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 4.3_CO2 after the peak in the CO₂ trace. The portion of the O₂ trace used to determine O₂ consumption was therefore terminated at 4.3_O2 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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