Muscular fitness
Surveys show that a large segment of the U.S. population, including recreational athletes, lack muscular fitness. This aspect of fitness actually is a combination of strength and endurance. Strength is the greatest force a muscle can exert in one effort—for example, the ability to lift an object one time. Endurance is the muscle's ability to make repeated efforts.

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Even if you have no intention of becoming an Olympic weight-lifter, there's still reason to care about muscular fitness. It influences your ability to do everyday chores, like housework and yard work. It affects how easily you can carry a bag of groceries or lift a young child. It's also at the core of physical skill and sports performance, affecting how hard you swing a softball bat or how long you last on the tennis court.

Muscle-strengthening exercises are likely to improve your stamina and your energy. Equally important, they increase resistance to injury. People with strong muscles are less likely to suffer everyday muscle aches and pains. They also have less strain on their hearts.

Resistance training. Building muscular fitness involves resistance training, progressively overloading your muscles so that they get stronger to meet the challenge. This can be done with exercises that use your body to exert force, like push-ups, chin-ups, and sit-ups. Commonly, people use weight training, also called weight lifting, to provide resistance.

Strength gains come from resistance—how much weight you lift. Endurance is achieved through repetition—how many times you lift a weight in succession. Both are important to develop.

Experts advise you to start any weight-training program with light weights and easy repetition. Start with a weight that you can lift comfortably eight to 12 times. Try to do a second set of each exercise after a break of a few minutes. Do at least one exercise for each muscle group, moving from the larger muscles (the legs) down to smaller ones (arms and biceps).

Strength gains come when you work with close to the heaviest weight that you can lift comfortably. This is the overload principle. You'll see the quickest benefits if you lift the maximum amount during fewer repetitions of each exercise. Using a weight that's too heavy, however, can lead to injury. And if you're interested in all-around conditioning, it's best to start with low amounts and progress gradually.

Guidelines for strength training

• Learn the proper form. If you use the wrong technique, you could injure a muscle. Proper technique involves slow, controlled lifts—three or four seconds for each movement through the entire range of motion—to provide the most resistance.
• Breathe while you lift. Don't hold your breath because that can cause your blood pressure to rise. Breathe out when you are doing the most strenuous part of the exercise.
• Weight train on days when you're not doing hard aerobic workouts.
• Allow at least one day between weight-training sessions to let your muscles recover. Proper recovery will help you build strength faster.

In 1990, the American College of Sports Medicine added strength training to its list of exercise guidelines, recommending that people do at least two sessions a week. Make sure not to work the same muscles on two consecutive days.

Flexibility
Flexibility, sometimes called the forgotten fitness factor, refers to the range of motion in joints. Flexibility depends on the condition of muscles that control a joint. If muscles are too taut, the joint fails to move through its full range. As people age, they tend to lose range of motion. Inactivity can contribute too. If you're out of shape and inactive, you often may feel stiff or have trouble moving, bending, and straightening.

Exercises to improve flexibility can help you bend, turn, and reach more easily. They also improve agility and posture, as well as protect against injury from sudden movements. If you're flexible, you may be saved from a backache the next time you reach quickly to pick up something from the floor.

Stretching exercises. Stretching exercises are used to lengthen muscles and improve flexibility. Experts usually recommend static stretching, a method that involves slow, continuous stretches held in position. Here are some guidelines:

• Stretch slowly and gradually. Stretch until you feel tension in the muscle. Don't bounce. Bouncing can cause small tears and muscle soreness.
• Don't stretch to the point of feeling pain, a signal that you've pushed too far or are doing something incorrectly and possibly injuring muscle tissue.
• Hold your stretch for at least 15 seconds. Holding for 30 seconds is even better. Breathe naturally as you stretch: Don't hold your breath. Repeat the same stretch one or more times.

Try to find time throughout the day to stretch. For more comprehensive stretching, plan a longer routine that targets major muscles after your aerobic workouts. That's when muscles are warmed up and pliable, which will make stretching most effective. If you do stretches before activities, make sure to warm up first for five to 10 minutes with walking or easy stationary cycling.

The American College of Sports Medicine advises stretching at least three times a week. Each stretch should be done for 10–30 seconds, to a position of mild discomfort, and repeated three to five times.

Several forms of exercise lead joints through an extended range of motion and can improve flexibility, too. These include yoga, t'ai chi, and many aquatic exercises.