It's not enough to build muscle and achieve aerobic fitness. You need to think about flexibility, too. Stretching can help. You may think of stretching as something performed only by runners or gymnasts. But we all need to stretch in order to protect our mobility and independence. "A lot of people don't understand that stretching has to happen on a regular basis. It should be daily," says David Nolan, a physical therapist at Harvard-affiliated Massachusetts General Hospital.