Objective: Correlation of muscle function, muscle mass and endurance, and exercise tolerance in chronic obstructive pulmonary disease (COPD).
Design: Sixteen COPD patients (forced expiratory volume during the first second 38 +/- 15% predicted) and 6 controls underwent magnetic resonance imaging of the thigh, muscle strength and endurance, and exercise tolerance assessments.
Results: Thigh mass distribution was bimodal (cutoff 19.0 kg m). Six COPD patients (16 +/- 2.5 kg m(-2)) (P < 0.05) presented reduced thigh mass (COPDLQ), whereas 10 patients with normal quadriceps mass (COPDNQ) and all controls had identical mass distribution (22 +/- 2.4 kg m(-2)). COPDLQ patients had lower muscle function and lower exercise tolerance than both COPDNQ and controls (P < 0.05 each), but muscle strength corrected by mass was similar between COPD patients (COPDLQ 0.59 +/- 0.12 and COPDNQ 0.55 +/- 0.10 Nm kg(-1) m(-2)) and controls (0.62 +/- 0.04 Nm Kg(-1) m(-2)). In contrast, endurance to muscle mass ratio was lower in COPD (COPDLQ and COPDNQ 0.91 +/- 0.15 and 0.89 +/- 0.15 J kg(-1) m(-2)) than in controls (1.07 +/- 0.11 J kg(-1) m(-2)) (P < 0.05). Half-time phosphocreatine recovery (COPDLQ 66 +/- 14 and COPDNQ 55 +/- 9 secs, not significant) was also slower than in controls (43 +/- 10 secs) (P < 0.01).
Conclusions: Impaired muscle strength was explained by reduced muscle mass, but it did not account for abnormal muscle endurance. The latter seems associated to impaired O2 transport/O2 utilization, resulting in altered muscle bioenergetics.