BACKGROUND Delivery of oxygen to the mitochondria is essential for oxidative phosphorylation and the production of ATP during rest and during muscle contraction. Despite widespread application of near infrared spectroscopy (NIRS), to our knowledge it has not been used to evaluate whether oxygen delivery to the mitochondria, as assessed by post-ischemic reoxygenation parameters, is related to the oxidative capacity of the muscle. PURPOSE To determine the relation between microvascular reactivity and mitochondrial oxidative capacity in the knee joint musculature using NIRS. METHODS Bilateral measurements of the vastus lateralis (VL) and medial hamstring (MH) muscles were taken in 14 moderate to highly active healthy adults (7M/7F, age: 22 ± 3 yrs, BMI: 21.3 ± 3.5 kg/m2, physical activity levels: 4190 ± 1324 MET minutes/week). Two NIRS probes (Portamon, Artinis) were secured over the respective muscle bellies, which collected data during a 30 second bout of surface electrical stimulation that caused an increase in metabolism followed by brief arterial occlusions to determine the metabolic rate of the muscle over time. A final resting metabolism measurement was taken 5 minutes after the arterial occlusions to obtain the time to half recovery of metabolic rate (NIRS k), which is a valid estimate of mitochondrial oxidative capacity. Microvascular reactivity was measured as 1) the 10-second reoxygenation slope of the oxygen saturation signal (StO2) and 2) the half time to peak oxygenation following 5 minutes of arterial occlusion in each muscle. RESULTS There was a significant positive association between NIRS kand the reoxygenation slope in both the VL (r = 0.64, p = 0.025) and the MH (r = 0.53, p = 0.049) muscles. NIRS kvalues were also positively associated with the time to half peak oxygenation in the VL (r = 0.65, p = 0.021) and the MH (r = 0.60, p = 0.026) muscles. There were no differences between the VL and MH muscles for NIRS kvalues (Mean ± SD: 1.78 ± 0.24 vs. 1.78 ± 0.33, p = 0.99), 10-second reoxygenation slope (2.16 ± 0.84 vs. 1.99 ± 0.96, p = 0.65), and half time to peak oxygenation (8.4s ± 1.6 vs. 9.1s ± 2.2, p = 0.38). CONCLUSION Our results suggest that microvascular reactivity is positively associated with the mitochondrial capacity of the knee joint musculature in this sample of healthy adults. Whether these associations exist in different clinical populations warrants future investigation.