Crossbridge and non-crossbridge contributions to force in shortening and lengthening muscle

Abstract

Analysis of tension responses to ramp length changes in muscle can provide important information about the crossbridge cycle. During a ramp length change, the force response of an active muscle shows an early change in slope (the P1 transition) followed by a later, gradual change in slope (the P2 transition). Modeling shows that the first transition reflects the tension change associated with the crossbridge power stroke in shortening and with its reversal in lengthening; the reduction in slope at the second transition occurs when most of the crossbridges (myosin heads) that were attached at the start of the ramp become detached; the steady tension during shortening is borne mainly by poststroke heads whereas tension during lengthening is borne mostly by pre-stroke heads. After the P2 transition, the tension reaches a steady level in the model whereas in the experiments the tension continues to increase during lengthening or to decrease during shortening; this tension change is seen at a wide range of sarcomere lengths and even when active force is reduced by a myosin inhibitor. It appears that some non-crossbridge components in muscle fibers stiffen upon activation and contribute to the continued tension rise during lengthening; release of such tension leads to tension decline during shortening. Thus, non-crossbridge visco-elasticity in sarcomeres may also contribute to energy storage and release during in situ muscle function. © Springer Science+Business Media, LLC 2010.