A normal muscle at rest does not show any electrical activity on an electromyography, or EMG recording, but when the muscle contracts, it shows electrical activity as a smooth action potential, or wave line, states Healthgrades. The action potential's size and shape reveals the muscle's response to its controlling nerve.
Abnormal muscle activity is revealed when the muscle shows electrical activity at rest. When the muscle contracts, it shows an abnormal action potential pattern in the size or shape of the wave changes, notes Healthgrades. For example, a muscle that is constantly active may be spastic.The electromyography reveals what is occurring in the muscle, and a neurologist determines why it is occurring, after reading the results and information from other sources. The results may mean that the muscle is damaged or that a problem exists with the nerves that control the muscle.