Meeting Abstract
Previous studies demonstrated dissociation of muscle activation and force during in vivo treadmill running over obstacles in the lateral gastrocnemius (LG) muscles of guinea fowl. There was a long (90-150 ms) and variable delay between EMG onset and force onset, and the correlation between peak EMG and peak force amplitude (r2 = 0.30) was small compared to the correlation between muscle length at foot contact and peak force amplitude (r2 = 0.63). To further demonstrate decoupling between force and activation, we used ex vivo extensor digitorum longus muscles (EDL) of mice as an “avatar” for the in vivo guinea fowl LG. Instead of sinusoidal or triangular length inputs typically used in workloop experiments, we used length records from the guinea fowl LG recorded in vivo using sonomicrometry as inputs for ex vivo workloop experiments. Muscles were stimulated submaximally with similar onset and duration to in vivo guinea fowl muscles. Work loops recorded ex vivo in EDL strongly resembled those from the guinea fowl LG during in vivo treadmill running. Both ex vivo EDL and in vivo LG workloops deviate substantially in shape from traditional ex vivo workloop experiments, demonstrating the importance of small high-frequency perturbations to muscle force production. A titin-based muscle model accurately predicted muscle force for both in vivo and ex vivo muscles (r2 > 0.80). While these studies demonstrate that muscle length and velocity regulate muscle force production, the results suggest that neither the isometric force-length relationship nor the isotonic force-velocity relationship describes their roles during in vivo force production.
