BACKGROUNDSkeletal muscle dysfunction is one of important systemic manifestations of chronic obstructive pulmonary disease (COPD) and is associated with mortality in patients with COPD, thus quantifying its strength is of great clinical interest and of particular value. Quadriceps maximal volitional contraction (MVC) is often used for the routine measurements of this muscle's strength; while twitch tension (TwQ) evoked by magnetic stimulation of the femoral nerve has been employed for measurement of quadriceps strength non-volitionally. We aimed to investigate the prevalence and severity of skeletal muscle dysfunction in COPD patients by measurement of quadriceps strength with volitional and non-volitional techniques, and to probe into some methodological issues.

METHODSWe recruited 71 COPD patients and 60 control subjects. Quadriceps strength was measured with both maximality of TwQ and MVC force. The reproducibility for TwQ and MVC was investigated using within-occasion variability from three repeated maneuvers.

RESULTSMaximal TwQ was achieved in 121 participants at a mean of 90% of the stimulator's maximum output. The mean maximality of TwQ was decrease by about 44% - 47% in COPD patients as compared with controls (P < 0.05), so was MVC. There was a significant correlation between quadriceps TwQ and MVC, and the mean ratio of TwQ/MVC was 0.29 in controls and 0.33 in patients. The coefficient of variation showed that TwQ yielded lower within-occasion variability than MVC in both groups.

CONCLUSIONSQuadriceps strength is commonly and substantially impaired in patients with COPD, in terms of MVC as well as TwQ. The magnetic stimulation of the femoral nerve presents a higher reproducibility and is a better technique for measurement of quadriceps strength for the general population, especially for those who are too unwell to perform a full MVC; while it may not be applied to subjects who are over-weighted.