CARR, JA; ELLERBY, D; HOOGENDYK, TA; RUBENSON, J; MARSH, RL; Northeastern University; Northeastern University; Northeastern University; Northeastern University; Northeastern University: Differential Strain in an Architecturally Complex Muscle

Understanding the mechanical function of muscles with extensive origins and insertions is challenging. The Iliotibialis lateralis pars postacetabularis (ILPO) is the largest muscle in the hindlimb of the guinea fowl (Numida meleagris). The ILPO has a broad fleshy origin that spans the ilium and ischium with relatively short anterior fascicles inserting on an aponeurotic tendon and longer posterior fascicles. The moment arm of the fascicles at the hip increases anteriorly to posteriorly whereas the moment arm at the knee remains constant across the muscle. Using sonomicrometry and electromyography, we examined whether the ILPO experiences differential strain between the anterior and posterior fascicles and among the proximal, central and distal portions of the posterior fascicle. Histological techniques combined with sonomicrometry were used to normalize the measured strain patterns to sarcomere length. The posterior fascicles experience greater strains than the anterior fascicles, both being lengthened and shortening while active. Differences in the moment arm at the hip do not completely explain the differential strain between anterior and posterior fascicles. We hypothesize that the differences in strain patterns are caused by the large aponeurosis to which the anterior fascicles attach. The aponeurosis, in series with the anterior fascicle, is predicted to stretch when the muscle is active thus reducing the strain of the anterior fascicles. Strain increases proximally to distally in the posterior fascicles when the segments are lengthened actively. These proximal to distal differences may be caused by length-tension effects or differences in the timing of activation relative to foot down. Supported by NIH AR47337.