Clinical assessment of peripheral muscle function in patients with chronic obstructive pulmonary disease

Correlation of muscle function, muscle mass and endurance, and exercise tolerance in chronic obstructive pulmonary disease (COPD).

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.

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).

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.