What is a myotome?

A myotome is defined as the group of muscles supplied by one spinal nerve root level. There are 31 pairs of spinal nerves (S2.13) each contributing to the innervations of many muscles (e.g. C₅ innervates parts of supraspinatus, infraspinatus, deltoid and biceps). The muscles supplied by a single nerve root level are generally involved in a common muscle action/s and it is this muscle action that is assessed. The list below identifies a simplified version of the actions associated with each spinal nerve root level (Grieve 2004).

C1/C2 – neck flexion/neck extension (see Fig. 31.1)

Figure 31.1 Testing for myotome C3.

C3 – neck side flexion

C4 – shoulder elevation

C5 – shoulder abduction (see Fig. 31.2)

Figure 31.2 Testing for myotome C5.

C6 – elbow flexion or forearm supination

C7 – elbow extension or wrist flexion

C8 – thumb extension or adduction and ulna deviation

T1 – finger abduction/adduction

L2 – hip flexion or adduction

L3 – knee extension (see Fig. 31.3)

Figure 31.3 Testing for myotome L3.

L4 – dorsi flexion at the ankle

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