What Happens During Stretching
If a muscle is not used often enough, it tends to become shorter, tighter and much weaker. Often, this happens gradually over time, leaving many adults vulnerable to injuries, back pain or chronic muscle overuse or overstress.
A regular flexibility program (which isolates and gently stretches tight muscles and their tendons) can help correct the problem and improve your overall posture and balance in the process. Stretching works primarily by causing the elastic connective tissue surrounding a muscle and its tendons to lengthen slightly. Repeated stretching (on a daily or near-daily basis) can make the entire muscle longer.
For a stretching session to have a permanent lengthening effect, however, the connective tissue must be warmed up first, either through physical activity or by a heating device. Many experts suggest stretching after a brief warm up. For the best results, then, you should stretch a bit before starting to exercise, but then stretch again during or immediately after your workout or just after taking a hot bath or shower. Stretching "cold" before a workout will produce a short-term increase in muscle length (helpful for "working the kinks out" before you get started), but it is not as effective as stretching after a warm up.