Skeletal muscle oxygenation during constant work rate exercise.

Abstract

We compared the slow rise in VO2 during heavy exercise (i.e., greater than lactic acidosis threshold (LAT)) with changes in muscle oxyhemoglobin+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 VO2. 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 VO2 (r = -0.69, P < 0.0001). These studies support the hypothesis that the slow rise in VO2 during 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).