Is it time, once again, to stretch? For decades, many of us stretched before a workout, usually by reaching toward our toes or leaning against a wall to elongate our hamstrings, then holding that pose without moving until it felt uncomfortable, a technique known as static stretching. Most people, including scientists and entire generations of elementary-school P.E. teachers, believed that static stretching lengthened muscles and increased flexibility, making people better able to perform athletically.