This study investigated heterogeneity of muscle O2 consumption (diffusive m-VO2) and muscle oxygenation difference (m-O2 difference) within the forearm flexor muscles using multi-optical fibers near-infrared continuous wave spectroscopy (NIRcws) during incremental exercise. Nine healthy male subjects performed incremental dynamic handgrip exercise until exhaustion. The workload was increased by 5% maximal voluntary contraction (MVC) every 1 min, starting at 10% MVC. The NIRcws probes (10 channels) were placed on the right forearm flexor muscles to monitor muscle oxygenation. The diffusive m-VO2 and the m-O2 difference were evaluated at each exercise stage. The diffusive m-VO2 at the medial site was significantly greater than the lateral site at 25% MVC (p < 0.05). Similarly, m-O2 difference at the medial site increased significantly over the lateral site (p < 0.05), whereas there were no significant differences in diffusive m-VO2 or m-O2 difference between the proximal and distal sites. These results in forearm muscle were different from the previous study which found that there were longitudinal differences in muscle VO2 in the femoral muscle. © Springer Science+Business Media, LLC 2010.