HOOGENDYK, T.A.*; CARR, J.A.; HENRY, H.T.; RUBENSON, J.; MARSH, R.L.; Northeastern University, Boston, Massachusetts: Mechanical and neural determinants of differences in fascicle strain between functionally distinct compartments in M. Iliofibularis during terrestrial locomotion in guinea fowl (Numida meleagris).

Although the gross morphology of M. Iliofibularis (IF) in Galliformes is suggestive of an architecturally simple two-joint muscle with largely parallel fibers, recent findings point to a more complex situation in which the whole muscle is sub-divided into at least two functionally distinct compartments. Here we present results from sonomicrometry, fine-wire electrode, and muscle moment arm experiments to explain the mechanical and neural basis of observed differences in strain between anterior and posterior fascicles. Sonomicrometry results show two lengthening-shortening cycles per stride for anterior fascicles and one for posterior fascicles. Anterior and posterior fascicles of the IF are differentially activated, with posterior activity occurring during stance phase and anterior during swing. The broad, fleshy origin of the IF leads to a wide range of extensor moment arms at the hip. Posterior fascicles have the greatest mechanical advantage and anterior fascicles have the least. Both compartments share the same tendon of insertion at the knee. This musculoskeletal arrangement results in anterior fascicle strain being largely influenced by knee angular excursion while posterior fascicles are affected by that of both the hip and knee. Fascicle strain heterogeneities arise from differences in mechanical advantage and activation between anterior and posterior fascicles. Supported by NIH AR47337 to RLM.