"255 more on muscular endurance. The effects of muscle - strengthening activity are limited to the muscles doing the work. It's important to work all the major muscle groups of the body: the le gs, hips, back, abdomen, chest, shoulders, and arms. Muscle - strengthening activities provide additional benefits not found with aerobic activity. The benefits of muscle - strengthening activity include increased bone strength and muscular fitness. Muscle - s trengthening activities can also help maintain muscle mass during a program of weight loss. Muscle - strengthening activities make muscles do more work than they are accustomed to doing. That is, they overload the muscles. Resistance training, including wei ght training, is a familiar example of muscle - strengthening activity. Other examples include working with resistance bands, doing calisthenics that use body weight for resistance (such as push - ups, pull - ups, and sit - ups), carrying heavy loads, and heavy ga rdening (such as digging or hoeing). Muscle - strengthening activities count if they involve a moderate to high level of intensity or effort and work the major muscle groups of the body: the legs, hips, back, chest, abdomen, shoulders, and arms. Muscle stre ngthening activities for all the major muscle groups should be done at least 2 days a week. No specific amount of time is recommended for muscle strengthening, but muscle - strengthening exercises should be performed to the point at which it would be diffic ult to do another repetition without help. When resistance training is used to enhance muscle strength, one set of 8 to 12 repetitions of each exercise is effective, although two or three sets may be more effective. Development of muscle strength and endur ance is progressive over time. Increases in the amount of weight or the days a week of exercising will result in stronger muscles. Flexibility Flexibility is the ability of moving a joint through the range of motion. Flexibility is an important part of phy sical fitness. Some types of physical activity, such as dancing, require more flexibility than others. Stretching exercises are effective in increasing flexibility, and thereby can allow people to more easily do activities that require greater flexibility. For this reason, flexibility activities are an appropriate part of a physical activity program, even though they have no known health benefits and it is unclear whether they reduce risk of injury. Time spent doing flexibility activities by themselves does not count toward meeting the aerobic or muscle - strengthening Guidelines. Although there are not specific national guidelines for flexibility, adults should do flexibility exercises at least two or three days each week to improve range of motion. This can be done by holding a stretch for 10 - 30 seconds to the point of tightness or slight discomfort. Repeat each stretch two to four times, accumulating 60 seconds per stretch. Body composition"