Stretch and shortening of skeletal muscles activated along the ascending limb of the force-length relation

Abstract

There is a history dependence of skeletal muscle contraction. When muscles are activated and subsequently stretched, they produce a long lasting force enhancement. When muscles are activated and subsequently shortened, they produce a long-lasting force depression. The purposes of the studies shown in this chapter were (1) to evaluate if force enhancement and force depression are present along the ascending limb of the force-length (FL) relation, (2) to evaluate if the history-dependent properties of force production are associated with sarcomere length (SL) non-uniformity, and (3) to determine the effects of cross-bridge (de) activation on force depression. Isolated myofibrils were activated by either Ca2+ or MgADP and were subjected to consecutive stretches or shortenings along the ascending limb of the FL relation, separated by periods (~5 s) of isometric contraction. Force after stretch was higher than force after shortening when the contractions were produced at similar SLs. The difference in force could not be explained by SL non-uniformity. After shortening, MgADP activation produced forces that were higher than Ca 2+ activation. Since MgADP induces the formation of strongly bound cross-bridges, the result suggests that force depression following shortening is associated with cross-bridge deactivation. © Springer Science+Business Media, LLC 2010.