RESUMO
Previous observations have shown that, in isolated perfused dog gastrocnemii in situ, stimulated to aerobic rhythmic isotonic tetanic contractions (at about 40% of maximal isometric force), only about 20% of the overall metabolic power (proportional to the rate of O2 consumption, VO2) was converted into mechanical power (W). Here we report that, in the same preparation, the maximal velocity during the shortening phase of each tetanus (v, mm s(-1)) increased with the rate of energy dissipation, as given by the difference between VO2 and W (W kg(-1)). The relationship between these variables was described by: v=2.85(VO2-W)(1.24) (R2=0.85; n=17). A mathematical analysis of this equation shows that the overall mechanical efficiency (eta=WVO2(-1)) decreased with increasing v (at constant VO2), whereas it increased with increasing VO2 (at constant v). The net effect of this state of affairs was that the decrease of eta over the entire range of work intensities was relatively minor (from 0.22 to 0.15), in spite of a large increase of v, (from 40 to 120 mm s(-1)), thanks to the concomitant increase of VO2 (from 10 to 25 W kg(-1)). So, under these experimental conditions, the energetics of work performance seems to be governed by two conflicting needs. The need for a sufficiently high shortening speed (and hence power output), itself requiring a sufficiently large energy dissipation rate, which, however, brings about a fall in eta. This is counteracted by the increased VO2, which in turn leads to an increased efficiency at the expense of a fall in shortening speed.
