Watch the Left Hand Stretching 2 online guitar lesson by Brad Carlton from Essential Techniques

You could label this lesson "guitar gymnastics" as it deals with the issues of stretching the left hand along the string axis as well as the fret axis. Stretching is the best warmup that exists as it prevents injuries, promotes flexibility, and allows for higher speed with a very relaxed touch. And of course it allows you to reach a greater portion of the fingerboard.