The effects of beta-blockade on electrically stimulated contraction in fatigued human triceps surae muscle.

The contractile characteristics of the triceps surae muscle group were examined before and after repeated isometric contractions in two groups of eight healthy young males. Single twitches and trains of stimuli at 10, 20, 50 and 100 Hz were delivered to the muscle using supramaximal voltages. Subjects were treated with beta-blockade (2 X 80 mg oral propranolol, beta-b) or matched placebo in a double-blind crossover design. Four different exercise conditions were studied: (I) maximal voluntary contraction (MVC); (II) MVC during circulatory occlusion; (III) electrical stimulation at 20 Hz using 50% of voltage required for maximal torque production; and (IV) electrical stimulation with occlusion. Each contraction was for 5 s with 5 s recovery. Total duration of exercise was 10 min for non-occluded contractions and to a 50% decline in torque output with occlusion. At rest prior to exercise, maximal voluntary contraction was significantly reduced (5.7%) by beta-b during 40 observations in 16 subjects. Following exercise without occlusion (I and III), the reduction in torque output of the muscle at 10 and 20 Hz stimulation was generally greater during beta-b than placebo. This low frequency fatigue was longer-lasting with beta-b. The shorter lasting reduction in torque at 50 and 100 Hz was generally not different between beta-b placebo. After exercise with occlusion (II and IV), the torque output at all stimulation frequencies was reduced to a similar extent in both placebo and beta-b at most comparison points. Twitch responses after exercise with occlusion showed decreases in peak tension and time to peak tension and a lengthening of one-half relaxation time in both placebo and beta-b. It was concluded that the greater reduction in torque output of the triceps surae muscle group at low frequencies during beta-b was probably a consequence of a reduction in blood flow relative to the placebo treatment. This relative low frequency fatigue could be responsible for the increased perception of effort in patients exercising during beta-blocker therapy.