Muscle-strengthening activities make muscles do more work than they are accustomed to during activities of daily life. Muscle strengthening includes strength training, resistance training, and muscular endurance exercises.
Research has shown that strength training increases power and endurance and also:
In addition, in women age 40 or older, strength training helps prevent loss of muscle mass.
A strength training program may include free weights or weight-training machines. But you don't need extensive equipment to do strength-training. You can do Pilates mat work, using simply a stability ball, or do exercises that use your own body weight for resistance, such as push-ups, sit-ups, lunges, squats, and dips.
The Physical Activity Guidelines for Americans  recommend incorporating muscle strengthening activities at least two times per 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. Development of muscle strength and endurance is progressive over time. This means that gradual increases in the amount of weight or the days per week of exercise will result in stronger muscles.