Meeting Abstract
The passive mechanical properties (i.e. passive stiffness) of muscle have been shown to influence the region of the force-length (F-L) curve over which it operates. Previous work in anurans has shown that muscles with distinct in vivo functions (i.e. energy production vs. dissipation) vary in passive mechanical properties, but it remains unclear whether this pattern is broadly held across terrestrial vertebrates or restricted to highly-specialized muscles. Here we use results of inverse dynamics analyses in juvenile alligators to deduce the in vivo functions of limb muscles during walking, and subsequently characterized the F-L properties of two different limb muscles in situ. Results from inverse dynamics suggest that elbow extensor muscles undergo a period of negative work (eccentric contraction) and ankle extensors muscles primarily produce positive (concentric contraction) or zero (isometric contraction) work during stance phase of walking. Based on these results and previously published anatomical data (Allen et al. 2010, 2014), triceps longus lateralis (TLL) and lateral gastrocnemius (LG) were chosen for muscle preparations. Our preliminary in situ results show that TLL passive force reaches 20% maximum isometric force (L20) at shorter relative lengths when compared to LG. We also find that the slope of log-transformed TLL data is greater than that of LG (ANCOVA p<0.05), indicating an increase in stiffness of muscle that dissipates mechanical energy. These results suggest a causal link between a muscle’s expected in vivo function and its passive stiffness, and this work in crocodilians expands the range of muscle passive stiffness literature to include reptiles and animals with semi-erect postures. Future work will determine the in vivo operating lengths of TLL and LG followed by in situ F-L characterizations to map where on their respective force-length curves these muscles operate at during walking.
