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

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^-2). Six COPD patients (16 ± 2.5 kg m^-2) (P < 0.05) presented reduced thigh mass (COPD_LQ), whereas 10 patients with normal quadriceps mass (COPD_NQ) and all controls had identical mass distribution (22 ± 2.4 kg m^-2). COPD_LQ patients had lower muscle function and lower exercise tolerance than both COPD_NQ and controls (P < 0.05 each), but muscle strength corrected by mass was similar between COPD patients (COPD_LQ 0.59 ±0.12 and COPD _NQ 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 (COPD_LQ and COPD_NQ 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 (COPD_LQ 66 ± 14 and COPD_NQ 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. American Journal of Physical Medicine & Rehabilitation