Brief periods of inactivity reduce leg microvascular, but not macrovascular, function in healthy young men. 
What is the central question of this study?  We aimed to examine leg vascular responses to brief periods of inactivity.  What is the main finding and its importance?  We demonstrate that a mere 10 min of sitting is sufficient to impair leg microvascular function (reactive hyperaemia).  However, conduit artery vasodilatation (flow-mediated dilatation) was unaffected, indicating maintained macrovascular function.  Interestingly, immobile supine rest also resulted in a reduction in microvascular function alone that was prevented when calf muscle contractions were performed.  Collectively, these data highlight the susceptibility of the microcirculation to short periods of inactivity and the beneficial role of skeletal muscle contraction for vascular health.  Prolonged sitting for 1-6 h has been shown to impair leg macrovascular [i.e. reduced flow-mediated dilatation (FMD)] and microvascular (i.e. reduced reactive hyperaemia) function.  These impairments appear to be mediated through reductions in shear stress.  Interestingly, a reduction in shear rate has been observed as early as 10 min into sitting.  However, it is unknown whether this acute reduction in shear stress is sufficient to affect vascular function.  Accordingly, we studied 18 young men and assessed popliteal artery FMD and reactive hyperaemia before (Baseline) and after (PostSit) a 10 min sitting period.  Popliteal artery shear rate was significantly reduced during sitting (Baseline, 62 ± 35 s-1 ; 10 min sitting, 27 ± 13 s-1 ; P < 0.001).  Macrovascular function was unaffected by 10 min of sitting (Baseline, 4.4 ± 2.1%; PostSit, 4.3 ± 2.3%; P = 0.97), but microvascular function was reduced (Baseline, 4852 ± 2261 a.u.;  PostSit, 3522 ± 1872 a.u.;  P = 0.02).  In a subset of individuals, we extended the recovery period after sitting and demonstrated that resting shear rate and reactive hyperaemia responses remained low up to 1 h post-sitting (P < 0.001), whereas FMD was unchanged throughout (P = 0.99).  Additionally, time control experiments were performed with participants in an immobile supine position, which demonstrated no change in macrovascular function (P = 0.94) but, unexpectedly, a reduction in microvascular function (P = 0.008).  Importantly, when calf muscle contractions were performed during supine rest, reactive hyperaemia responses were maintained (P = 0.76), along with FMD (P = 0.88).  These findings suggest that the leg microcirculation might be more vulnerable to short periods of inactivity, whereas conduit artery vasodilatation appears well maintained.  Moreover, intermittent skeletal muscle contractions are beneficial for microvascular function.