The spinal cord carries information between your brain and your body, specifically your skin and voluntary muscles.
Each of your 31 pairs of spinal nerves controls a set of muscles and an area of skin.
A myotome is the group of muscles controlled by one pair of spinal nerves. Myo means muscle. Tome means section. Testing your ability to move joints, which are part of a particular myotome, can tell your doctor a lot about the severity of your Spinal Cord Injury (or SCI).
This is because the SCI disrupts signals that are normally sent between your brain and voluntary muscles.