Changes in isometric function following rhythmic exercise.

Abstract

Seven male subjects exercised for 1, 3, 10 and 20 min on a cycle ergometer at 20, 60 and 80% VO2max, and then held to fatigue a sustained contraction of the quadriceps at 40% maximal voluntary contraction in order to determine what influence various levels of dynamic exercise would have on isometric function of the same group of muscles. Muscle temperature was measured before and within 15 s of the completion of the cycling to determine whether changes in muscle temperature might influence the subsequent isometric performance. Isometric endurance was shorter as the severity of the cycling increased beyond 20% VO2max, and as the duration of cycling increased up to 10 min. There were discrete linear relationships between muscle temperature and isometric endurance associated with cycling at 60% and 80% VO2max. There was a direct inverse relationship between quadriceps strength after cycling and muscle temperature, yet a significant reduction in strength occurred only after cycling at 80% VO2max. These results suggest that the encroachment on endurance and strength are controlled by different mechanisms. The heart rates during the isometric contractions were dependent on the preceding rhythmic exercise and decreased after exercise at 60 or 80% VO2max. In contrast, the blood pressure always increased during the isometric contractions, reaching similar values at the point of fatigue, regardless of the severity of the previous rhythmic exercise. These data provide additional evidence that separate mechanisms control changes in heart rate and blood pressure.