Muscle work enhancement by stretch - Passive visco-elasticity or cross-bridges?

Tetanized frog muscle fibers subjected to ramp stretches on the plateau of the tension-length relation, followed by an isotonic release against a load equal to the maximum isometric tension (T-o), exhibit a well defined transient shortening against T-o which was attributed to the release of mechanical energy stored during stretching within the damped element of the cross-bridges(1,2). However, this interpretation has recently been challenged(3), and 'transient shortening against T-o' has instead been attributed to elastic elements strained because of non-uniform distribution of lengthening within the fibre volume. The 'excess length change', resulting from the recoil of these elastic elements, was found i)to increase continuously with stretch amplitude up to 50 nm per h.s. with a 100 nm per h.s. strain, ii) to decrease steadily with the decrease in force during stress relaxation after the ramp stretch, and iii) to increase on the descending limb of the tension-length relation where sarcomere inhomogeneity is greater. In contrast, the transient shortening against T-o: i) reaches a plateau at 8 nm per half sarcomere after about 50 nm per half sarcomere strain, ii) remains constant during the temperature dependent, fast phase of stress relaxation(4), when the excess in force above isometric reduces to about one half, iii) also occurs on the ascending limb of the tension-length relation where sarcomere inhomogeneity is drastically reduced(5). As a consequence of these differences we conclude that transient shortening(2) and 'excess length change'(3) do not "reflect the same underlying process"(3)