Influence of an Acute Bout of Sitting on Leg Vascular Function

Abstract

Following an extended sitting challenge of 1-6 hours, endothelial function measured via flow-mediated dilation (FMD) is impaired in the leg. Further, this impairment has been shown to be mediated by the reduction in shear stress that occurs in the legs with prolonged sitting. Interestingly, previous findings have demonstrated that prolonged periods of sitting result in a reduction in shear rate as early as 10 minutes into the sitting period. However, it is unknown whether this acute reduction in shear stress is sufficient to alter conduit artery endothelial function or if the decline in shear stress must be maintained for a longer period. PURPOSE: We tested the hypothesis that 10 minutes of sitting would result in a reduction in popliteal artery shear stress and an impairment in FMD. METHODS: Popliteal artery diameter and blood velocity were continuously recorded via duplex Doppler ultrasound in ten healthy men before, during, and after a 10 minute sitting period. In addition, popliteal artery FMD was performed before and after the acute sitting period. Shear rate was calculated as [8 x mean blood velocity/diameter]. FMD was calculated as [(peak diameter – base diameter)/base diameter x 100] and was ANCOVA-corrected for hyperemic shear rate AUC. RESULTS: Popliteal artery shear rate was significantly reduced with 10 minutes of sitting (PreSit: 57.1±11.2 s-1 vs. PostSit: 37.4±5.1 s-1; P = 0.034); however popliteal artery FMD was unaffected (PreSit: 4.5±0.6% vs. PostSit: 4.6±0.8%; P = 0.738). ANCOVA-corrected FMD yielded similar results (P = 0.715). Interestingly, reactive hyperemia, a measure of microvascular dilator function, tended to be lower following the acute bout of sitting (PreSit: 59,243±9,642 a.u. vs. PostSit: 41,201±6,578 a.u.; P = 0.06). CONCLUSIONS: These preliminary findings demonstrate that shear rate is diminished within the first 10 minutes of sitting, yet this stimulus is not sufficient to affect conduit artery endothelial function. However, microvascular function may be more sensitive to this brief reduction in shear stress, suggestive of a distinction between sitting-induced impairments in macrovascular and microvascular responsiveness. Supported by UTA College of Nursing and Health Innovation.