Evaluation of electromyographic activity and heart rate responses to isometric exercise. The role played by muscular mass and type

The purpose of the present study was to examine the relationship between th e electromyographic (EMG) activity and heart rate (HR) responses induced by isometric exercise performed by knee extension (KE) and flexion (KF) in me n. Fifteen healthy male subjects, 21 +/- 1.3 years (mean +/- SD), were subm itted to KE and KF isometric exercise tests at 100% of maximal voluntary co ntraction (MVC). The exercises were performed with one leg (right or left) and with two legs simultaneously, for 10 s in the sifting position with the hip and knee flexed at 90 degrees. EMG activity (root mean square values) and KR (beats/min) were recorded simultaneously both at rest and throughout the sustained contraction. The HR responses to isometric exercise in KE an d KF were similar when performed with one and two legs. However, the HR inc rease was always significantly higher in KE than KF (P<0.05), whereas the E MG activity was higher in KE than in KF (P<0.05), regardless of the muscle mass (one or two legs) involved in the effort. The correlation coefficients between HR response and the EMG activity during KE (r = 0.33, P>0.05) and KF (r = 0.15, P>0.05) contractions were not significant. These results sugg est that the predominant mechanism responsible for the larger increase in H R response to KE as compared to KF in our study could be dependent on quali tative and quantitative differences in the fiber type composition found in each muscle group. This mechanism seems to demand a higher activation of mo tor units with a corresponding increase in central command to the cardiovas cular centers that modulate HR control.