Peak torque (PT), contraction work (CW), mean power (CP), and time for contraction (t) were recorded for each of 200 full range-of-motion repetitive isokinetic (maximum effort) plantar flexions at the preset angular velocity 60 degrees s-1 (degrees X s-1). The subjects were 12 healthy males (42-46 years). Simultaneous recordings of electromyographic activity of the three parts of the triceps surae muscle were made. The electromyograms were truly integrated and the mean power frequency was also calculated for each of the contractions. The mechanical variables decreased steeply during approximately the first 70 contractions (the fatigue phase) followed by a steady-state level (the endurance level) with no further significant decrease. The fatigue phase was composed of two parts. The first was characterized by decreases in CW, MPF and in the ratio CW/iEMG, while iEMG and iEMG/t were unaltered. During the second part decreases in iEMG and iEMG/t occurred while MPF and CW/iEMG had stabilized. During the endurance level all the variables were stable but lowered when compared with the initial contractions of the fatigue phase. The results indicated that the mechanical fatigue went through two parts before the steady-state level was established.
The soleus, a slow-contracting, and the extensor digitorum longus (EDL), a fast-contracting skeletal muscle from guinea-pig were prepared for isometric recording of sub-tetanic contractions in vitro. The contents of adenosine-triphosphate (ATP) and creatinephosphate (CP) together with their metabolites and the contents of lactate, pyruvate and cyclic adenosine-monophosphate (c-AMP) in the muscles were determined. It was found that the energy and redox state of the isolated soleus and EDL muscles is very stable and does not significantly differ from the normal state in vivo. Moreover, there were no consistent changes in these variables after treatment with terbutaline (a beta 2-adrenoceptor agonist) or propranolol or both. Thus, effects on energy metabolism do not seem to cause the changes in muscle contraction, characteristic for beta-adrenoceptor stimulation. On the other hand, the functional effects were accompanied by elevation of the c-AMP level of the muscles.
The metabolic changes in blood, red (m. soleus) and white (m. vastus lateralis) skeletal muscle fibres were investigated after short-term (3 min) infusion of adrenaline with or without prior treatment with propranolol or metoprolol. The adrenaline-induced increase in plasma lactate levels was totally prevented by prior treatment with metoprolol or propranolol, whilst the beta-blockers had no effect on blood glucose levels. Similar effects on lactate levels were found in the m. soleus, while metoprolol was less effective than propranolol in m. vastus lateralis. Adrenaline decreased the level of muscle creatinine phosphate and ADP, causing the equilibrium of the creatinine kinase reaction to change in the direction of ATP synthesis, although the level of ATP usually decreased. This effect was more pronounced in m. vastus lateralis compared with m. soleus. The [ATP]/[ADP] [Pi]-ratio tended to increase during infusion of adrenaline. This effect was counteracted by metroprolol but not by propranolol. The effects on the "phosphate potential" ([ATP]/[ADP] [Pi]) and the equilibrium within the creatine kinase were more pronounced in m. vastus lateralis than in m. soleus. The results demonstrate the possible role of receptors other than beta-receptors, i.e. alpha-receptors, in mediating changes in plasma glucose levels, while plasma lactate levels are regulated by the beta-adrenergic system. The role of beta-receptors in mediating changes in muscle lactate levels may differ in m. soleus and m. vastus lateralis, with a relative predominance of beta 2-receptors in m. vastus lateralis. Quantitative and qualitative differences in the adrenergic control of the energy state in the two types of muscle fibre were obvious, although it was not possible to distinguish clearly between the relative importance of alpha, beta 1 and beta 2-receptors.
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