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The shortening velocity of skeletal muscle fibres is determined principally by actomyosin cross-bridges. However, these contractile elements are in parallel with elastic elements, whose main structural basis is thought to be the titin filaments. If titin is stretched, it may contribute to sarcomere shortening simply because it can recoil 'passively'. The titin-based contribution to shortening velocity (Vp) was quantified in single rabbit psoas myofibrils. Non-activated specimens were rapidly released from different initial sarcomere lengths (SLs) by various step amplitudes sufficient to buckle the myofibrils; Vp was calculated from the release amplitude and the time to slack reuptake. Vp increased progressively (upper limit of detection, ~60 µm s-1 sarcomere-1) between 2.0 and 3.0 µm SL, albeit more steeply than passive tension. At very low passive tension levels already (< 1-2 mN mm-2), Vp could greatly exceed the unloaded shortening velocity measured in fully Ca2+-activated skinned rabbit psoas fibres. Degradation of titin in relaxed myofibrils by low doses of trypsin (5 min) drastically decreased Vp. In intact myofibrils, average Vp was faster, the smaller the release step applied. Also, Vp was much higher at 30 °C than at 15 °C (Q10: 2.0, 3.04 or 6.15, for release steps of 150, 250 or 450 nm sarcomere-1, respectively). Viscous forces opposing the shortening are likely to be involved in determining these effects. The results support the idea that the contractile system imposes a braking force onto the passive recoil of elastic structures. However, elastic recoil may aid active shortening during phases of high elastic energy utilization, i.e. immediately after the onset of contraction under low or zero load or during prolonged shortening from greater physiological SLs.
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