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Instantaneous cardioacceleration in response to high-intensity, short-duration isometric contractions Kitagawa, Eiji

Abstract

The purpose of this study was to investigate on a beat-by-beat basis, the nature of the instantaneous cardioacceleration resulting from short-duration, high-intensity, isometric contractions; to determine the relationship between relative muscular tension and cycle time, actual change in cycle time and relative change in cycle time; and to determine the effect of short-duration, high-intensity, isometric contractions on the recovery rate of cycle time. Muscular tensions and electrocardiograms were recorded before, during and after the isometric task and relative muscular tension, cycle time, actual change in cycle time and relative change in cycle time were calculated from these recordings. The sample population consisted of thirty university males. Each subject was randomly assigned to one of three groups. The task consisted of either a 50% MVC, 75% MVC or 100% MVC to be held for 10-seconds. Dynamometer and ECG recordings were monitered for the 15-seconds immediately before the contraction, the 10-seconds during the contraction and the 15-seconds immediately following the contraction. The hypotheses were: at the onset of an isometric contraction, the initial rate of increase of the relative change in cycle time is directly related to the magnitude of the percent maximal voluntary contraction; during the isometric contraction, the relative muscular tension is more closely related to the relative change in cycle time than either cycle time or actual change in cycle time; and, upon release of the isometric contraction, the initial rate of decrease of the relative change in cycle time is directly related to the magnitude of the percent maximal voluntary contraction. The trend analysis of the relative change in cycle time, during contraction, indicated that the linear and quadratic trends were different between the three groups. However, trend analyses of paired comparisons between the three groups indicated that the trends of the 50% MVC and 75% MVC groups were significantly different from the trend of the 100% MVC group, but, were not significantly different from each other. The post-hoc Newman-Keuls analysis indicated that the heart responded differently over the first five beats than over the last five beats, suggesting that there are at least two phases in the cardio-acceleratory response to isometric contractions and these two phases may be a result of a central and a peripheral heart-trigger mechanism. The trend analysis of the relative change in cycle time, during recovery, indicated a significant linear trend in beats with a significant difference in this trend between groups. However, observation of the graphic plot of means indicated that the data did not support the hypothesis, that the initial rate of decrease of relative change in cycle time was directly related to the magnitude of the %MVC. The non-support may have been due to the masking effect of respiration.

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