In vitro and in vivo contractile properties of the vastus lateralis muscle in males with COPD

Peripheral muscle weakness is common in chronic obstructive pulmonary disease (COPD) but it is still under debate whether weakness is due to atrophy or contractile dysfunction. In vitro and in vivo contractile properties of the vastus lateralis muscle were studied in 16 patients with stable COPD (forced expiratory volume in one second 39+/-16% of predicted, age 67+/-4yrs (mean+/-SD)) and nine sedentary control subjects. Isometric knee extensor strength was measured while mid-thigh muscle cross-sectional area (MTMCSA) was obtained using computed tomography. Muscle strips from the vastus lateralis obtained through open biopsy were rapidly suspended in an oxygenated Krebs-Ringer solution that was maintained at 35degreesC with a pH of 7.40 to study their contractile properties. The isometric 2 knee extensors strength/MTMCSA ratio was 0.50+/-0.08 versus 0.58+/-0.06 kg-cm(-2) for COPD and control subjects, respectively. The muscle bundle 2 cross-sectional area (CSA) was 4.6+/-2.1 and 4.4+/-3.1 mm(-2), the length at which active tension was maximum was 15+/-4 and 15+/-3 mm, and maximal isometric peak forces normalised for CSA were 4.3+/-2.7 and 4.8+/-2.6 N cm(-2) for COPD and control subjects, respectively. The force/frequency relationship tended to be shifted to the right in patients with COPD, meaning that a higher stimulation frequency was necessary to produce the same relative force. Patients with COPD had a lower proportion of type I fibre than controls (26+/-12% versus 39+/-11%) with reciprocal significant increase in type IIb fibre proportion (20+/-16% versus 8 +/-4%). The proportion of type IIa fibres was similar between the two groups. These results suggest that the contractile properties of the vastus lateralis are preserved in patients with chronic obstructive pulmonary disease. Therefore, the reduction in the quadriceps strength in patients with chronic obstructive pulmonary disease cannot be explained on the basis of an alteration of the contractile apparatus.