Heart rate (HR) response to isometric exercise (handgrip) was investigated in 7 normal males of sedentary habits before and after endurance training involving the muscles of the lower limbs. Thirteen additional sedentary individuals and 7 middle-distance runners were also studied. Isometric exercise was performed at 100, 75 and 50% of maximum voluntary contraction (MVC) during 10, 20 and 40 s, respectively. Training produced a 15.6 +/- 1.4% (mean +/- SEM) increase in VO2max and a reduction in resting HR from 69 +/- 1.9 to 58 +/- 1.7 bpm. HR was monitored throughout each period of isometric exercise. 2. The pattern of HR response to static effort performed by untrained muscles was comparable before and after training as well as in athletes and sedentary individuals during the first 10 s of contraction, a period during which tachycardia is mainly mediated by vagal release. After the first 10 s, when the sympathetic influence on tachycardia becomes evident, athletes and trained individuals showed a slight but nonsignificant tendency toward lower HR increases. 3. These results do not demonstrate any appreciable alteration in the efferent activity of autonomic components induced by aerobic training of the leg muscles when isometric exercise is performed with untrained muscles.
