Meeting Abstract

5.4  Thursday, Jan. 3  Spring properties of muscle during active shortening and lengthening MONROY, JA*; HOKANSON, J; NISHIKAWA, KC; Northern Arizona University; SUNY Cortland jenna.monroy@nau.edu

When its load is reduced, an active muscle will recoil elastically to a shorter length. When its load increases, an active muscle will lengthen. In this study, we describe and compare the spring properties of mouse soleus muscle during active shortening and lengthening. Using load-clamp and dynamic stiffness experiments performed on a servo-motor force-lever, we measured the changes in muscle length that resulted from changes in muscle force. Using these data, we calculated muscle stiffness across a range of muscle forces and lengths. Our results show that muscle stiffness increases with force (i.e., up to maximum isometric force) and is greatest when changes in load are small (i.e., isotonic shortening). The relationship between change in length and force is exponential during active shortening. Thus, muscle stiffness decreases non-linearly as the change in load increases. Minimum stiffness, reached during unloaded shortening, is independent of the initial force. As active muscles are lengthened, both peak and steady-state forces increase linearly, so that muscle stiffness remains constant and is likewise independent of the initial force. Remarkably, the steady-state stiffness of actively lengthening muscle approaches the minimum stiffness observed during active shortening at zero load. These observations support the hypothesis that titin plays a role in elastic recoil of actively shortening muscle as well as in steady-state enhancement of force with stretch. Supported by NSF IOS-0623791, IOS-0732949, NIH R25-GM56931, the TRIF Fund for Biotechnology and Science Foundation Arizona.