Glucocorticoid-induced muscle atrophy. The role of exercise in treatment and prevention.

Glucocorticoids have catabolic effects on skeletal muscle. These effects demonstrate fiber type specificity: Type IIB fibers are most susceptible and type I fibers least susceptible to the atrophy effects of glucocorticoids. Exercise training has been used frequently to counteract glucocorticoid-induced muscle atrophy. Studies have demonstrated that both resistance exercise and endurance exercise are effective in attenuating this atrophy. The cellular mechanisms mediating the attenuating effect of exercise on glucocorticoid-induced atrophy are not completely understood. More research is needed to further elucidate the details of these mechanisms and to determine the most efficacious exercise protocols for deterring glucocorticoid-induced muscle atrophy. Exercise evaluation of patients with glucocorticoid-induced myopathy and subsequent exercise prescription must be individualized and based on the patient's initial muscle function and medical status.

Alterations in intramuscular connective tissue after limb casting affect contraction-induced muscle injury.

This study examined the effect of alterations in rat intramuscular connective tissue (CT), secondary to limb immobilization, on the muscle's susceptibility to contraction-induced injury. Hindlimbs were casted for 3 wk with the extensor digitorum longus muscle fixed in a shortened (IM-SP) or lengthened position (IM-LP). An age-matched control group remained uncasted. Extensor digitorum longus muscles were injured in vivo by using a motorized foot pedal that repeatedly flexed and extended the foot while the muscle was electrically stimulated during plantar flexion. Four hours postinjury, maximum isometric tetanic force (Po) was measured in vitro and was used as a functional index of muscle injury. Muscles were fixed, sectioned, and stained for later analysis. Intramuscular CT concentration, expressed as the ratio of CT area to muscle fiber area, was significantly higher in both IM-SP (0.153 +/- 0.003) and IM-LP (0.174 +/- 0.003) groups compared with controls (0.104 +/- 0.003). Po values of injured muscles both IM-LP and IM-SP were higher than the injured controls' Po of 9.41 +/- 0.63 N/cm2 (P < 0.05). Injured IM-LP muscle forces were significantly higher than those of IM-SP. This study demonstrated that limb immobilization increases intramuscular CT concentration, which is accompanied by attenuation of muscle injury. We conclude that remodeling of intramuscular CT affects the muscle's resistance to contraction-induced injury.