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<or=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.