The Molecular Mechanism of Skeletal Muscle Visco-Elasticity

The molecular spring titin is the primary structure providing passive force inskeletal muscle sarcomeres1, and titin has been said to hold the key to the visco-elasticproperties of muscles2. Specifically, it has been argued that unfolding/refolding ofthe Ig domains of titin causes the visco-elastic behavior, but Ig domain refoldingonly occurs at specific sarcomere lengths and forces2. Therefore, the purpose of thisstudy was to test the hypothesis that muscle is highly visco-elastic in regions of Igdomain unfolding/refolding while it is virtually elastic when refolding is prevented.Ten myofibrils from rabbit psoas were isolated and prepared for mechanical testingas described previously3. Testing involved passive stretch release cycles of variousmagnitudes. First, three stretch shortening cycles were performed between averagesarcomere lengths of 2.6 to 4.6 mm. Then, myofibrils were rested for ten minutes atslack length (1.8 mm) and the initial stretch-shortening cycles were repeated two moretimes. Following another rest of ten minutes, myofibrils were stretched to an averagesarcomere length of approximately 4.6 mm and then shortened and stretched by 0.5,1.0, or 1.5 mm/per sarcomere ten times. When stretched and released from 2.6 to4.6 mm and back, myofibrils exhibited a highly visco-elastic behavior and Ig domainun/refolding is known to occur2,3. However, when myofibrils were cycled ten timesby a short magnitude (e.g 0.5 mm) starting at an average sarcomere length of 4.6 mm,titin behaved virtually elastic and Ig domain refolding was prevented2,3. We concludefrom these results that myofibrils behave visco-elastically in regions where Ig domainunfolding/refolding is known to occur, while they behave essentially elastic whenoperating in regions where Ig domain refolding is prevented. Therefore, a muscle’spassive properties may change from highly visco-elastic to virtually purely elasticdepending on the kinetics of Ig domain un/re folding