lift the torso up from a standing forward bend, 'l'hen as you slowly lower back down into the bend, the back muscles resist the force of gravity that is pulling you forward, lengthening eccentrically to smooth your descent.
isotonic and isometric activity
Most readers are already familiar with the terms "isotonic" and "isometric." Strictly speaking, the term isotonic refers solely to shortening of a muscle under a constant load, but this never happens in reality except in the case of vanishingly small ranges of motion. Over time, however, the term isotonic has become corrupted to apply generally to exercise that involves movement, usually under conditions of moderate or minimal resistance. Isometric exercise, on the other hand, refers to something more precise— holding still, often under conditions of substantial or maximum resistance. Raising and lowering a book repetitively is an isotonic exercise for the biceps brachii and its synergists, and holding it still, neither allowing it to fall nor raising it, is an isometric exercise for the same muscles. Most athletic endeavors involve isotonic exercise because they involve movement. Japanese sumo wrestling between equally matched, tightly gripped, and momentarily immobile opponents is one obvious exception. And isometric exercise is also exemplified by any and every hatha yoga posture which you are holding steadily with muscular effort.
relaxation, stretch, and mobility
If few or no nerve impulses are impinging on muscle fibers, the muscle tissue will be relaxed, as when you are in the corpse posture (fig. 1.14). But if a relaxed muscle is stretched, the situation becomes more complex. Working with a partner can make this plain. If you lie down and lift your hands straight overhead, and then ask someone to stretch you gently by pulling on your wrists, you will notice that you can easily go with the stretch provided you have good flexibility. But if your partner pulls too suddenly or if there is any appreciable pain, the nervous system will resist relaxation and keep the muscles tense; or at the least, you will sense them tightening up to resist the stretch. Finally, if you allow yourself to remain near your limit of passive but comfortable stretch for a while longer, you may feel the muscles relax again, allowing your partner to pull more insistently.
Many of these same responses are apparent if you set up similar conditions of stretching on your own, as when you place your hands overhead against a wall and stretch the underside of the arms. This is more demanding of your concentration than relaxing into someone else's work, however, because you are concentrating on two tasks at the same time: creating the necessary conditions for the stretch, and relaxing into that effort. But the
28 ANATOMY OF HATHA Y<XJA
same rules apply. If you go too far and too quickly, pain inhibits lengthening, prevents relaxation, and spoils the work.
muscular activity in a lunging posture
To discover for yourself' how skeletal muscles operate in hatha yoga, tiy a warrior posture (warrior I) with the feet spread wide apart, the hands stretched overhead, and the palms together (figs. 1.2 and 7.20). Feel what happens as you slowly pull the arms to the rear and lower your weight. To pull the arms up and back, the muscles facing the rear have to shorten concentrically, while antagonist muscles facing the front passively resist the stretch and possibly completion of the posture. As you lower your weight the quadriceps femoris muscle on the front of the flexed thigh resists gravity and lengthens eccentrically. Finally, as you hold still in the posture, muscles throughout the body will be in a state of isometric contraction.
Several important principles of musculoskeletal activity cannot be addressed until we have considered the nervous system and the connective tissues in detail. For now, it is enough to realize that all muscular activity, whether it be contraction of individual cells, isotonic or isometric exercise, agonist or antagonist activity, concentric shortening, or eccentric lengthening, takes place strictly under the guidance of the nervous system.
muscles facing the front resist pulling of the arms to the rear
muscles facing the front resist pulling of the arms to the rear right quadriceps femoris muscle lengthens eccentncally
Figure 1.2. Warrior I pose
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