Muscle Work Enhancement by Stretch
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Tetanized frog muscle fibres 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 (To), 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-bridges1,2. However, this interpretation has recently been challenged3, 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 relaxation4, 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 reduced5. As a consequence of these differences we conclude that transient shortening2 and ‘excess length change’3 do not “reflect the same underlying process”3.
KeywordsSarcomere Length Force Enhancement Slack Length Tension Recovery Stretch Amplitude
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