DALEY, M.A.; BIEWENER, A.A.: In vivo analysis of work performed by two ankle extensors of the guinea fowl during locomotion: How do similar agonist muscles fulfill different mechanical roles?

How well do we understand the relationship between the architecture of a muscle-tendon system and its dynamic function during locomotion? What allows a muscle-tendon unit to act as a motor, strut or brake? We address these questions by studying the in vivo performance of two ankle extensors of the guinea fowl. During steady level running the m. gastrocnemius of turkeys contracts relatively isometrically while its associated tendon stores and releases strain energy, similar to the pattern in wallaby ankle extensors. This force-spring behavior is attributed to the architecture of the muscle; a short fibered, pinnate muscle belly with a long tendon. As agonists of the gastrocnemius, the digital flexor muscles possess grossly similar architecture. Although one might expect these muscles to function similarly, we have found substantial differences in the work performed by each of these muscles during locomotion. While the gastrocnemius generates force isometrically, the digital flexor(IV) absorbs energy by lengthening during force generation. The difference in work performed by each of these muscles is consistent with the summed work at all joints about which they act. The gastrocnemius crosses the knee and ankle where little net work production occurs during support, whereas the digital flexors cross the ankle and TMP joints. Energy is absorbed at the TMP joint. Consistent with this, the digital flexor absorbs more energy than the gastrocnemius. Thus, despite their similar architecture, these two ankle extensors fulfill different mechanical roles; indicating that we do not yet fully understand the relationship between muscle-tendon architecture and dynamic function. (Supported by NSF IBN 9923699, and a HHMI Predoctoral Fellowship)