Exercise performance in 6-to-11-year-old boys with Duchenne muscular dystrophy

To determine effects of severe muscular dystrophy on the performance of dynamic exercise, cardiorespiratory responses to incremental work on a bicycle ergometer and isokinetic limb strength measurements were compared for 13 dystrophic boys and 13 normal, untrained boys. The dystrophic boys (D) were matched to the normal boys (N) on the basis of age (8.4 yr), height (125 cm) and weight (25.7 kg). At rest, the dystrophic group had higher heart rates (HR) (D = 102; D = 31; N = 39 ml), with no difference in oxygen uptake (VO2), calculated cardiac output (Q), pulmonary ventilation (VE), or respiratory exchange ratio (R). During submaximal work, VO2, SV, Q and VE were lower in D. During maximal work, D had lower peak values for work rate (D = 400; N = 600 kg/min), endurance (D = 41; N = 60 ml), Q (D = 5.2; N = 11.0 liters/min), VE (D = 8.2; N = 36.9 liters/min), and R (D = 0.84; N = 0.99). Arm and leg strengths (four flexion and four extension motions) were lower in D, but muscle girths were not necessarily smaller. The findings indicate exercise performance in D was below normal and limited by low cardiorespiratory capacities, diminished leg strength, and perhaps reduced peripheral oxygen utilization. Duchenne muscular dystrophy, even in its early stages, apparently affects the work capacity of cardiac and pulmonary muscles as well as limb muscles.