Meeting Abstract

60.2  Tuesday, Jan. 6  Muscle fiber length operating ranges reflect disparate functions between muscles ENG, CM*; HIGHAM, TE; BIEWENER, AA; Harvard Univ., Cambridge, MA; Clemson Univ., Clemson, SC; Harvard Univ., Cambridge, MA cmeng@fas.harvard.edu

Animals move in complex environments that place constantly shifting demands on the locomotor system. To accommodate these demands, hindlimb muscles may be functionally integrated or disparate. For example, the lateral gastrocnemius (LG) and medial gastrocnemius (MG) in the guinea fowl are both synergists (in ankle extension) and antagonists (in knee flexion and extension, respectively). There are many factors that may be modulated to facilitate changes in demand (e.g. force-velocity properties, fiber type recruitment). Furthermore, a muscle may modulate the fiber lengths over which it operates, and hence, active force, a relationship described by the muscle-specific length-tension (L-T) curve. We measured the L-T properties of the LG and MG of helmeted guinea fowl (Numida meleagris) and examined how different demands affected the operating range of each muscle relative to its L-T curve. Sonomicrometry crystals were used to measure fiber length during walking and running on level and incline. Following in vivo experiments, the muscles were isolated in an in situ preparation and stimulated at incremental lengths to construct a muscle-specific L-T curve. Both LG and MG operate at lengths shorter than optimal (determined in situ) during all training conditions. The range of fiber strain increased for the LG on the incline, but not with speed. However, the LG operated closer to optimal length as speed increased, indicating a greater capacity for active force generation. There was no change in the operating range in MG across training conditions. These results demonstrate: 1) there are multiple ways in which a muscles operating range can be modulated, and 2) LG may be more important than MG in increasing hindlimb work as running speed increases.