MacIntosh, Brian R.MacDougall, Keenan Barry2020-04-012020-04-012020-03-30MacDougall, K. B. (2020). Additional in-series compliance does not affect the length dependence of activation in rat medial gastrocnemius (Master's thesis, University of Calgary, Calgary, Canada). Retrieved from https://prism.ucalgary.ca.http://hdl.handle.net/1880/111768Background: The force-length relationship describes the amount of force a muscle can produce as a function of its length, typically during maximal isometric contractions. When activation is submaximal, it has been shown that the length at which force production is highest (the optimum length) is shifted towards longer lengths. The usual explanation for this shift is a length-dependent increase in calcium sensitivity due to an increased proximity of the myofilaments with increases in length, which has been dubbed the “length dependence of activation” (LDA). Some recent research has suggested that shortening-induced force depression, (a phenomenon which describes the relative reduction in muscle force when a muscle is actively shortening to a given length compared to contracting isometrically at that same length) has the potential to alter a muscle’s optimum length in an activation-dependent way, thereby affecting any activation dependent shifts in optimum length. Purpose: The purpose of this study was to determine whether shortening-induced force depression would impact the relative shift in optimum length that occurs with submaximal activation. Methods: Rat medial gastrocnemius muscle was isolated and stimulated at 200 Hz triplet pulses and single pulse stimulation over a range of lengths, both with and without additional in-series compliance provided by a small piece of silicon tubing in series with the muscle tendon unit. Peak force and optimum length were calculated by curve-fitting of the force-length data, and these data were compared between low- and high-compliance conditions as well as with maximal and submaximal activation. Results: Additional in-series compliance led to a significant decrease in force at both maximal and submaximal activation, but did not impact the optimum length for either condition. Therefore, in our model, shortening-induced force depression did not impact the length dependence of activation. Conclusion: The additional in-series compliance allowed greater muscle shortening upon activation, which was accompanied by a reduction in force, however the post-shortening length at which the muscle produced the most force was unchanged. The best explanation for the length dependence of activation in skeletal muscle remains a length dependent increase in calcium sensitivity.engUniversity of Calgary graduate students retain copyright ownership and moral rights for their thesis. You may use this material in any way that is permitted by the Copyright Act or through licensing that has been assigned to the document. For uses that are not allowable under copyright legislation or licensing, you are required to seek permission.length dependence of activationforce-velocity relationshipforce-length relationshipshortening-induced force depressiontendon complianceAnatomyAnimal PhysiologyPhysiologymaster thesis10.11575/PRISM/37658