EFFECT OF SELECTIVE AND NONSELECTIVE BETA-BLOCKADE ON SKELETAL-MUSCLEEXCITABILITY AND FATIGUABILITY

The effects of beta-blockade on skeletal muscle excitability and fatig uability during exercise were examined. Ten healthy males (mean age 21 .9 +/- 1.1 yr) performed a 4-min fatigue protocol consisting of interm ittent isometric voluntary contractions of the knee extensors in one l eg. Subjects performed the exercise after treatment with placebo, 100 mg metoprolol, or an equipotent dose of propranolol (60 mg, n = 1; 80 mg, n = 8; 100 mg, n = 1) twice a day for 76 h before testing accordin g to a randomized double-blind design. The evoked twitch torque, maxim al voluntary torque, and maximal M-wave amplitude were unaffected by t he beta-blockade treatments before fatigue. During the placebo trial, there were significant reductions in the evoked and voluntary torques (77 +/- 15 and 55 +/- 11%, respectively) after the fatigue protocol; h owever, both the voluntary electromyogram and evoked M waves were well maintained throughout fatigue. The beta-blockade treatments had no si gnificant effect on torque or electromyogram activity over the course of the exercise. Thus, despite evidence for an impairment of dynamic e xercise performance with beta(1)- and beta(1,2)-blockade, there appear s to be no effect of these agents on muscle excitability and fatiguabi lity during isometric muscle activity.