(MayoClinic.com) Your friends enjoy using the weight machines and free weights at the fitness center. And you see the results of their hard work — toned muscles and an overall improved physique. You'd like to start a weight training program, but you're not sure you have the time. Think again.

Weight training is a type of strength training that uses weights for resistance. Weight training provides a stress to the muscles that causes them to adapt and get stronger, similar to the way aerobic conditioning strengthens your heart. Weight training can be performed with free weights, such as barbells and dumbbells, or by using weight machines.

You don't have to be in the weight room for 90 minutes a day to see results. For most people, short weight training sessions a couple of times a week are more practical than are extended daily workouts.

You can see significant improvement in your strength with just two or three 20- or 30-minute weight training sessions a week. That frequency also meets activity recommendations for healthy adults, which call for strength training at least twice a week — in addition to at least 150 minutes a week of moderate aerobic activity.

Weight training offers important health benefits when done properly. But it can lead to injuries, such as sprains, strains and fractures, if it's not done correctly.

For best results, consider these basic weight training principles:

• Learn proper technique. If you're a novice, work with a trainer or other fitness specialist to learn correct technique. Even experienced athletes may need to brush up on their form from time to time.
• Do a single set of repetitions. Theories on the best way to approach weight training abound, including countless repetitions and hours at the gym. But research shows that a single set of 12 repetitions with the proper weight can build muscle efficiently in most people and can be as effective as three sets of the same exercise.
• Use the proper weight. How do you know what's the proper weight? It's one that's heavy enough to tire your muscles after about 12 to 15 repetitions. You should be just barely able to finish the last repetition.
• Start slowly. If you're a beginner, you may find that you're able to lift only a few pounds. That's OK. Once your muscles, tendons and ligaments get used to weight training exercises, you may be surprised at how quickly you progress. Once you can easily do 12 repetitions with a particular weight, gradually increase the weight.
• Take time to rest. To give your muscles time to recover, rest one full day between exercising each specific muscle group. You might choose to work the major muscle groups at a single session two or three times a week — or plan daily sessions for specific muscle groups. For example, on Monday work your arms and shoulders, on Tuesday work your legs, and so on.

Lean muscle mass naturally decreases with age. If you don't do anything to replace the muscle loss, it'll be replaced with fat. But weight training can help you reverse the trend — at any age. As your muscle mass increases, you'll be able to work harder and longer before you get tired. You'll maintain joint flexibility, increase bone density and better manage your weight. So don't wait. Get started today.

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