Force-length properties in stable skeletal muscle fibers--theoretical considerations.

Differences in the force-length (F-L) properties between sarcomeres and fibers have been associated with the supposed unstable nature of the sarcomere F-L relation on the descending limb (i.e. at sarcomere lengths greater than optimal length). Recently, it has been suggested that sarcomere behavior in a fiber is stable during contractions on the descending limb of the F-L relation; therefore, a factor other than sarcomere instability must be responsible for the observed differences in the F-L relations of sarcomeres and fibers. The purpose of this study was to determine theoretically the F-L relation of a muscle fiber when sarcomeres were at a stable, steady-state length. Three models of muscle fibers are presented; each model contains sarcomeres with different mechanical properties which have been observed experimentally. Results of these theoretical considerations demonstrate that sarcomeres with the classic F-L properties as measured by Gordon et al. (J. Physiol. 184, 170-192, 1966) cannot predict the F-L relation exhibited by fibers. The addition of cross-bridge stiffness properties to the classic sarcomere F-L relation still does not explain the differences between the sarcomere and fiber F-L relations. However, if history dependent sarcomere properties are used, the fiber F-L relation exhibits an elongated plateau and greater forces on the descending limb compared to the classic sarcomere F-L relation; and the fiber F-L relation corresponds qualitatively to experimental findings.