BELARDINELLI, R., T. J. BARSTOW, J. PORSZASZ, and K. WASSERMAN. Skeletal muscle oxygenation during constant work rate exercise.Med. Sci. Sports Exerc., Vol. 27, No. 4, pp. 512–519, 1995. We compared the slow rise in &OV0312;O2during heavy exercise (i.e., greater than lactic acidosis threshold (LAT)) with changes in muscle oxyhe-moglobin + oxymyoglobin (O2Hb/O2Mb) saturation by reflectance near infrared spectroscopy. Ten subjects performed four 6-min cycle ergometer tests with two constant work rates less than and two greater than the LAT, equivalent to 20,40,65 and 75% peak &OV0312;O2. During less than LAT exercise, O2Hb/O2Mb saturation decreased to a minimum by 2 min and then remained constant or rose slightly. For greater than LAT work rates, the initial fall in O2Hb/O2Mb saturation was greater the higher the work rate and continued to decrease with time after 3 min. Between minutes 3 and 6, the rate of decrease in O2Hb/O2Mb saturation correlated with the increase in &OV0312;O2(r = −0.69,P< 0.0001). These studies support the hypothesis that the slow rise in &OV0312;O2during heavy constant work rate exercise is associated with a progressive decline in O2Hb/O2Mb saturation in the contracting muscles themselves that may be facilitated by capillary oxyhemoglobin dissociation owing to tissue lactic acidosis (Bohr effect).