Priming of human skeletal muscle microcirculation is intensity-dependent and sex-specific

The priming effect, whereby prior exercise alters responses to subsequent exercise, has been characterized mainly using systemic measures (e.g., pulmonary oxygen uptake kinetics), whereas muscle microvascular mechanisms and sex-specific responses remain unclear. We used diffuse correlation spectroscopy combined with near-infrared spectroscopy (DCS-NIRS) to quantify relative microvascular blood flow index (rBFI) and tissue oxygenation (StO₂) and to derive relative oxygen extraction fraction (rOEF) and relative muscle oxygen consumption (rMRO₂) during rhythmic handgrip exercise in the flexor digitorum superficialis. Twenty-seven healthy young adults (13 males, 14 females) performed two 5-min bouts (Ex1, Ex2) at 10% and 30% maximal voluntary contraction (MVC); females also completed 50% MVC on a separate day. In the overall cohort, between-bout differences were modest at 10% MVC but clearer at 30% MVC, with Ex2 showing higher rBFI and rMRO₂ and lower StO₂ with higher rOEF during the pre-exercise and early-to-mid exercise phases (p < 0.05). In males, priming was evident at 30% MVC: Ex2 showed higher rBFI and rMRO₂ and lower StO₂ with higher rOEF during exercise (p < 0.05). In females, differences at 30% MVC were limited: StO₂ and rOEF did not differ during exercise and rMRO₂ differed only at an isolated early time point. At 50% MVC in females, Ex2 showed higher rBFI across exercise and higher rMRO₂ during the pre-exercise and early exercise phases, with discrete mid-exercise differences (p < 0.05), whereas StO₂ and rOEF did not differ during exercise. These findings indicate that the expression of priming-related microvascular responses is intensity-dependent and sex-specific.