Meeting Abstract
39.2 Sunday, Jan. 5 08:15 Compliance, activation and the force-length relationship in skeletal muscle HOLT, NC*; ABBOTT , EM; AZIZI, E; Univ. of California, Irvine; Univ. of California, Irvine; Univ. of California, Irvine natalie.c.holt@gmail.com
In vivo muscle performance is modulated by varying the level of muscle activation, both by changing activation frequency and the number of motor units recruited. However, much of our understanding of muscle physiology comes from studies using maximal activation. This discrepancy limits our understanding of in vivo muscle function. A muscle’s optimal length (L0) has been shown to increase when activation level is reduced by decreasing stimulus frequency. Length-dependent calcium effects have been implicated as a potential mechanism to explain this increase in L0. However, this pattern may also be explained by the fact that the effects of in-series compliance will vary with the muscle’s level of force. We distinguish between these alternate mechanisms by examining how different stimulation conditions affect L0 in a single muscle. Isometric force-length curves were constructed for frog plantaris muscles using: 1) a tetanic stimulus of supra-maximal amplitude (high activation, high calcium in all fibres and high whole muscle force); 2) a tetanic stimulus of sub-maximal amplitude (low activation, high calcium in a sub-set of fibres and low whole muscle force) and; 3) a twitch stimulus (low activation, low calcium in all fibres and low whole muscle force). If the optimal length increases solely with decreasing maximum force, compliance would be the most likely candidate to explain shifts in L0. If however, L0 was independent of maximum force, and only varied between supra-maximal tetanic and twitch contractions, calcium-dependent mechanisms would be implicated. Our findings suggest that compliance has a greater role than calcium in determining the optimal length of the force-length relationship and imply that the interaction between compliance and activation level may affect in vivo muscle performance more than is currently appreciated. Supported by NIH AR055295.
