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Fig. 2. CO2 production and O2 consumption associated with mechanical work. (A) Increase in the CO2 concentration of the air exiting the experimental chamber following 10 s of muscle stimulation and work output. The power output during the stimulation is shown in C. (B) The corresponding decrease in O2 concentration of the air leaving the experimental chamber. The output signal from the O2 analyzer was occasionally rebalanced between trials to compensate for drift. The background level for the O2 trace is therefore arbitrary. The delay between the increase in CO2 concentration and the decrease in O2 concentration is a consequence of the O2 analyzer being downstream from the CO2 analyzer. The dotted lines in A and B connect the means of the first 50 samples collected in each trace with the means of the last 50 samples. These lines are taken as baselines for estimating the increase in CO2 production or O2 consumption during the period represented by the trace. The extra CO2 produced as a result of stimulation was measured as the area between the CO2 trace and the calculated baseline (shaded portion of A). (D) The increase in CO2 emission above the calculated baseline and the corresponding decrease in O2 concentration on semi-logarithmic plots. The graph of O2 concentration has been displaced vertically from that for CO2 to avoid overlap of the O2 and CO2 traces. The traces are thickened where they are approximately linear in the semi-logarithmic presentation and, therefore, where the relaxation of the evoked gas production or consumption is approximately exponential. The time constants for the exponential portions of the traces were determined from the slopes of least-squares regression lines (dotted lines) fitted to the thickened portions of the traces. The time constant for the CO2 trace ( CO2) was 140 ms, that for the O2 (O2) 41 ms. To make the measurements of CO2 produced and O2 consumed as comparable as possible, the oxygen consumption was measured to the same end point relative to its signal decay rate as was the CO2 production. Here, the end of the recording, which is also the end of the interval used in determining CO2 production, came 603 s or 4.3CO2 after the peak in the CO2 trace. The portion of the O2 trace used to determine O2 consumption was therefore terminated at 4.3O2 s (marked by an arrow in B) beyond the peak of the O2 trace. In the example shown, the work output was 53.1 mJ, the extra CO2 production was 13.4 µl and the extra O2 consumption, corrected for the change in volume flow through the O2 analyzer caused by CO2 absorption, was 15.8 µl. Thus, the respiratory coefficient (RQ) was 0.85 and the efficiency, assuming an energy to oxygen ratio of 20.1 J ml–1, was 16.7 %.
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