Abstract
[Purpose] The objective of this study was to investigate the effect of the duration of muscle relaxation during intermittent isometric exercises (IIEs). [Subjects] Subjects were twelve healthy males.[Methods] The subjects performed 10 cycles of intermittent isometric knee extension at 60% maximum voluntary contraction for 8 seconds. One cycle of IIE consisted of a muscle contraction phase and subsequent muscle relaxation phase. To determine the durations of the muscle relaxation phase, two recovery periods (TR1.0 and TR0.5) of oxygenation level in the vastus lateralis muscle (VL) were measured with near-infrared spectroscopy immediately after the isometric knee extension. TR1.0 and TR0.5 were 30.6 ± 6.8 seconds and 15.4 ± 4.8 seconds, respectively. Therefore, IIEs were performed as two different types (IIE-TR1.0 and IIE-TR0.5). [Results] Deoxygenation during muscle contraction and relaxation phases in VL were significantly stronger in IIE-TR0.5 than in IIE-TR1.0. The reduction in median power frequency, an indirect measure of lactate accumulation in VL, was also significantly greater in IIE-TR0.5 than in IIE-TR1.0. [Conclusion] The results of this study suggest that the duration of muscle relaxation determines both deoxygenation and lactate accumulation in active muscles during IIE.
