title: Force produced by isolated sarcomeres and half-sarcomeres after an imposed stretch.
abstract: When a stretch is imposed to activated muscles, there is a residual force enhancement that persists after the stretch; the force is higher than that produced during an isometric contraction in the corresponding length. The mechanisms behind the force enhancement remain elusive, and there is disagreement if it represents a sarcomeric property, or if it is associated with length nonuniformities among sarcomeres and half-sarcomeres. The purpose of this study was to investigate the effects of stretch on single sarcomeres and myofibrils with predetermined numbers of sarcomeres (n = 2, 3. . . , 8) isolated from the rabbit psoas muscle. Sarcomeres were attached between two precalibrated microneedles for force measurements, and images of the preparations were projected onto a linear photodiode array for measurements of half-sarcomere length (SL). Fully activated sarcomeres were subjected to a stretch (5-10% of initial SL, at a speed of 0.3 μm·s(-1)·SL(-1)) after which they were maintained isometric for at least 5 s before deactivation. Single sarcomeres showed two patterns: 31 sarcomeres showed a small level of force enhancement after stretch (10.46 ± 0.78%), and 28 sarcomeres did not show force enhancement (-0.54 ± 0.17%). In these preparations, there was not a strong correlation between the force enhancement and half-sarcomere length nonuniformities. When three or more sarcomeres arranged in series were stretched, force enhancement was always observed, and it increased linearly with the degree of half-sarcomere length nonuniformities. The results show that the residual force enhancement has two mechanisms: 1) stretch-induced changes in sarcomeric structure(s); we suggest that titin is responsible for this component, and 2) stretch-induced nonuniformities of half-sarcomere lengths, which significantly increases the level of force enhancement.
authors: Departments of Kinesiology and Physical Education, Physiology, Physics, McGill University, Montreal, Quebec, Canada. dilson.rassier@mcgill.ca
authors: Dilson E
authors: DE
authors: Rassier
authors: Ivan
authors: I
authors: Pavlov
mesh_terms: D000818:Animals; D002469:Cell Separation / Q000295:instrumentation; D002478:Cells, Cultured; D007537:Isometric Contraction / Q000502:physiology*; D008846:Micromanipulation / Q000295:instrumentation; D009470:Muscle Spindles / Q000502:physiology*; D009339:Needles; D016658:Psoas Muscles / Q000166:cytology* / Q000502:physiology*; D011817:Rabbits; D012518:Sarcomeres / Q000502:physiology*; D013997:Time Factors