Key result
Stretch of activated frog muscle fibers resulted in higher steady-state force than isometric contractions, and administration of BDM further increased this force enhancement.
Force enhancement after stretch in muscle fibers may be partially associated with the recruitment of weakly bound cross bridges into a strongly bound state.
May inform basic muscle models but should not alter clinical practice; leaves open cross-bridge mechanisms in mammalian systems.
The purpose of this study was to evaluate the relationship between force and stiffness after stretch of activated fibers, while simultaneously changing contractility by interfering with the cross-bridge kinetics and muscle activation. Single fibers dissected from lumbrical muscles of frogs were placed at a length 20% longer than the plateau of the force-length relationship, activated, and stretched by 5 and 10% of fiber length (speed: 40% fiber length/s). Experiments were conducted with maximal and submaximal stimulation in Ringer solution and with the addition of 2 and 5 mM of the myosin inhibitor 2,3-butanedione monoxime (BDM) to the solution. The steady-state force after stretch of an activated fiber was higher than the isometric force produced at the corresponding length in all conditions investigated. Lowering the frequency of stimulation decreased the force and stiffness during isometric contractions, but it did not change force enhancement and stiffness enhancement after stretch. Administration of BDM decreased the force and stiffness during isometric contractions, but it increased the force enhancement and stiffness enhancement after stretch. The relationship between force enhancement and stiffness suggests that the increase in force after stretch may be caused by an increase in the proportion of cross bridges attached to actin. Because BDM places cross bridges in a weakly bound, pre-powerstroke state, our results further suggest that force enhancement is partially associated with a recruitment of weakly bound cross bridges into a strongly bound state.
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Rassier et al. (2005) studied this question. Stretch of activated fibers and administration of 2,3-butanedione monoxime (BDM) vs. Isometric contractions was evaluated on Steady-state force and stiffness after stretch. Stretch of activated frog muscle fibers resulted in higher steady-state force than isometric contractions, and administration of BDM further increased this force enhancement.
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