Contractile properties of the human triceps surae following prolonged exercise and β-blockade

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Abstract
Sixteen healthy males volunteered to perform both an incremental maximal and prolonged submaximal treadmill test with .beta.-blockade (2 .times. 80 mg oral propranolol per day) or matched placebo in a blind crossover design. Prior to and following the prolonged exercise, electrical stimulation of the triceps surae was performed to examine contractile properties. During the maximal test, the heart rate (HR) was reduced at all times by .beta.-blockade. The time to exhaustion in this test was significantly reduced by .beta.-blockade (P < 0.03), while the maximal oxyen uptake .ovrhdot.VO2max was not significantly lower (P=0.06). In response to prolonged treadmill walking at 60% of .ovrhdot.VO2max, the HR was reduced but .ovrhdot.VO2, respiratory quotient and ventilation were not affected by .beta.-blockade relative to placebo. Plasma concentrations of free fatty acids increased during execise in the placebo but not .beta.-blocked treatment (P < 0.0001). Plasma noradrenalin and adrenalin increased with exercise; the increase in adrenalin with .beta.-blockade was greater than that with placebo (P < 0.0001). The RPE obtained at interval during the prolonged exercise were greater for .beta.-blockades than placebo. Eight of 16 subjects were unable to complete full 90 min with .beta.-blockade; but all 16 completed the test with placebo. The electrically evoked twitches in the triceps surae muscle group after exercise did not differ in peak torque or one-half relaxation time compared to pre-exercise. The time to peak twitch torque was significantly shorter after exercise. No differences in twitch were observed due to .beta.-blockade. The tetanic responses at 10, 20, 50 and 100 Hz were not affected by either exercise or the .beta.-blockade. In conclusion, an increased subjective estimate of fatigue (RPE) was observed during prolonged exercise with .beta.-blockade. This subjective fatigue did not relate to altered peripheral muscle force production during electrical stimulation. The results suggest either a central rather than peripheral origin of fatigue, or fatigue in a muscle group not examined by stimulation of the triceps surae.