Effects of shortening velocity and of oxygen consumption on efficiency of contraction in dog gastrocnemius

Abstract

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, V̇O₂) was converted into mechanical power (Ẇ). 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 V̇O₂ and Ẇ (W kg⁻¹). The relationship between these variables was described by: v=2.85(V̇O₂−Ẇ)^1.24 (R ²=0.85; n=17). A mathematical analysis of this equation shows that the overall mechanical efficiency (η=ẆV̇O₂ ⁻¹) decreased with increasing v (at constant V̇O₂), whereas it increased with increasing V̇O₂ (at constant v). The net effect of this state of affairs was that the decrease of η 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 V̇O₂ (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 η. This is counteracted by the increased V̇O₂, which in turn leads to an increased efficiency at the expense of a fall in shortening speed.