The aim of this study was to investigate the performance of in vivo quantitative near- infrared spectroscopy (NIRS) in skeletal muscle at various workloads. NIRS was used for the quantitative measurement of O2 consumption (m _VO2) in the human flexor digitorum superficialis muscle at rest and during rhythmic isometric handgrip exercise in a broad range of work intensities (10–90% MVC¼maximum voluntary contraction force). Six subjects were tested on three separate days. No significant differences were found in m _VO2 measured over different days with the exception of the highest workload. The within-subject variability for each workload measured over the three measurements days ranged from 15Æ7to25Æ6% and did not increase at the high workloads. The m _VO2 was 0Æ14 ± 0Æ01 mlO2 min–1 100 g–1 at rest and increased roughly 19 times to 2Æ68 ± 0Æ58 mlO2 min–1 100 g–1 at 72% MVC. These results show that local muscle oxygen consumption at rest as well as during exercise at a broad range of work intensities can be measured reliably by NIRS, applied to a uniform selected subject population. This is of great importance as direct local measurement of m _VO2 during exercise is not possible with the conventional techniques. The method is robust enough to measure over separate days and at various workloads and can therefore contribute to a better understanding of human physiology in both the normal and pathological state of the muscle. Introduction