Cardiovascular adjustments to isometric contractions: The role of intensity of contraction, endurance time, and different muscle groups

This study was designed to investigate the influence of the intensity of contraction, endurance time, and the contracting muscle group on the cardiovascular responses to static exercise. Three primary questions were addressed: (1) Are the cardiovascular responses to isometric contractions dependent on the relative intensity of contraction?; (2) Are the cardiovascular responses to isometric contractions to fatigue with the knee extensors and finger flexors different?; (3) Are the cardiovascular responses to isometric contractions at each of the selected intensities and muscle groups different at the moment of fatigue?. A three-way within-subjects design was employed in order to examine the role of three different factors: time, intensity of contraction, and distinct muscle groups, on the circulatory adjustments to static exercise.;The present study was conducted at the Physical Fitness Research Laboratory of the Department of Kinesiology at the University of Illinois. Participants in the study were 12 healthy, untrained, young (18 to 30 years old), Caucasian males. Maximal voluntary contractions were measured with the finger flexors and knee extensors. Isometric contractions were performed to fatigue at 20%, 40%, 60%, and 80% MVC. Cardiovascular variables were measured before, during, and after the exercise period. Heart rate, blood pressure, and stroke volume were recorded through electrocardiography, sphygmomanometry, and impedance cardiography, respectively. From these measures, cardiac output, mean arterial pressure, and total peripheral resistance were calculated. A three-way ANOVA for repeated measures was used in the analyses of the data. Measures of the effect size were also reported as an additional measure of the treatment effect.;In regard to the cardiovascular responses to isometric contractions from rest to fatigue with different intensities of contraction and muscle groups, the results of this study, for the most part, confirm findings commonly observed in the literature. In addition, with respect to the primary questions addressed in this study, the results support the following conclusions: (1) The intensity of isometric contractions appears to play an important role in the cardiovascular adjustments during static exercise. This effect is apparent in the responses of blood pressure, heart rate, and total peripheral resistance to isometric contractions. This pattern of response was similar for the two muscle groups investigated in the present study. (2) The findings of the present study provide evidence that heart rate responses to isometric contractions are dependent on the contracting muscle group, being greater with knee extensors than with finger flexors. For contractions above 20% MVC, blood pressure responses were also related to the active muscle group, presenting higher values with knee extensors than with finger flexors. The participation of cardiac output and total peripheral resistance in the elicitation of blood pressure responses varied with the intensity of contraction and contracting muscle group. (3) The graphical representation of the interactions between muscle groups and intensity of contraction demonstrates for some dependent variables a somewhat different response between muscle groups. However, systolic blood pressure was the only variable to demonstrate statistical significance for the muscle x intensity effect. (Abstract shortened by UMI.)...