Myotomes are generally more difficult to test than dermatomes because each skeletal muscle in the body often develops from more than one somite and is therefore innervated by nerves derived from more than one spinal cord level (Fig. 1.37).
Testing movements at successive joints can help in localizing lesions to specific nerves or to a specific spinal cord level. For example:
Muscles that move the shoulder joint are innervated mainly by spinal nerves from spinal cord levels C5 and C6.