Abstract
When a muscle is stretched during a contraction, the resulting steady-state force is higher than the isometric force produced at a comparable sarcomere length. This phenomenon, also referred to as residual force enhancement, cannot be readily explained by the force-sarcomere length relation. One of the most accepted mechanisms for the residual force enhancement is the development of sarcomere length non-uniformities after an active stretch. The aim of this study was to directly investigate the effect of non-uniformities on the force-producing capabilities of isolated myofibrils after they are actively stretched. We evaluated the effect of depleting a single A-band on sarcomere length non-uniformity and residual force enhancement. We observed that sarcomere length non-uniformity was effectively increased following A-band depletion. Furthermore, isometric forces decreased, while the percent residual force enhancement increased compared to intact myofibrils (5% vs. 20%). We conclude that sarcomere length non-uniformities are partially responsible for the enhanced force production after stretch.
Highlights
When a muscle is stretched during a contraction, the resulting steady-state force is higher than the isometric force produced at a comparable sarcomere length
We investigated the effect of sarcomere length non-uniformities on the residual force enhancement by inducing sarcomere length non-uniformities in isolated myofibrils—preparations in which we can track all sarcomeres during activation and stretch
We developed a new technique in our laboratory that can target selected sarcomeres to be treated with a high ionic strength s olution[16,17], causing a depletion of sarcomere thick filaments
Summary
When a muscle is stretched during a contraction, the resulting steady-state force is higher than the isometric force produced at a comparable sarcomere length. When a muscle fiber is stretched while activated, the steady-state force stabilizes at a level that is higher than that produced during isometric contractions at the corresponding length[5,6,7,8] The mechanism behind this phenomenon, known as residual force enhancement, remains elusive as it cannot be explained by the force–length relation[9,10]. The long sarcomeres will equilibrate the total force by increasing the passive force In this way, an increase in sarcomere length non-uniformity beyond levels that are present during isometric contractions will lead to an increased total force produced after stretch. In a recent study performed in our laboratory, we observed that force enhancement was directly linked to the non-uniformity of half-sarcomeres; when half-sarcomere length non-uniformity was not present, the levels of force enhancement were substantially decreased or even inexistent[8]. We hypothesized that the increase in sarcomere length non-uniformity would lead to increased levels of residual force enhancement
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