Dynamic response characteristics of hyperaemia in the human calf muscle : effect of exercise intensity and relation to electromyographic activity

To clarify the structure of the muscle hyperaemic response during submaximal exercise in the supine position, we tested the hypotheses that this response measured in human calf muscle is biphasic or triphasic (growth only) at low-moderate or high forces, respectively. Ten subjects performed four series of 5-min bout of intermittent contractions from a resting baseline to 30, 60 and 90% of peak force, as well as from an exercise baseline to higher forces. For each exercise transition, leg blood flow (LBF: plethysmography) and leg vascular conductance (LVC) were measured between contractions and averaged across four trials. Six 'growth-only' and 'growth and decay' models were fitted to these averaged responses and significant differences between their goodness-of-fit were tested statistically. For rest-exercise transitions, triphasic or quadphasic 'growth and decay' models provided the best fit to the majority of LBF and LVC responses. The intensity dependent growth in hyperaemia was due mainly to a significant increase in amplitude of the rapid growth phase. A fast decay in LBF and LVC occurred at all intensities (mean t = 4-5 s, mean TD = 9-14 s). A slower decay appeared at the lowest intensity (mean t = 18-28 s, TD = 90 s) that coincided with a monoexponential decline in EMG activity (mean t = 23, TD = 87 s). Thus, although biphasic growth is an essential feature of muscle hyperaemia, rapid and slow decay phases also exist that highlight additional mechanisms which contribute to this dynamic response during exercise.