Flexibility is important for runners because it provides power in the hip muscles and stretches the torso, making it more passive and effective. Muscular strength is defined as the capacity of a muscle group to exert maximal force against a resistance. Strength training is an example of muscular strength in running. The more flexible you are, the more energy you can conserve when running down hills or over obstacles. Studies show that flexibility decreases as we age, so it's important to maintain flexibility if you want to keep running into your 60s and beyond. Regular exercise such as yoga, stretching, and weightlifting can help increase your range of motion and muscular strength, which will help you run better. Research published in 2004 by the American College of Sports Medicine found that people who completed a series of stretching exercises three times a week for eight weeks were able to stretch farther during a fitness test than those who didn't participate in the program. They also used less oxygen during the test, which indicates that greater flexibility leads to improved running form and may even reduce the risk of injury. In addition to being important for aging runners, strong muscles and joints provide support for healthy bones. Studies show that individuals who carry out regular strength training have stronger bones than those who don't exercise regularly.