Meeting Abstract
The relationship that exists between muscle length, and the force that it can produce, is a defining feature of skeletal muscle. It is widely accepted that this relationship, where maximum force is produced at an intermediate length (the optimal length), is a result of the overlap between the contractile proteins actin and myosin. However, optimum length increases with decreasing muscle activation level suggesting that there are additional determinants of the force-length relationship. We have previously suggested that at the level of the whole muscle, the amount of internal work that must be done to overcome compliance within the muscle and allow force to be effectively generated. Therefore, at lower activation levels, where less work is done by cross-bridges, muscles may be able to produce more force at longer lengths due to reduced internal work requirements. Muscle ergometry and sonomicrometry were used in frog plantaris muscles to determine the effect of activation level on optimum fiber length in whole muscles and muscle fiber bundles. The fiber bundles will have reduced structural complexity and no in-series compliance thereby decreasing internal work requirements compared to whole muscles. We test that hypothesis that at lower levels of activation, optimum length will shift to significantly longer lengths in the whole muscle compared to the fiber bundle. Understanding the effect of internal compliance and muscle shortening on the force-length relationship will not only highlight other determinants of the force-length relationship, but will also provide insight into this relationship in vivo.
