PHYSIOLOGICAL-EFFECTS OF MICROPAUSES IN ISOMETRIC HANDGRIP EXERCISE

The physiological response to continuous and intermittent handgrip exercise was evaluated. Three experiments were performed until exhaustion at 25% of maximal voluntary contraction (MVC): experiment 1, continuous handgrip (CH) (n = 8); experiment 2, intermittent handgrip with 10-s rest pause every 3 min (IH) (n = 8); and experiment 3, as IH but with electrical stimulation (ES) of the forearm extensors in the pauses (IHES) (n = 4). Before, during, and after exercise, recordings were made of heart rate (HR), arterial blood pressure (BP), exercising forearm blood flow, and concentrations of potassium [K+] and lactate [La-] in venous blood from both arms. The electromyogram (EMG) of the exercising forearm extensors and perceived exertion were monitored during exercise. Before and up to 24 h after exercise, observations were made of MVC, of force response to electrical stimulation and of the EMG response to a 10-s test contraction (handgrip) at 25% of the initial MVC. Maximal endurance time (t(lim)) was significantly longer in IH (23.1 min) than in CH (16.2 min). The ES had no significant effect on t(lim). During exercise, no significant differences were seen between CH and IH in blood flow, venous [K+] and [La-], or EMG response. The HR and BP increased at the same rate in CH and IH but, because of the longer duration of IH, the levels at exhaustion were higher in this protocol. The subjects reported less subjective fatigue in IH. During recovery, return to normal MVC was slower after CH (24 h) than after IH (4 h). However, the frequency content of the EMG during the 10-s test contractions was still reduced 24 h after IH, but only 4 h after CH. This may be explained by the larger amount of potassium lost up to the first hour of recovery in IH. None of the measured physiological parameters could provide a satisfactory explanation, either of the 43% difference in t(lim) between CH and IH, or of the sense of relief reported by all subjects during the short rest pauses in IH. Therefore, a plausible explanation was that the prolongation of t(lim) in IH may have been related to differences at a sensory level. It was concluded that short rest periods, if introduced in exercise with long cycle-times, could give an immediate sense of relief, postponing the subjective threshold of fatigue. Thus the results of this study provided experimental support for the hypothesis that the introduction of micropauses may create an increased risk of musculoskeletal disorders.