In order to identify peripheral biomarkers of impaired oxidative metabolism during exercise following a 10-day bed rest, 10 males performed an incremental exercise (to determine peak pulmonary V˙O 2 (V˙O 2 p)) and moderate-intensity exercises, before (PRE) and after (POST) bed rest. Blood flow response was evaluated in the common femoral artery by Eco-Doppler during 1 min of passive leg movements (PLM). The intramuscular matching between O 2 delivery and Lucrezia Zuccarelli completed her BSc and MSc in Sport Science at the University of Milan. During her masters she was a visiting student at the European Space Agency in Cologne. She moved to Udine and completed her PhD in Biomedical Sciences at the University of Udine. Her research activity focuses on the functional evaluation of oxidative metabolism during exercise along the oxygen pathway from the ambient air to skeletal muscle mitochondria in physiological and non-physiological conditions. O 2 utilization was evaluated by near-infrared spectroscopy (NIRS). Mitochondrial respiration was evaluated ex vivo by high-resolution respirometry in isolated muscle fibres, and in vivo by NIRS by the evaluation of skeletal muscle V˙O 2 (V˙O 2 m) recovery kinetics. Resting V˙O 2 m was estimated by NIRS. Peak V˙O 2 p was lower in POST vs. PRE. The area under the blood flow vs. time curve during PLM was smaller (P = 0.03) in POST (274 ± 233 mL) vs. PRE (427 ± 291). An increased (P = 0.03) overshoot of muscle deoxygenation during a metabolic transition was identified in POST. Skeletal muscle citrate synthase activity was not different (P = 0.11) in POST (131 ± 16 nmol min-1 mg-1) vs. PRE (138 ± 19). Maximal ADP-stimulated mitochondrial respiration (66 ± 18 pmol s-1 mg-1 (POST) vs. 72 ± 14 (PRE), P = 0.41) was not affected by bed rest. Apparent K m for ADP sensitivity of mitochondrial respiration was reduced in POST vs. PRE (P = 0.04). The V˙O 2 m recovery time constant was not different (P = 0.79) in POST (22 ± 6 s) vs. PRE (22 ± 6). Resting V˙O 2 m was reduced by 25% in POST vs. PRE (P = 0.006). Microvascular-endothelial function was impaired following a 10-day bed rest, whereas mitochondrial mass and function (both in vivo and ex vivo) were unaffected or slightly enhanced. Abstract figure legend Potential sites of impairment of oxidative metabolism during exercise following microgravity-inactivity. Key points r Ten days of horizontal bed rest impaired in vivo oxidative function during exercise. r Microvascular impairments were identified by different methods. r Mitochondrial mass and mitochondrial function (evaluated both in vivo and ex vivo) were unchanged or even improved (i.e. enhanced mitochondrial sensitivity to submaximal [ADP]). r Resting muscle oxygen uptake was significantly lower following bed rest, suggesting that muscle catabolic processes induced by bed rest/inactivity are less energy-consuming than anabolic ones.