Shortening during stimulation vs. during relaxation: How do the costs compare?

White muscle fibres from dogfish were used to investigate the energetic cost of shortening by fully active muscle and by relaxing muscle. The muscle preparation was tetanized for 0.6 s and shortened by 1 mm (about 15% L0) at 7 mm/s (about 30% V0) either during stimulation or during relaxation. Isometric tetani at L0 were also investigated. Mechanical work was calculated from force and length change. Work + heat was taken as a measure of energetic cost. Both work and energetic cost were higher for shortening during stimulation than during relaxation. We also evaluated separately the work and heat associated with the contractile component and with the series elastic component. Work stored in the series elasticity could be completely recovered as external work when the shortening occurred during relaxation.