To any informed observer, it is plain that the musculoskeletal system executes all our acts of will, expresses our conscious and unconscious habits, breathes air into the lungs, articulates our oral expression of words, and implements all generally recognized forms of nonverbal expression and communication. And in the practice of hatha yoga, it is plainly the musculoskeletal system that enables us to achieve external balance, to twist, bend, turn upside down, to be still or active, and to accomplish all cleansing and breathing exercises. Nevertheless, we are subtly deceived if we think that is the end of the story. Just as we see munchkins sing and dance in The Wizard of Oz and do not learn that they are not autonomous until the end of the story, we'll find that muscles, like munchkins, do not operate in isolation. And just as Dorothy found that the wizard kept a tether on everything going on in his realm, so we'll see that the nervous system keeps an absolute rein on the musculoskeletal system. The two systems combined form a neuro-musculoskeletal system that unifies all aspects of our actions and activities.
To illustrate how the nervous system manages posture, let's say you are standing and decide to sit. First your nervous system commands the flexor muscles (muscles that fold the limbs and bend the spine forward) to pull the upper part of the trunk forward and to initiate bending at the hips, knees, and ankles. A bare moment after you initiate that movement, gravity takes center stage and starts to pull you toward the sitting position. And at the same time—accompanying the action of gravity—the nervous system commands the extensor muscles (those that resist folding the limbs) to counteract gravity and keep you from falling in a heap. Finally, as soon as you are settled in a secure seated position, the nervous system permits the extensor muscles and the body as a whole to relax.
The musculoskeletal system does more than move the body, it also serves as a movable container for the internal organs. Just as a robot houses and protects its hidden supporting elements (power plant, integrated circuits, programmable computers, self-repairing components, and enough fuel to function for a reasonable length of time), so does the musculoskeletal
system house and protect the delicate internal organs. Hatha yoga postures teach us to control both the muscles that operate the extremities and the muscles that form the container.
The term "muscle" technically includes both its central fleshy part, the belly of the muscle, and its tendons. The belly of a muscle is composed of individual muscle fibers (muscle cells) which are surrounded by connective tissue fibers that run into a tendon. The tendon in turn connects the belly of the muscle to a bone.
Under ordinary circumstances muscle cells contract, or shorten, only because nerve impulses signal them to do so. When many nerve impulses per second travel to most of the individual fibers in a muscle, it pulls strongly on the tendon; if only a few nerve impulses per second travel to a smaller population of fibers within the muscle, it pulls weakly on the tendon; and if nerve impulses are totally absent the muscle is totally relaxed.
(Technical note: One of the most persistent misconceptions doggedly surviving in the biomedical community is that all muscles, even those at rest, always keep receiving at least some nerve impulses, fifty years of electromyography with fine-wire needle electrodes is at odds with this belief, documenting from the 1950s on that it's not necessarily true, and that with biofeedback training we can learn to relax most of our skeletal muscles completely.]
A muscle usually operates on a movable joint such as a hinge or a ball and socket, and when a muscle is stimulated to contract by the nervous system, the resulting tension is imparted to the bones on both sides of the fulcrum of the joint. In the case of a hinge such as the elbow that opens to about 180°, any muscle situated on the face of the hinge that can close will decrease the angle between the two bones, and any muscle situated on the back side of the hinge will open it up from a closcd or partially closed posit ion. For example, the biceps brachii muscle lies on the inside of the hinge, so it acts to flex the forearm (by definition, the segment of the upper extremity between the wrist and the elbow), pulling the hand toward the shoulder. The triceps brachii is situated on the back side of the arm (the segment of the upper extremity between the elbow and the shoulder) on the outside of the hinge, so it acts to extend the elbow, or unfold the hinge (fig. 1.1).
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