Human tibialis anterior contractile responses following fatiguing exercise with and without beta-adrenoceptor blockade.

The effects of oral propranolol (2 x 80 mg/day) on the contractile responses to twitch and tetanic electrical stimulation were examined in the tibialis anterior (TA) muscle of seven healthy young males. The TA muscle was fatigued by four forms of repeated isometric contractions: (1) maximal voluntary contractions (MVC), (2) MVC with circulation occluded, (3) electrically evoked contractions with 20 Hz supramaximal voltage stimulation and (4) electrically evoked contractions with circulation occluded. Each contraction was sustained for 10 s with 5 s recovery. Duration of exercise was 10 min for intact circulation and 4 min for circulatory occlusion. Pre-exercise, both the twitch contraction time and the 1/2 relaxation time were significantly (P less than 0.05) longer with beta-blockade than placebo. beta-blockade did not affect torque output during tetanic stimulation or MVC. Immediately post-exercise, the peak twitch torque was reduced in all beta-blocked and placebo conditions except electrically induced exercise with intact circulation. The 1/2 relaxation time was significantly lengthened by repeated MVC with circulation intact; beta-blockade caused a greater lengthening than placebo (P less than 0.05). The tetanic torque was reduced immediately post-exercise at each of 10, 20, 50 and 100 Hz for both beta-blockade and placebo for each form of exercise. There were no significant beta-blockade effects. Torque output at 10 Hz was still reduced up to 10 min post-exercise. In contrast, 100 Hz torque output recovered by 5 min post-exercise. The changes in tetanic responses were qualitatively similar with intact circulation and with circulatory occlusion. In the tibialis anterior muscle, the effects of fatiguing exercise are not accentuated by beta-blockade. These data in the TA are notably different from those in the triceps surae, where greater fatigue has been shown with beta-blockade.