Stability of Skeletal Muscle on the Descending Limb of the Force-length Relationship

INTRODUCTION There is continuing controversy surrounding force production and stability of muscle contraction on the descending limb of the force-length (F-L) relation. Many believe that force production on the descending limb is unstable. This notion is correct if the negative slope of the F-L relationship, which is obtained in a series of static, independent contractions, would also be negative for continuous dynamic contractions, such as stretching or shortening of the muscle. However, unstable behavior during muscle contraction has not been demonstrated by direct observation during dynamic muscle contraction, but has been inferred based on theoretical considerations or by measurement of average sarcomere length during contraction. In fact, there is evidence that sarcomeres may actually be stable on the descending limb of the F-L relation. While a muscle is stretched, force increases, thus establishing positive stiffness. If the residual force enhancement after stretch is larger than the force produced at the initial length, then force production in the muscle is stable. This has not been investigated using a whole muscle preparation, nor has it been discussed in the context of stability on the descending limb. Thus the purpose of this study was to investigate stability on the descending limb of the force length curve in an in situ muscle preparation.