The elasticity of muscle tissue contributes to its passive property, as shown in Figure 12.23. When a muscle is stimulated to contract at different lengths, the relationship between contractile force and muscle length changes. Initially, contractile force increases with increasing muscle length until it reaches the optimal length (LO). Beyond this point, contractile force decreases. This pattern aligns with the sliding filament theory, as discussed previously. When the sarcomere length becomes very long (3.7 µm), actin filaments do not overlap with myosin filaments, resulting in no contraction. However, as the sarcomere length decreases towards LO, the amount of overlap increases, leading to a progressive increase in contractile force. When the sarcomere length decreases below 2 µm, the thin filaments collide in the middle of the sarcomere, disrupting the actin-myosin interaction and causing a decrease in contractile force. To construct length-tension curves, muscles were kept at a specific length, and the contractile force was measured during isometric contraction.

The force-velocity relationship of skeletal muscle, as depicted in Figure 12.24, demonstrates that the velocity at which a muscle shortens depends on the amount of force it needs to generate. When there is no load, the muscle's shortening velocity is at its maximum (V0). V0 corresponds to the highest cycling rate of cross-bridges and is proportional to the maximal rate of energy turnover (ATPase activity) by myosin. Consequently, fast-twitch muscles have a higher V0 compared to slow-twitch muscles. Increasing the load reduces the muscle's shortening velocity until, at maximum load, the muscle cannot lift the load and therefore cannot shorten (zero velocity). Further increases in load cause the muscle to stretch, resulting in negative velocity. The maximal isometric tension, where the shortening velocity is zero, depends on the number of active cross-bridges between actin and myosin. Generally, fast-twitch motor units exhibit greater maximal isometric tension due to the larger diameter of fast-twitch muscle fibers and the higher number of muscle fibers in a typical fast-twitch motor unit. The power-stress curve in Figure 12.24 represents the rate of work done at each load and reveals that the maximum rate of work occurs at a submaximal load, approximately 30% of the maximal tetanic tension. The calculation of this curve simply involved multiplying the x and y coordinates and plotting the product against the x-coordinate.
Fig. 12.23 ). This purely passive property is due to the elasticity of the muscle tissue. If the muscle is stimulated to contract at these various lengths, a different relationship is obtained. Specifically, contractile force increases as muscle length is increased up to a point (designated LO to indicate optimal length). As the muscle is stretched beyond LO, contractile force decreases. This length-tension curve is consistent with the sliding filament theory, described previously. At a very 0 1 201 Length (fraction of optimum) Sarcomere length (µm) •Fig. 12.23. Length-Tension Relationship in Skeletal Muscle. A, Experimentalsetupinwhichmaximalisometrictetanictensionismeasuredatvariousmusclelengths.B, Howactivetensionwascalculatedatvariousmusclelengths(i.e.,bysubtractingpassivetensionfromtotaltensionateachmusclelength).C, Plotofactivetensionasafunctionofmusclelength,withthepredictedoverlapofthickandthinfilamentsatselectedpoints.
•Fig. 12.24. Force-Velocity Relationship of Skeletal Muscle. Theexperimentalsetupisshownontheright.Theinitialmusclelengthwaskeptconstant,buttheamountofweightthatthemusclehadtoliftduringtetanicstimulationvaried.Whilethesevariousamountsofweightwerelifted,muscle-shorteningvelocitywasmeasured.Seetextfordetails.F,force;V,velocity.
long sarcomere length (3.7 µm), actin filaments no longer overlap with myosin filaments, and so there is no contraction. As muscle length is decreased toward LO, the amount of overlap increases, and contractile force progressively increases. As sarcomere length decreases below 2 µm, the thin filaments collide in the middle of the sarcomere, the actin-myosin interaction is disturbed, and hence contractile force decreases. For construction of the length-tension curves, muscles were maintained at a given length, and then contractile force was measured (i.e., isometric contraction).
Thus the length-tension relationship supports the sliding filament theory of muscle contraction.
The velocity at which a muscle shortens is strongly dependent on the amount of force that the muscle must develop (
Fig. 12.24 ). In the absence of any load, the shortening velocity of the muscle is maximal (denoted as V0). V0 corresponds to the maximal cycling rate of the cross-bridges (i.e., it is proportional to the maximal rate of energy turnover [ATPase activity] by myosin). Thus V0 for fast-twitch muscle is higher than that for slow-twitch muscle. Increasing the load decreases the velocity of muscle shortening until, at maximal load, the muscle cannot lift the load and hence cannot shorten (zero velocity). Further increases in load result in stretching the muscle (negative velocity). The maximal isometric tension (i.e., force at which shortening velocity is zero) is proportional to the number of active cross-bridges between actin and myosin, and it is usually greater for fast-twitch motor units (because of the larger diameter of fast-twitch muscle fibers and greater number of muscle fibers in a typical fast-twitch motor unit). In
Fig. 12.24 , the power-stress curve reflects the rate of work done at each load and shows that the maximal rate of work was done at a submaximal load (namely, when the force of contraction was approximately 30% of the maximal tetanic tension). To calculate the latter curve, the x-and y-coordinates were simply multiplied, and then the product was plotted as a function of the x-coordinate.
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