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Fig. 6 | Journal of NeuroEngineering and Rehabilitation

Fig. 6

From: Skeletal muscle mechanics: questions, problems and possible solutions

Fig. 6

Stress vs. average sarcomere length traces for experiments in single myofibrils stretched way beyond actin-myosin filament overlap while activated (Active), while passive (Passive), and after elimination of titin (Passive no titin). In the region beyond actin-myosin filament overlap (beyond the grey shaded area), one would expect the force in the passively and actively stretched sarcomeres to be the same as cross-bridge based active forces are eliminated in this region. However, this was not the case and sarcomeres stretched beyond actin-myosin filament overlap had titin-based forces that were 3–4 times greater in actively compared to passively stretched myofibrils when stretching started at a sarcomere length of 2.0 μm. When stretching started at average sarcomere length of 3.4 μm (that is halfway down the descending limb of the force-length relationship – Half force), the extra, titin-based force, was substantially reduced but still significantly greater than the corresponding forces obtained in passive stretching of myofibrils. When titin is eliminated from the myofibril preparation, all passive and active force production is eliminated as well, indicating that (i) titin is required for active force transmission, and (ii) that titin is the only force carrying structure in single sarcomeres once sarcomeres are stretched beyond actin-myosin filament overlap. Combined, these results suggest that titin produces more force in actively compared to passively stretched muscles. The mechanisms of how this titin-based increases in force are achieved remain unknown but are thought to occur through an increase in titin stiffness caused by calcium binding to titin upon activation as shown by Labeit and Duvall [29, 30], and by titin binding to actin as shown in our laboratory [16, 17]. (Adapted from Herzog and Leonard [31], with permission)

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