By preloading a muscle, the muscle is stretched and isometric twitch tension increases (Figure 9). Taking the peak tension of each twitch, a length-tension curve is developed (Figure 11).
Maximal tension (Lmax) is produced at initial length (Lo, usually not "rest" length), and tension decreases at either greater or lesser length. This is explained by an improvement of the interdigitation of the thick (myosin) and thin (actin, tropomyosin, troponin) myofilaments based on the sliding filament model of muscle contraction. Therefore, by slightly stretching a muscle, the more advantageous positioning of the cross-bridges (myosin heads) relative to the attachment sites on the thin myofilaments is achieved, and greater pulling can occur.
Note for cardiac muscle the narrow range of working lengths and the sharp increase in passive tension which occurs beginning below Lo. Cardiac and skeletal muscle are distinctly different in this respect.
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