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|Title:||The Effect of B-Blockade on Skeletal Muscle Excitability and Fatiguability During Exercise|
|Other Titles:||B-Blockade and Skeletal Muscle Function During Exercise|
|Abstract:||The purpose of this investigation was to examine the effects of selective and non-selective β-blockade on muscle excitability and fatiguability during exercise. Ten healthy males (x̄ age= 21.9 ± 7.1 yrs) participated in all phases of the study. The first stage was designed to determine equipotent doses of the selective (metoprolol) and non-selective (propranolol) β-blocking agents within each subject. Symptom limited, maximal graded exercise tests were performed on an electrically braked cycle ergometer during a control condition and after the administration of 100 mg of metoprolol. Following this, exercise tests were performed to establish a dose of propranolol equipotent to that of 100 mg of metoprolol within each subject. In 8 of the subjects, 80 mg of propranolol produced a heart rate attenuation equal to that of 100 mg of metoprolol. In contrast, 1 subject required 60 mg of propranolol while another required 100 mg of propranolol to match the effects of the metoprolol treatment. Significant reductions in the submaximal and maximal oxygen uptakes were observed during the metoprolol (9% ↓ ± 7%; 10% ↓ ± 4%, respectively) and the propranolol (9% ↓ ± 7%; 19% ↓ ± 4%, respectively) treatments. Similarly, the time to exhaustion was reduced significantly by 13% (± 8%) and 19% (± 8%) following the administration of metoprolol and propranolol, respectively. The reductions in the maximal oxygen uptake and the time to exhaustion elicited by the β-blocking agents were significantly greater following the non -selective versus the selective drug treatments. It was hypothesized that part of the impairment in exercise performance with β-blockade could be the result of an inhibition in the activity of the adrenergically cor,trolled Na⁺-K⁺ ATPase with a subsequent failure in muscle excitability during exercise. Thus, in the second stage of this investigation, a double blind design was utilized to investigate the effects of metoprolol and propranolol on muscle excitability and fatiguability. Subjects performed a 4 minute fatigue protocol consisting of intermittent, isometric voluntary contractions of the knee extensor muscles in one leg. The protocols were performed on three separate occasions following the administration of either placebo, 100 mg of metoprolol or an equipotent dose of propranolol. Surface electrodes were used to record the voluntary EMG activity and M-waves from the vastus medialis in the active and inactive legs throughout each of the drug trials. During the control trial, significant declines in the evoked twitch torque (77% ↓ ± 15%) and the voluntary torque (55% ↓ ± 11%) were observed but these recovered completely within 15 minutes following the exercise. In contrast, both the voluntary EMG activity and the M-waves recorded from the active and inactive legs were maintained throughout fatigue and recovery in the control state. Neither the evoked contractile properties for the voluntary muscle strength of the knee extensors were affected by the administration of metoprolol or propranolol. The fatiguability of the quadriceps was also unaffected by the β-blocking agents. Similarly, the β-blockade treatments did not alter the EMG activity or the M-waves measured from either of the legs at rest and over the course of fatigue and recovery. The results of this investigation suggest that although β-blocking agents do impair dynamic exercise performance, there is no effect of these agents on peripheral skeletal muscle function during single limb exercise. These observations have been explained in relation to the possible central and hemodynamic effects of β-blockade.|
|Appears in Collections:||Digitized Open Access Dissertations and Theses|
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