Purpose: The authors determined the degree of respiratory and peripheral muscle weakness in patients with moderate to severe chronic obstructive pulmonary disease (COPD). Differences in severity of muscle weakness among muscle groups may provide treatment options, such as selective muscle training, to adapt the exercise prescription in pulmonary rehabilitation programs. In addition, this information may add to the knowledge on the mechanisms of muscle weakness.
Methods: Respiratory and peripheral muscle force were quantified in 22 healthy elderly subjects and 40 consecutive COPD patients (forced expiratory volume in 1 second, percent of predicted value [% pred] 41 +/- 19; transfer factor for carbon monoxide, % pred 47 +/- 26) admitted to a pulmonary rehabilitation program. Lung function, diffusing capacity, isometric force of four peripheral muscle groups (handgrip, elbow flexion, shoulder abduction, and knee extension), neck flexion force, and maximal inspiratory and expiratory pressures were measured.
Results: Patients had reduced respiratory muscle strength (mean 64% of control subjects' value [% control]) and peripheral muscle strength (mean 75% control) compared to normal subjects. Inspiratory muscle strength (59 +/- 18% control) was significantly lower than expiratory muscle strength (69 +/- 25% control) and peripheral muscle strength (P < 0.01). Neck flexion force (80 +/- 19% control) was better preserved than maximal inspiratory pressure and shoulder abduction force (70 +/- 15% control, P < 0.01). Handgrip force (78 +/- 16% control) and elbow flexion force (78 +/- 14% control) were significantly less affected than shoulder abduction force (70 +/- 15% control, P < 0.01). Finally, shoulder abduction force and knee-extension force (72 +/- 24% control) were not significantly different.
Conclusions: Muscle weakness in stable COPD patients does not affect all muscles to a similar extent. Inspiratory muscle force is affected more than peripheral muscle force, whereas proximal upper limb muscle strength was impaired more than distal upper limb muscle strength.