Intermittent vs. Continuous Handgrip Exercise and Peripheral Endothelial Function: Impact of Shear Rate Fluctuations.

Abstract

Sustained exercise-induced elevations in shear rate (SR) has been well established as beneficial for improving endothelial function. However, the impact of intermittent fluctuations in SR is not understood. We investigated the effect of intermittent SR elevations compared to sustained elevations on peripheral endothelial function. Brachial artery flow-mediated dilation (FMD) was assessed in thirteen adults (9M/4F; 22±4 yr) before and after 30 min of handgrip exercise. Three different rhythmic forearm exercise interventions were performed at a rate of 20 contractions/min. Intermittent exercises (6 x 3 min exercise interspersed by 2 min of rest) were performed at 25% (INT-25%) and 15% (INT-15%) maximum voluntary contraction (MVC), and continuous exercise was completed at 15% MVC. Brachial artery diameter and velocity were measured using Doppler ultrasound. The total increase in SR above baseline throughout exercise was greater during INT-25% (4441±516·s-1) and continuous (4070±407 s-1) compared with INT-15% (2811±342 s-1, P<0.05). %FMD increased following all exercises (INT-25%: 5.7±1.2% to 8.1±1.2%; INT-15%: 5.2±1.2% to 7.0±1.1%; continuous: 5.5±1.3% to 6.8±1.3%, P<0.05 for all). The increase following INT-25% was significantly greater than INT-15% and continuous (P<0.05 for both). Normalized FMD to shear rate area under the curve increased with intermittent exercise (INT-25%: 2.2±0.2% to 3.4±0.3%; INT-15%: 2.1±0.2% to 3.2±0.2%, P<0.05 for both) but did not following continuous (2.1±0.2% to 2.5±0.1%, P=0.06). The increase in normalized FMD with intermittent exercises were greater than continuous (P<0.05 for both). These findings suggest intermittent fluctuations in SR during handgrip exercise may be more beneficial than sustained elevations on improving peripheral endothelial function.