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SKELETAL MUSCLE AND EXERCISE |
1 Laboratory for Biomechanics, ETH Zürich, 8093 Zürich, Switzerland
2 Institute of Vegetative Physiology, University of Cologne, 50931 Cologne, Germany
3 Department of Physiology, University of Bristol, Bristol BS8 1TD, UK
We examined length changes of individual half-sarcomeres during and after stretch in actively contracting, single rabbit psoas myofibrils containing 10–30 sarcomeres. The myofibrils were fluorescently immunostained so that both Z-lines and M-bands of sarcomeres could be monitored by video microscopy simultaneously with the force measurement. Half-sarcomere lengths were determined by processing of video images and tracking the fluorescent Z-line and M-band signals. Upon Ca2+ activation, during the rise in force, active half-sarcomeres predominantly shorten but to different extents so that an active myofibril consists of half-sarcomeres of different lengths and thus asymmetric sarcomeres, i.e. shifted A-bands, indicating different amounts of filament overlap in the two halves. When force reached a plateau, the myofibril was stretched by 15–20% resting length (L0) at a velocity of 
0.2 L0 s–1. The myofibril force response to a ramp stretch is similar to that reported from muscle fibres. Despite the 2.5-fold increase in force due to the stretch, the variability in half-sarcomere length remained almost constant during the stretch and A-band shifts did not progress further, independent of whether half-sarcomeres shortened or lengthened during the initial Ca2+ activation. Moreover, albeit half-sarcomeres lengthened to different extents during a stretch, rapid elongation of individual sarcomeres beyond filament overlap (‘popping’) was not observed. Thus, in contrast to predictions of the ‘popping sarcomere’ hypothesis, a stretch rather stabilizes the uniformity of half-sarcomere lengths and sarcomere symmetry. In general, the half-sarcomere length changes (dynamics) before and after stretch were slow and the dynamics after stretch were not readily predictable on the basis of the steady-state force–sarcomere length relation.
(Received 23 January 2006;
accepted after revision 7 March 2006;
first published online 9 March 2006)
Corresponding author J. Denoth: Laboratory for Biomechanics, ETH Zürich, ETH Hönggerberg, HCI E 357.1, CH-8093 Zürich, Switzerland. Email: jdenoth{at}ethz.ch
Re-use of this article is permitted in accordance with the Creative Commons Deed, Atribution 2.5, which does not permit commercial exploitation.
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