6.2 Wednesday, Jan. 4 Length, force and changes in the elastic behavior of active muscle. GILMORE, L. A.*; NISHIKAWA, K. C.; Northern Arizona University email@example.com
Previous studies have examined how muscle force and shortening velocity change with length changes in active muscle. However, few studies have examined how elastic behavior changes with muscle length. Using mouse soleus muscle in load clamp experiments, we examined the elastic behavior of actively shortening muscle at optimal length and at lengths 2.5%, 8% or 13% greater than optimal length. All length changes were made prior to activating the muscle. In each load clamp experiment, the muscle was maximally stimulated prior to a period of rapid unloading. By examining the changes in force and length during this period of elastic recoil we were able to describe the elastic behavior at each length. Our results show that elastic behavior is length dependent, but is not predicted by the active, passive or total force alone. Muscles were more compliant at 13% above optimal length, although there was no difference in total force at this length and at optimal length. At 13% above optimal length the ratio of passive and active force changes, however, there was no correlation between the elastic behavior and passive or active force. This suggests that neither a passive elastic element nor the cross-bridge action alone can describe the observed elastic behavior. Instead, these results suggest an internal, length-dependent spring which experiences changes in stiffness during activation. A similar mechanism has been proposed to explain force enhancement during active stretch. Our results are consistent with the winding filament model of muscle contraction, in which titin is engaged mechanically during Ca++ influx, and then winds upon the thin filaments as force develops. Supported by NSF IOS-1025806, IOS-0732949, II5-0827688, TRIF Fund for Biotechnology and Science Foundation Arizona.