One of the definitions of pain is the lack of oxygen to a muscle.

Causes:

Muscle being in a short or compressed  condition, repetitive actions, impacts, chemical imbalance, disease etc.

Pain is a signal God put in the body to tell us that we are in danger of injuring body parts or tissue. The greater the pain, the greater the risk or danger of the injury occurring. Stop activities that produce pain.

Tight muscles put too much tension on the points where they attach. Loosening muscles back to their designed resting length will reduce the strain or pull on the attachment points thus reducing the pain signal.

 What reduces the tension? Reciprocal inhibiton will restore the normal length of the muscle- however the brain needs to learn (be re-educated) that it is safe for the musle to go to its normal length without danger of pain (range of motion of arm for example). You need to stretch following treatments .

 Pain is like a credit card and movement (use) is the only acceptable payment- pills, shots & surgery are only postponments of payment- a day of reckoning is beckoning.