This study explored the accuracy with which venous occlusion plethysmography (VOP) assesses the hyperaemic response during calf exercise. Using Doppler ultrasound (DU) as a criterion standard technique, we tested the hypotheses that leg blood flow during contraction is not greater than at rest and that VOP provides similar estimates of the hyperaemic response between contractions as DU. Eleven subjects performed several bouts of calf exercise across a wide range of forces (50-400 N ≅ 6-45%MVC). Each bout consisted of 2 min of intermittent contractions preceded and immediately followed by sustained (40 s) contractions. DU estimates of leg blood flow during the sustained contractions were never significantly greater (P > 0.05) than those measured at rest. Paired (DU and VOP) estimates of leg blood flow (n = 488) were obtained between intermittent contractions and ranged between ~50-900 ml min(-1). There was a strong correlation between these DU and VOP estimates (Pearson r = 0.91; P < 0.05). Ordinary least products regression analysis, with VOP as the y variable, showed a relatively small proportional bias (slope = 0.942; CI = 0.938-0.946) and fixed bias (y intercept = -13.3 ml min(-1); CI = -14.4 to -12.2 ml min(-1)) between the two measurement techniques. Since these small biases can be explained by the slight differences in vascular regions which the two techniques assess, these data suggest that VOP can accurately assess the hyperaemic response to exercise.
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 τ = 4-5 s, mean TD = 9-14 s). A slower decay appeared at the lowest intensity (mean τ = 18-28 s, TD ≅ 90 s) that coincided with a monoexponential decline in EMG activity (mean τ = 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.
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