3a

is loosely and poorly defined, and there is much debate as to what it actually means and what is actually happening when we stretch tissue.

Officially, the stretch of the collagen fiber is known as strain, which is the measurement of how much the tissue has deformed. Although commonly seen as a change in length, strain can refer to a change in any dimension—something that is important to keep in mind, as we see only two dimensions on the printed page. When a muscle is eccentrically contracting, it increases not only in length but also in width and in depth. We frequently measure myofascial strain as a change in length, but compare the images in figure 3.2a—a passive stretch of the tissue simply lengthens it—an eccentric, active loading of myofascial tissue causes the tissue to expand in every direction, a feature of some biological tissues known as auxeticity.

Myofascia wraps around individual muscle fibers, bundles of fibers, and the whole muscle, effectively intertwining it and making it part of its structure. Because of this, its three-dimensional structure will strain in every direction, more like a balloon than an elastic band. The auxetic nature of myofascia therefore helps disperse eccentric loading forces in every direction rather than focusing them between the polar ends of a muscle. This means that fascial containment of fiber bundles improves muscle force output, as the containment creates a stiffened environment that transfers muscle force more effectively and efficiently.

Our movement naturally exploits that fact by preparing for most actions by first going in the opposite direction. To continue with the example of throwing, it is the body’s flexors that will be recruited to produce most of the force into the projectile as the arm goes back. A summary of those flexors would include rectus abdominis, rectus femoris, the psoas, and iliacus, along with the anterior adductors of the back leg and, of course, pectoralis major. Each of these muscles has to work to decelerate extension during the preparatory cocking, and this active eccentric muscle contraction creates lengthening and expansion of the fascial tissues. It is commonly thought that muscle fibers decelerate countermovement by staying close to isometric, which keeps them within their optimum force-length ratio. This idea is a simplification of the force-length curve seen in many textbooks (figure 3.3a). A number of researchers have shown that muscle fibers adjust their lengthtension ratio to work on the so-called “ascending limb” of the curve (figure 3.3b). Muscle sarcomeres change their length in response to the forces acting through them—the higher the force, the shorter they will become. The result is that when muscles have to work eccentrically, they are moving toward the peak of the force-length curve.

These findings show that the force-length curve is not fixed and that muscles use a feedback loop to reset their sarcomere length to optimize the system. That optimization allows muscles to become stronger as they lengthen during the eccentric phase of a movement and, despite lengthening, prevent the fiber length