"When the total CPK level is very high, it usually means there has been injury or stress to the heart, the brain, or muscle tissue. For example, when a muscle is damaged, CPK leaks into the bloodstream. Determining which specific form of CPK is high helps doctor's determine which exact tissue has been damaged."

http://www.nlm.nih.gov/medlineplus/ency/article/003503.htm

The part about muscles releasing proteins when working out is definitely true, I just don't know about the rest. It's something I was told by my sports doctor though.