Answered by Malachy P. McHugh on Monday, June 1, 2009
There are no exercises that can permanently elongate your muscles—unless you're actually growing. Muscle length is determined by the joint (or joints) across which the muscle is attached. The hamstring, for example, originates from the ischial tuberosity (the bone that you sit on) and inserts behind the knee on the fibula and tibia bones. You can figure out the maximum length of your hamstring by performing a straight leg raise (SLR), stretching it to the point where it is still comfortable. Having tight hamstrings will limit reaching maximum, but doing daily stretching exercises over several weeks can increase the functional length of the hamstrings and give you a greater range of motion. Conversely, it's said that strength training shortens muscles. This is, in fact, true when the strengthening exercises involve small movements in which the muscle is primarily in a shortened position. For example, doing biceps curls from the 90° position to full flexion (where your arm is fully bent) will lead to an adaptive shortening of the muscle if repeated daily over several weeks to months. Therefore, to prevent this adaptive shortening, it's important to exercise the muscle through the full range of motion (i.e., fully extend and fully bend your arm).