Novel Characterisation of In-Exercise Vascular Shear Rate

Abstract

PURPOSE: Exercise induced changes in vascular shear stress act as the stimulus for improvements in endothelial function. Similar improvements in endothelial function have been found with interval (IT) compared to continuous (CON) exercise. This study aimed to distinguish the differing patterns of shear produced by these protocols and determine whether these patterns differentially influence acute changes in endothelial function. METHODS: 9 healthy (23 ± 3 yrs, 23.9 ± 2.1 kg/m2, 7 male vs. 2 female) participants completed a ramp incremental test on a supine ergometer. Participants then completed 4 different intensity and duration matched protocols consisting of 24 min at work rates (WR) equivalent to 125% lactate threshold (LT): (i) continuous exercise (CON), (ii) 180s of work interspersed with 180s of active recovery (10 W: LONG IT), (iii) 60s of work with 60s active recovery (SHORT IT) and (iv) 180s second work with 180s recovery at WR equivalent to 70% LT (LONG IT 70). During each protocol heart rate and breath by breath gas exchange was measured in addition to Doppler ultrasound assessment of shear rate in the brachial artery. Flow Mediated Dilation (FMD) using ultrasound was used as an index of endothelial function pre and 15 min post each protocol. Data were assessed via two-way repeated measures ANOVA for protocol and time. RESULTS: Anterograde shear increased continuously during CON and SHORT IT but mirrored changes in WR during both LONG protocols. Retrograde shear remained low in CON and oscillated with WR during IT. Peak retrograde shear was greater in IT than in CON (P<0.05) but did not differ between IT protocols. However, total volumes of anterograde and retrograde shear (all protocol mean respectively: 557113 ± 180159 s−1 and 180074 ± 80960 s−1) and the oscillatory shear index (0.26 ± 0.07) were similar across all protocols (P>0.05). FMD increased by an average of 2.45 ± 3.83 % (P<0.05) following exercise with no difference between protocols (P>0.05). CONCLUSION: When exercise sessions are matched for intensity and duration, variable shear rate patterns during IT and CON protocols do not differentially affect acute FMD responses. IT exercises of varying duty cycle length are equally as effective at improving endothelial function as traditional CON exercise.