A major concern with exercise in individuals with NMD is overwork weakness. Overwork weakness was first suggested in DMD by muscle histologic examination, demonstrating that most degeneration occurs in muscles typically used during sustained physical activity. Muscle enzymes, used as a marker of muscle breakdown, were reduced by bedrest in dystrophic patients and elevated to a greater degree than normal controls after a vigorous strengthening program. Traditionally, high resistance exercise has been discouraged in NMD due to fears of overworking the weakened muscle. However, a survey of the research literature indicates studies with results ranging from no untoward effects to beneficial effects, with slowly progressive NMDs obtaining better results than DMDs.
Stronger muscles appear to strengthen more than very weak ones.
Thus, starting an exercise regimen early in the disease when muscle fiber degeneration and weakness are minimal is generally recommended. Measurement of exercise capacity is feasible in children and adolescents with NMD if they are able to climb stairs with the aid of a railing. The limitation appears to be in oxygen uptake at the muscle cell level, which is proportional to the loss of active muscle tissue. Likewise, cardiac output seems to increase proportionally to increased O2 consumption during exercise in muscular dystrophies, as in normal individuals. Little work has been done on the response of diseased muscle to endurance exercise.