The physical activity guidelines for Americans clearly state that both aerobic (endurance) and muscle strengthening (resistance) physical activity are equally beneficial and important to any exercise regime. There are benefits that are achieved from strength training that do not occur with aerobic activity alone.
Strength training is defined as muscle-strengthening or bone-strengthening activity and includes activities that work all the different parts of the body - legs, hips, back, chest, stomach, shoulders and arms. Exercises for each muscle group should be repeated 8 to 12 times per session. Muscle-strengthening activities count if they involve a moderate to high level of intensity or effort and 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 difficult 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 is progressive over time.
Source: ihrsa.org/publicpolicy/HHS_GR4_overview.pdf
