YOO, E.H.; MCGUIGAN, M.P.; BIEWENER, A.A.; Harvard University, Concord Field Station; Harvard University, Concord Field Station; Harvard University, Concord Field Station: In vivo strain and activation patterns of an ankle extensor in pygmy goats (Capra hircus) during incline versus level locomotion

When animals move at different gaits and different slopes, their leg muscles change function to meet different demands in work performance. Whereas level locomotion favors muscle force economy and energy savings, locomotion on an incline requires positive work to raise the animal�s center of mass whereas a decline requires absorption of energy and therefore some limb muscles must perform negative work. Can the ability to modulate work performance be attributed to specializations in muscle-tendon architecture? In order to answer this question, we used sonomicrometry, tendon force buckles, and EMG to study lateral (LG) and medial (MG) gastrocnemius in vivo length, force production, and activity. Data were collected together with high-speed video as goats moved on a treadmill at a moderate walk, trot, and gallop (1.5 m/s, 2.5 m/s, and 4.5 m/s) under decline, level, and incline conditions (-14.5°, 0°, and +14.5°). LG and MG contract under nearly isometric conditions during level walking (35.72±5.42 mJ and 45.13±7.10 mJ) and trotting (35.38±9.58 mJ and 30.66±4.58 mJ), generating little positive work. In contrast, during level galloping, both LG and MG generate significantly more positive work (107.01±51.26 mJ and 157.06±64.87 mJ) due to greater active shortening and increased force during stance. LG decreases work production on a decline surface (20.05±18.27 mJ) and both LG and MG increase work production on an incline surface (115.01±19.99 mJ and 266.60±39.58 mJ), when compared to level while walking. Our results suggest that both heads of the goat gastrocnemius are able to modulate work performance in response to changes in gait and slope (Supported by NIH AR-047679).