Reduced exercise hyperaemia in claf muscles working at high contraction frequencies.

The effects of muscle contraction frequency on blood flow to the calf muscle (Qcalf) 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. The Qcalf 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 mean Qcalf 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 which Qcalf 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.