It increases muscular strength. It reduces tension and stress. It has a low potential for injury, and it doesn't even look like exercise. Why, then, don't more people practice yoga. People think of yoga as being passive and mystical - an otherworldly activity that doesn't relate to their lives. People are experiencing a vacuum because of all the outward directed activity, and they are going to have to go back to the experience of self. Although the Indian discipline of yoga has been practiced for more than 5,000 years, in this country there are few followers. Almost half the American adult population swims and close to a quarter runs or jogs, yet only 2 percent practices yoga. The word yoga derives from the Sanskrit root yuj, meaning to yoke or connect. Through yoga's various techniques, one is said to arrive at mental and physical equilibrium, better health and inner peace. It has been described as providing, in effect, a ''work-in'' rather than a workout. There are at least eight main branches of yoga and several offshoots of each, but essentially there are only two concerned with exercise: hatha yoga and kundalini yoga. Hatha is the most popular type of yoga in the Western world. It is a slow-paced discipline that emphasizes controlled breathing and assuming various physical poses. It is said to aid the nervous system, the glands and the vital organs. Kundalini, which was introduced to this country in 1969 by Yogi Bhajan, is more active, combining various modes of breathing, movement and meditation. It is based on the idea that body energy that is coiled below the base of the spine can be tapped so that it travels upward through different energy centers or chakras until it reaches the head. At this point one arrives at one's highest potential. Classically, there are 84 basic yoga positions, or asanas, which are coordinated with special breathing techniques. The asanas range from simple bends and twists to pretzel-like contortions reserved for the most advanced practitioners. The various poses elongate the muscles and build flexibility. Along with the proper breathing, they help rid the body of tension. Static holds isolate and strengthen particular muscles. Asanas have been evolved over the centuries so as to exercise every muscle, nerve and gland in the body. They secure a fine physique, which is strong and elastic without being muscle-bound, and they keep the body free from disease. They reduce fatigue and soothe the nerves. But their real importance lies in the way they train and discipline the mind.