Reduced exercise hyperaemia in calf muscles working at high contraction frequencies

Summary

The effects of muscle contraction frequency on blood flow to the calf muscle (Q _calf) were studied in six female subjects, who performed dynamic plantar flexions at frequencies of 20, 40, 60, 80 and 100 contractions · min⁻¹, in a supine position. TheQ _calf measured by a mercury-in-rubber strain gauge plethysmograph, increased as contraction frequency increased and reached a peak at 60–80 contractions · min⁻¹. After 100 plantar flexions at 60 contractions · min⁻¹, the meanQ _calf was 30.95 (SEM 4.52) ml · 100 ml⁻¹ · min⁻¹. At 100 contractions · min⁻¹, however, it decreased significantly compared with that at 60 contractions · min⁻¹ at a specified time (2 min or exhaustion) or after a fixed amount of work (100 contractions). The contraction frequency at whichQ _calf reached a peak depended on the duration of exercise. The heart rate showed its highest mean value at 60 contractions · min⁻¹ and decreased significantly at 100 contractions · min⁻¹. The mean blood pressure was lower at 100 contractions · min⁻¹ than at 60 contractions · min⁻¹. The relaxation period between contractions, measured by recording the electromyogram from the gastrocnemius muscles, shortened markedly as the frequency increased; the mean value at 100 contractions · min⁻¹ was 0.14 (SEM 0.02) s, which corresponded to 35.7% of the contraction time. This shortened relaxation period between contractions should have led to the inhibition of exercise hyperaemia at the higher contraction frequencies.