Fig. 2. Records of the partial pressure (P_O2) of oxygen in the chamber during the course of one series of measurements in two different muscles (top and bottom traces). The initial decline in P_O2 as a result of resting metabolism in the muscle is followed by a more rapid decline when the muscle is stimulated to do work, which is then followed by a return to the resting rate when the muscle ceases working. The vertical separation between regressions fit to the initial and final resting rates, evaluated about half way between the onset and termination of work (line with arrowheads), is used to measure the change in P_O2 that occurred while the muscle was working.