The word yoga (from the same root as the English "yoke") is Sanskrit for "union." The system originated over 4,000 years ago in India, where it was traditionally practiced by Hindu ascetics, or yogis. Patanjali, the father of yoga, defined the path of the yogi in his Yoga Sutras (or aphorisms) in the 3rd century B.C. Yoga was introduced to the West in the 19th century, when scholars translated ancient Hindu religious texts. Initially it attracted few followers, but has grown enormously in popularity since the 1960s, along with many other Eastern disciplines promoting well-being. By 1996, 500,000 people in the UK alone were doing yoga. It is part of some American health care programs, such as Dr. Ornish's regime for reversing heart disease.

In its purest form, yoga is a fully integrated system controlling all aspects of life. In the "Eight Limbs of Yoga," the yogi Patanjali described a series of increasingly spiritual stages on the path to enlightenment. These begin with ethical guidelines, including healthy eating habits and high levels of personal hygiene, and progress through the practice of asanas (physical postures) and pranayama (breathing techniques) to meditation and, eventually, withdrawal to the supreme level of pure consciousness.

Today there are many types of yoga, including yoga therapy to maintain health and help specific medical conditions, and ashtanga, or power yoga. While meditation is central to some forms of devotional yoga, the most popular form in the West is hatha yoga, using asanas and pranayama. Breath is seen as the outward form of prana ("life energy"), and controlled breathing regulates prana in the body.

Hatha means "balance," reflecting the balance of mind and body; a serene mind produces regular breathing and a relaxed body. Conversely, a relaxed body promotes steady breathing and a calm mind. Asanas are designed to benefit both mind and body; performed slowly and deliberately, they are coordinated with the breathing.

Each asana has an individual shape and it is important to maintain these shapes correctly. Regular practice is preferred, since it helps to maximize the benefits of performing the asanas.

Most research has been carried out using gentle forms of yoga; so far no medical evidence exists to substantiate claims for more advanced forms. Some reputable studies have, however, confirmed that yoga can benefit a range of conditions. One well-­known study from the 1960s was done at the Menninger Foundation in Texas, where researchers developing biofeedback techniques found that yoga meditation clearly affects the heart and circulation.

A 1990 study in The Lancet showed that yoga breathing reduced the frequency of asthma attacks, and at Oxford University in 1993, psychologists found yoga breathing more effective for restoring energy than relaxation or visualization. In the British Journal of Rheumatology, a 1994 study indicated that yoga therapy could benefit people with rheumatoid arthritis.

The medical establishment is skeptical of some of the claims of yoga (for example, that yoga can affect the nervous system and hormone levels), but most doctors would not object to it as an exercise or relaxation technique, and many would acknowledge that it deserves closer examination as a treatment for conditions such as heart disease. The development of yoga therapy, which uses specific asanas and pranayama to treat medical complaints, is likely to be followed with considerable interest.

Most people find it helpful to join a beginners' class before continuing on their own. It is advisable to attend classes given by a qualified teacher. You should bring lightweight clothing to wear during the class (leotards and tights or cotton shorts and tops are fine). It is not necessary to wear sneakers, since yoga is best done barefoot. The class should be held in a warm room with no drafts. You may need a rug or blanket to cover yourself while in the relaxation posture, and a mat if the floor is hard or slippery.

Most yoga classes last 40-90 minutes. After guiding you through gentle warm­up exercises, your yoga teacher will show you the correct way to perform yoga asanas, or postures, which you will then be asked to practice with the other pupils in the class. Sitting and standing asanas are held for between 20 seconds and 2 minutes and are designed to stretch and strengthen the body. Inverted postures, such as head or shoulder balance, can be held for longer. You will be encouraged to relax physically and mentally throughout your practice, to be conscious of your breathing and not to hold your breath.

Class sizes vary widely, but around 5-20 people is common. The atmosphere in the class should be focused but relaxed. When practicing the asanas, you should never be tempted to push yourself too far or to compete with your classmates.

If practicing yoga for medical reasons, always consult a teacher who is trained in yoga therapy. Certain asanas and breathing exercises have proved effective in relieving individual medical conditions, but it is important that you do the right ones.

Allow 2-3 hours after a meal before exercising.

Take care if you are practicing yoga during pregnancy or menstruation. Certain asanas, such as the headstand, should be avoided during pregnancy and, according to some experts, during menstruation.

Headstands and some other asanas are not advised if you have a neck or back injury, high blood pressure, circulatory problems, heart disease, or disorders of the brain, ears, or eyes.