The impact of handgrip exercise duty cycle on brachial artery flow-mediated dilation

Endothelial function is essential for vasoprotection and regulation of vascular tone. Using handgrip exercise (HGEX) to increase blood flow-associated shear stress is an increasingly popular method for assessing brachial artery endothelial function via flow-mediated dilation (FMD). However, different exercise duty cycles [ratio of handgrip relaxation: contraction (seconds)] produce different patterns of brachial artery shear stress with distinct antegrade/retrograde magnitudes. To determine the impact of HGEX duty cycle on brachial artery %FMD, three distinct duty cycles were employed while maintaining a uniform mean shear stress. Brachial artery diameter and mean blood velocity were assessed via echo and Doppler ultrasound in 16 healthy male subjects. Shear stress was estimated as shear rate (SR = blood velocity/brachial artery diameter) and the target mean SR during HGEX was 75 s(-1). Subjects performed three 6-min HGEX trials on each of 2 days (like trials averaged). In each trial, subjects performed one of the three randomly ordered HGEX duty cycles (1:1, 3:1, 5:1). %FMD was calculated from baseline to the end of HGEX and (subset N = 10) during each minute of HGEX. Data are mean +/- A SD. As intended, mean SR was uniform across duty cycles (6 min HGEX average: 72.9 +/- A 4.9s(-1), 72.6 +/- A 3.6s(-1), 72.8 +/- A 3.5 s(-1), p = 0.835), despite differences in antegrade/retrograde SR (p < 0.001). End-exercise %FMD (4.0 +/- A 1.3 %, 4.1 +/- A 2.2 %, 4.2 +/- A 1.4 %, p = 0.860) and %FMD during exercise (p = 0.939) were not different between duty cycles. These data indicate that the endothelium responds to the mean shear stress and is not specifically sensitive to the contraction/relaxation or retrograde shear stress created by a range of HGEX protocols.