Single location muscle monitoring does not reflect the heterogeneous activation of the muscle group(s) during a given exercise. Vastus lateralis and rectus femoris O2 consumption (VO2) was investigated, non-invasively, at rest and during maximal voluntary contractions (MVC) using a 12-channel near-infrared continuous wave spectroscopy (NIRcws) system (0.1 s acquisition time). VO2 either at rest or during MVC was found to be nonuniform in the 11 out of 12 measurement sites over a surface of 8 × 8 cm2. As expected, VO2 during exercise was significantly higher than VO2 at rest (P < 0.01). However, at each muscle measurement site no difference was found between the mean values (n = 12) of VO2 measured during a 5-s intermittent MVC and the VO2 values measured during 30-s continuous MVC (P = 0.25). These results strengthen the role of NIRcws as a powerful tool for investigating the spatial and temporal features of muscle oxygenation changes as well as muscle VO2. © 2001 Academic Press.