According to gate-control theory, a back massage would most likely reduce camaryn's physical aches and pains by causing the:

I believe the answer is: activation of large nerve fibers in your spinal cord

Gate control theory was popularized by Ronald Melzack and Patrick Wall,  it views that non-painful area input closes the doors or "gates" to the painful  area input, this exist in order to prevents pain sensation from coming through the central nervous system and being transferred across our body.