HENRY, H.T.*; MARSH, R.L.; ELLERBY, D.J.; Northeastern University; Northeastern University; Northeastern University: Function of the avian lateral gastrocnemius in jumping

The function of the same muscle may vary greatly during different locomotor behaviors. The main function of the avian lateral gastrocnemius (LG) during running is to produce force. During running the LG is active on the ascending limb of the length tension curve where force production is sub-optimal. We asked whether this seemingly suboptimal length was due to compromises in limb design that accommodate a different function of the LG during jumping. Kinematic and force plate studies document that guinea fowl produce very high work and power outputs during jumping and likely use prestorage of elastic energy. The LG is a pinnate muscle with a long tendon that passes around the ankle making it likely to play a significant role in jumping. We measured the active in vivo length trajectory of the LG using sonomicrometry and electromyography. Subsequent in situ contractile measurements allowed us to determine the length-tension curve. During jumping the LG began shortening with a length of approximately 1.25 Lo and shortened to approximately 0.7 Lo at lift-off. The in vivo strain trajectory showed a triphasic pattern of shortening similar to that recently described in jumping frogs. A period of high velocity shortening is followed by a period of low velocity shortening which leads to a period of high velocity shortening coinciding with the highest power output of the jump. The initial rapid shortening precedes ankle extension and likely functions to produce significant stretching of the tendon and storage of energy. The slow period of shortening should allow high forces to build up prior to the final high velocity shortening that allows release of energy stored in the tendon as well as direct work production by the muscle. Supported by NIH AR47337 to RLM.