Stability of muscle fibers on the descending limb of the force-length relation. A theoretical consideration.

For the past 40 years it has generally been accepted that the descending limb of the force-length (F-L) relation of muscle fibers is unstable; strong sarcomeres are thought to shorten onto the ascending limb of the F-L relation at the expense of weak sarcomeres which are stretched beyond the thin-thick myofilament overlap. This unstable behavior has not been demonstrated by direct observation but has been inferred indirectly, based on the negative slope of the F-L relation. Intuitively, the idea that the sarcomere length within skeletal muscle is unstable at any length within the normal working range would not be advantageous. Therefore, the purpose of this study was to identify analytically the mechanical conditions for which sarcomeres in a muscle fiber are stable. It was found that the mechanical conditions required for sarcomere and fiber stability could be associated with known properties of skeletal muscle fibers. Furthermore, it could be demonstrated that a fiber composed of a large number of sarcomeres can be stable, and still exhibit an apparently unstable (negatively sloped) descending limb of the F-L curve.