Muscles function optimally when in a flexible state.  Dynamic stretching before a workout and static stretching following a workout is my recommendation.  Dynamic stretching prepare our muscles for work while static stretching reward our muscles for the work while reminding the muscles to rest up, but in a lengthened position.

Muscles need flexibility.  Joints need stability or mobility depending on its function.  The shoulder and hip joints are designed for mobility and should be trained to be mobile.  The lumbar spine in comparison, is a family of joints that requires stability and should be strengthened to stay strong and stable.  (See my facebook page for videos on increasing spine mobility.)

The root of injury is often due to dysfunction at the joint above or below.  Take the low back for example.  Most of the low back pain patients I see have tight and weak hips.  When the hip has lost its ability to provide mobility, the lumbar spine has to compensate to move effectively.  Similarly, when the Thorasic spine is not moving as it should, the lumbar or cervical spine/shoulder girdle tries to step up which often results in disc disease and annular tears in the spine.  

Tennis players and golfers gain power from hip rotation and if the motion is not available at the hips, the lumbar spine will try to provide the rotation and over time the rotational stress placed on the lumbar spine will manifest itself in the form of injury.