Pain can initiate or increase stress by activating the neural pathways that stress and pain share.
Similarly stress can initiate or amplify the experience of pain by firing up their shared neural pathways and triggering or adding to the pain signal.
The body can stay tensed in a guarding reaction long after a traumatic incident has passed. Toxic metabolic by-products of constant muscle tension irritate nerves causing pain. In this way memories of injuries or traumatic incidents can perpetuate stress and pain.
Long term stress can damage the body. Permanently tense muscles can bend bone and pull the skeleton out of alignment causing distortions in gait and posture which lead to more damage and more pain.
The stress pages outline the damage that stress causes with examples of exercises to relax.