Nine healthy men, aged between 25 and 35 years, performed sustained maximal voluntary contractions (MVC) of foot plantar, foot dorsal, and finger flexor muscles. Contractions lasted 10 min and were followed by short test contractions at 30% MVC during recovery. Two positions of the working extremity high or low were established by different body postures (supine or sitting). Under these conditions, studies of force, integrated electromyogram (iEMG), blood pressure, and heart rate showed firstly that force decreased throughout the first few minutes of maximal contraction but reached a near steady-state value after 5 to 6 min. Secondly, force decay and steady-state level depended on muscle group and body position. When sitting (low leg), muscles with a high incidence of slow twitch fibres (plantar flexors) showed a slower force decay and a higher relative steady-state force than fast dorsal flexor muscles. When supine (high leg), plantar and dorsal flexor muscles reached about the same low level of relative steady-state force. Changes in iEMG, blood pressure, and heart rate did not differ in the two positions. Thirdly, during recovery, plantar flexor muscles showed higher iEMG values as well as higher values of blood pressure and heart rate when supine than when sitting. Recovery of dorsal flexor muscles was little affected by body posture. Fourthly, force development and recovery of predominantly fast finger flexor muscles were almost independent of arm position. It was concluded that muscle fibre composition was the main factor in determining endurance capacity. However, endurance was influenced by changes in the hydrostatic blood pressure component.(ABSTRACT TRUNCATED AT 250 WORDS)
1. Na-octanoate (in Ringer's solution) affects the membrane resistance of isolated frog skeletal muscle fibres in a biphasic way. Initially there is an increase followed by a more slowly developing decrease. The effect depends on both the concentration of the fatty acid (1-20 mM) and the time of exposure (2-20 min). 2. In muscles exposed either to isotonic K2SO4 or to Ringer's solution with Cl- substituted by methylsulphage, the octanoate-induced resistance drop is the same as in normal Ringer's solution whereas an initial increase is not observed. 3. In Na-free (Tris-) Ringer's solution, only an increase in membrane resistance is caused by octanoate. 4. The results suggest that Na-octanoate decreases the conductance of the resting muscle cell membrane for anions (Cl-1) whereas the permeability for cations (Na+, K+) is increased.
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