Benjamin C. Thompson

Role of retrograde flow in the shear stimulus associated with exercise blood flow

To test the hypothesis that retrograde flow influences the shear stimulus of exercise blood flow, eight healthy men [25·6 ± 3·1 years (SD)] performed 20 min of single‐leg knee‐extension exercise at two contraction velocities: fast (FR, 1·5 m s−1) and slow (SR, 0·4 m s−1). Contraction frequency (30 cpm) and workload (5 kg) were kept constant resulting in a work rate of 15·25 W for both contraction velocities. Common femoral artery diameter and blood velocity were measured at rest and during exercise using ultrasound Doppler. Mean blood flow was not different between contraction velocities while antegrade (2012·4 ± 379·9 versus 1745·6 ± 601·5 ml min−1; P = 0·05) and retrograde (121·7 ± 43·0 versus 11·2 ± 6·6 ml min−1; P<0·001) flows were higher during FR than SR contractions, respectively. Despite the similar mean blood flow response, vascular resistance was lower during FR than SR contractions (0·06 ± 0·01 versus 0·08 ± 0·03 units; P = 0·03) and was closely related to shear rate (pooled data: r = −0·77, P<0·01). Retrograde flow was associated with a lower vascular resistance during exercise (pooled data: r = −0·48, P≤0·05). In addition, calculated oscillatory flow indices were higher during FR than SR contractions and were significantly correlated to retrograde flow, shear rate and vascular resistance. These results indicate that retrograde blood flow influences the shear stimulus of exercise blood flow by enhancing the oscillatory behaviour of flow.

Muscle Strength and Pressor Response

The purpose of this study was to determine if muscle strength influences the hyperemic response to dynamic exercise. Men with low (n=8) and high (n=9) maximal forearm strength performed dynamic handgrip exercise as the same absolute workload increased in a ramp function (0.5 kg x min (-1)). Forearm blood flow (FBF) was measured instantaneously by ultrasound Doppler and blood pressure was measured by auscultation. The pressor response to exercise was greater (P<0.05) for low strength men at workloads >1.5 kg allowing volumetric FBF (ml x min (-1)) and vascular conductance to increase in proportion to absolute workload similar to high strength men. When FBF was expressed relative to forearm volume (ml x min (-1).100 ml (-1)) the hyperemic response to exercise (slope of relative FBF vs. workload) was greater in low strength men (3.2+/-1.5 vs. 1.7+/-0.4 ml x min (-1).100 ml (-1) x kg (-1), P<0.05) as was relative FBF at workloads >1.5 kg. However, when relative FBF was compared across relative work intensity, no difference was found between low and high strength groups. Together, these findings suggest men with low strength require a greater pressor response to match blood flow to exercise intensity as compared to high strength men.