4.1 Muscle Mechanical Behavior During Stretch
Representative publications: Lewin and Wyman 1927; Katz 1939; Abbott and Aubert 1952; Edman et al. 1978; Rassier 2009
When a fully activated muscle or a fiber is stretched with a moderate speed from one constant length to another, the force recorded on its end exceeds the maximum isometric force at the same muscle length (figure 4.2). At the end of stretch, the force can be 2 times larger than the maximum isometric force at the same length, a phenomenon known as dynamic force enhancement. This force enhancement is velocity dependent—the peak force typically increases with the stretch velocity (the force–velocity relation in the eccentric muscle actions is described later in the text, see figure 4.3 and section 126.96.36.199.) When the stretch is completed and the muscle length is kept constant at a new level, muscle force starts decreasing and reaches a value that is still larger than the force of isometric action at the same muscle length. This residual force enhancement after stretch lasts as long as the muscle is active.
4.1.1 Dynamic Force Enhancement
Representative publications: Sugi 1972; Flitney and Hirst 1978; van Atteveldt and Crowe 1980; Pinniger et al. 2006
Dynamic force enhancement during stretch is thought to be associated with increased strain of attached crossbridges between actin and myosin myofilaments (see figure 1.2). In this situation, crossbridges act like springs and develop force that increases with their strain. The time history of muscle force during stretch depends on several factors: rate of stretch, stretch magnitude, initial muscle length, stimulation intensity, muscle fatigue, and temperature. The rate of stretch is usually expressed in the units of optimal muscle length Lo per second, Lo/s.
When the (constant) rate of muscle stretch is low to moderate (e.g., up to 10 Lo/s for a bundle of muscle fibers from semitendinosus muscle of the frog at 20 °C, which roughly corresponds to the maximum shortening velocity of this muscle), the force of maximally stimulated muscle starts to increase at the beginning of stretch, initially faster and then more slowly, and reaches a maximum at the end of stretch. Then the force starts to decay to a level that often exceeds the isometric force at the same muscle length insofar as stimulation continues. The peak force developed during such stretches increases with stretch rate (figure 4.3).
As mentioned, this behavior of muscle force during stretch with low to moderate speeds is explained by deformation of crossbridges, which causes an increase in force. The initial higher rate of force increase results presumably from deformation of all attached crossbridges. The rate of force increase in this phase does not markedly depend on stretch velocity. A subsequent slowdown in the force increase (figure 4.2) can be explained by a partial detachment of some crossbridges and by formation of new crossbridges that are not as strongly deformed. The rate of force increase during this phase is stretch-velocity dependent.
188.8.131.52 Force–Velocity Relation for Lengthening Muscle
Representative publications: Katz 1939; Joyce et al. 1969; Otten 1987; Cole et al. 1996; Krylow and Sandercock 1974; Brown et al. 1999
The dependence of peak force on stretch velocity of maximally stimulated muscle during dynamic force enhancement does not follow the Hill force–velocity equation (equation 3.26b). When speed V becomes negative (see equation 3.26c), the predicted force increase is much smaller than observed in experiments. The predicted force (from the Hill equation) increases indefinitely when speed of muscle lengthening approaches a constant b, whereas experimentally observed peak forces converge to a constant force value of about 2F0 (see figure 4.3).
The force–velocity relation during lengthening of maximally stimulated muscles is not as well established as the force–velocity relation during muscle shortening (see section 3.2.2). There is no unique equation describing eccentric force–velocity relation that is accepted by all researchers. This could be explained by the fact that when the Hill force–velocity equation for shortening was proposed, it was thought that constants a and b in this equation had a fundamental physiological meaning related to energy production in the muscle, which later, after more accurate measurements, was determined to be incorrect.
Several equations have been proposed in the literature to describe the eccentric force–velocity relation (table 4.1).