severe foot-drop occurs infrequently. Other muscle groups such as the hip extensors and abductors and ankle plantar flexors are usually only mildly affected. 

Juvenile dermatomyositis (JDM) is perhaps the most uniform, in terms of clinical and histopathological features, of the whole PM/DM disease complex. Presentation may be before 5 years of age with peak incidence between 8 and 12 years. The disease may remit and recur until well into young adult life. The skin lesions include a facial rash in butterfly distribution across nose and cheeks. Erythematous skin changes are seen over extensor surfaces of joints, especially knees, knuckles and elbows. Muscle involvement is generally evident some time later and takes the form of weakness and stiffness, particularly affecting shoulder and pelvic musculature. Proximal muscles are often worse affected than distal muscles and extensors worse than flexors. In the absence of prompt and effective treatment contractures may occur at elbows, ankles, knees, and hips. Subcutaneous calcification and skin ulceration may be found calcification of deeper-lying connective tissue may be apparent on X-ray. 

Marsolais [38] reported improved motor function in patients with intramuscular stainless steel electrodes implanted in the quadriceps, hip flexors, extensors, and abductors. Though significant motor torque improvement was achieved in some of these patients, all of whom had some motor function preimplantation, patients required supervision for ambulation. 

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