First, the science. In any given area, muscles and bones which all work together to allow you to move and be functional. The nerves, which carry the brain's directions to the body and relay information back to the brain, travel closely in all of these areas as well. When we overuse our body in any way we get muscle tissue trauma or fatigue. Since our muscular system, the mover of our body, is supposed to work in sync and contract and relax in just the right time, when there is trauma or fatigue, it won't perform the way it's supposed to. This leads to a number of possibilities: 1) damage to the actual muscle or an associated muscle group (strain), 2) damage to the muscle-bone junction, and 3) stress to the joint (where two bones meet, allowing for motion) which creates dysfunction (in the spine, this is "subluxation", elsewhere this is "joint dysfunction").