Isometric fatigue induced by different levels of rhythmic exercise.

Three subjects were trained in leg extensor isometric contractions and in cycling. They then cycled for three consecutive bouts, each of 2.75 min at a constant level of VO2, from 20 to 80% VO2 max. Fifteen seconds after each bout of cycling the subjects exerted an isometric contraction of the right leg at 40% of the maximum voluntary contraction. In each experiment, the duration of the three successive isometric contractions decreased as in hand-grip contractions. There was also a linear reduction in isometric endurance as the severity of the preceding rhythmic exercise increased. In other experiments, after three bouts of rhythmic exercise at 20% VO2 max (each followed by a fatiguing contraction at 40% MVC), further bouts of cycling at increasing levels of severity up to 60% VO2 max resulted in a linear fall in isometric endurance which could be reversed by interposing a lighter level of cycling. The heart rates during these experiments showed a steady increase during the isometric exercise, to about 150 beats X min-1, as the bouts of preceding rhythmic exercise became progressively more severe. The isometric contractions had little influence on the heart rate during cycling. But the rhythmic cycling exercise markedly increased the heart rate achieved at the end of the isometric contractions while decreasing the increment in heart rate during the contraction.