Mechanical work, oxygen consumption, and efficiency in isolated frog and rat muscle.

The total work done during shortening, in repeated stretch-shortening cycles and the subsequent recovery oxygen consumption were measured in isolated frog (Rana esculenta) sartorius at 12 degrees C and rat (Wistar strain) extensor digitorum longus (EDL) and soleus at 20 degrees C. Two procedures were followed. In the first, the muscles were lengthened in the relaxed state and stimulated isometrically just before and during the first part of shortening. The peak efficiency (positive work done divided by the energetic equivalent of the oxygen consumed) was approximately 25% at 0.75-1.5 muscle lengths/s (Lo/s) in sartorius, 19% at 1.0 Lo/s in EDL, and 15% at 0.5 Lo/s in the soleus. In contrast to the measured efficiency values, the ratio between the tension-time integral and the oxygen consumption (the economy) is greater in soleus than in EDL. In the second procedure, stimulation began before stretching and continued during the first part of shortening. In this case, the efficiency attained values of approximately 35% in sartorius, 50% in EDL, and 40% in soleus. These values are in rough agreement with those measured in vivo during running.