The training stimulus responsible for blood pressure reductions after isometric training remains equivocal. It has previously been suggested that increased exposure to shear stress during isometric exercise may mediate endothelial adaptations, similar to that seen in dynamic exercise training. This may lead to a reduced peripheral resistance and thus a lower resting blood pressure (McGowan et al, 2007). However the role of shear stress in isometric training induced blood pressure reductions remains uninvestigated. The purpose of this study was to train participants at two intensities that either elicited a high or low shear stress stimulus to determine the role of conduit femoral artery shear stress in blood pressure reductions after an isometric leg training intervention.
35 male normotensive participants (age = 23.79 ± 6.30 yrs , height = 180 ± 5.71 cm, mass = 75.70 ± 11.11 kg ) were allocated to one of three groups: high shear stress (HI), low shear stress (LO) or control (CON). The HI and LO groups undertook an 8 week training programme of 4 x 2minute bilateral-leg isometric contractions, 3 times per week. Mean shear rate (MSR), peak shear rate (PSR) and change in shear rate (△SR) were used as an estimation of shear stress, and was measured using doppler ultrasound during training intervention in the common femoral artery. Resting systolic (SBP), diastolic (DBP) and mean (MAP) blood pressure parameters were measured at baseline, mid-point (4weeks) and post-training (8 weeks).
Repeated measure ANOVA demonstrated that the changes in SBP after training were not significant in any group. There were significant differences in DBP for the LO group pre-mid, mid-post and pre-post (P<0.05). Significant differences in MAP were observed in the LO group pre-mid and in the HI group mid-post (P< 0.05). However correlation coefficient analysis revealed no significant correlations between these sub-group blood pressure reductions and MSR, PSR, and △SR (P>0.05).
These results suggest that training according to different levels of shear stress in the common femoral artery does not provide a strong training stimulus for reductions in resting blood pressure after isometric leg exercise intervention. Although shear stress is accepted as an important training stimulus in dynamic exercise, these findings indicate that shear stress might not be the main stimulus for reductions in resting blood pressure after isometric training.