Fatigue and recovery of power and isometric torque following isotonic knee extensions

Abstract

The purpose of this study was to assess fatigue and recovery of isotonic power and isometric contractile properties after a series of maximal isotonic contractions. Using a Biodex dynamometer, 13 men [26 yr (SD 3)] performed isotonic [50% of isometric maximal voluntary contraction (MVC) every 1.2 s through 75° range of motion] single-limb knee extensions at the fastest velocity they could achieve until velocity was reduced by 35%. Time to task failure was 38 s, and, compared with baseline, power declined by ∼42% [741.0 (SD 106.0) vs. 426.5 W (SD 60.3) at task failure], and MVC declined by ∼26% [267.3 (SD 42.5) vs. 198.4 N·m (SD 45.7) at task failure]. Power recovered by 5 min, whereas MVC did not recover, and at 10 min was only ∼85% of baseline. Isometric MVC motor unit activation was ∼95% at rest and was unchanged at task failure (∼96%), but a small amount of failure was apparent between 1.5 and 10 min of recovery (∼87 to ∼91%). Half relaxation time measured from a 50-Hz isometric tetanus was significantly prolonged by ∼33% immediately after task failure but recovered by 1.5 min. A decline in the 10- to 50-Hz ratio of the evoked isometric contractions was observed at 5 and 10 min of recovery, which suggests excitation-contraction coupling impairment. Changes in velocity and half relaxation time during the protocol were strongly and negatively correlated ( r = −0.85). Thus mainly peripheral mechanisms were implicated in the substantial depression but relatively fast recovery of isotonic power. Furthermore, isometric muscle contractile properties were related to some, but not all, changes in isotonic function.