Strength training works muscles against resistance and has been the main activity at health and fitness clubs in the past. A great many people still do workouts that consist mostly of weight training, either with machines or with free weights, and most fitness experts still recommend this as part of an overall exercise program.
Weight lifting increases muscle bulk by a simple physiological principle: use any muscle more, and it will increase in size; stop using it, and it will decrease. This is an example of the body's quick responsiveness to the changing demands of the environment. In my experience many people who lift weights are attracted by the vision of having larger muscles and appearing more beautiful and more powerful. I find this focus on appearance unwise. In fact, some of the most powerful, fittest people I have met have bodies very unlike those of bodybuilders, with none of the muscle bulk and definition that are so fashionable in health and fitness clubs. I do not believe there is any necessary correlation between this appearance and the realities of strength, fitness, and health.
To my eye, the bodies of many weight lifters look more interesting than beautiful. Often the upper portions are overdeveloped and disproportionately large, especially the shoulders, upper arms, and chest. At its worst, bodybuilding actually sacrifices health for superficial appearance by encouraging unsound dietary practices (consumption of high-protein foods and protein and amino acid supplements), the use of dangerous drugs (anabolic steroids), and neglect of the real work of preventive health maintenance. The bodybuilders I have known are not as a group any healthier than other people, and some of them are less healthy because of the ways they think about and treat their bodies.
Nonetheless, if I find myself in a health club, I usually do a circuit of the weight machines and some work with free weights, because this activity feels good to me. You might want to do the same. If you are drawn to this kind of exercise, I would ask you to examine your motives carefully. If you do decide to lift weights on a regular basis, beware of instructors who try to persuade you to eat more protein or take protein powders, amino acids, or expensive herbal supplements that are supposed to give the muscle-bulking effects of steroids in a natural way without any side effects.
The Pilates Method is a popular and intense form of strength training based on the idea that the abdominal and pelvic muscles are the body's power center. Unlike weight training it does not isolate muscles. Pilates is done with an instructor or at a health club, using special machines or rolled-up towels for floor exercises. Movements are slow and aim to engage the mind as well as the body.
One benefit of working muscles against resistance is that it will increase your bone density, providing an antidote to the hormonal and metabolic changes that can cause calcium loss arid development of osteoporosis. Women become susceptible to this debilitating disease earlier than men, but women who have good habits of weight-bearing exercise have more protection than those who do not. Weight lifting is one kind of weight-bearing exercise; some of the aerobic activities discussed above, like running, dancing, and using aerobic machines, also strengthen bones while they condition your heart and lungs.
Another benefit of strength training is that increased muscle mass correlates with increased metabolism and ability to burn calories.
Learn more about exercise and fitness from Dr. Weil.