An excellent article summarising how poor posture can affect your sporting performance through common problems such as muscle imbalances, weakness and tightness.
Muscles shorten because they are weak, because you hold them in a shortened position for long periods of time or because they are having excessive constant loads placed on them that they are not designed to cope with. Shortened muscles can also inhibit opposing muscles, making them weak.
This article specifically addresses low back pain and shoulder pain caused by common imbalances that I see every day. Many people have these problems without pain but when they start exercise, these problems are revealed though pain or inability to perform gym exercises correctly.
