Influence of muscle length on human inspiratory and limb muscle endurance

The influence of muscle length on endurance of the inspiratory muscles and the flexors of the elbow was assessed in 12 healthy subjects who performed series of 18 maximal static voluntary contractions of 10 sec duration separated by rest intervals of 5 sec. Inspiratory contractions were performed at functional residual capacity (FRC) and at FRC plus half inspiratory capacity (FRC + 1/2 IC). Contractions of the flexors of the elbow were performed at 90 degrees and 45 degrees of flexion. Compared with values obtained at the control muscle length, maximal force at the short length was reduced to 78 +/- 10% for the inspiratory muscles and 74 +/- 6% for the flexors of the elbow (n.s.). For each test, indices of endurance were measured as the average (and peak) force attained in the better of the last two contractions expressed as a percentage of that in the best of the first three. For the flexors of the elbow endurance was significantly enhanced at the short length (45 degrees: 61 +/- 3.0%, mean +/- SE) compared with the control length (90 degrees: 55 +/- 2.0%, P less than 0.01). By contrast, this enhancement of endurance did not occur for the inspiratory muscles at the short length (FRC + 1/2 IC: 81 +/- 3.0%; FRC: 87 +/- 3.0%; P less than 0.02). Endurance of the inspiratory muscles was greater than that of the elbow flexors at both muscle lengths. The enhanced endurance of the elbow flexors, when tested at a short muscle length, is consistent with data obtained in isolated muscles.