Which muscle is NOT typically part of the C5–C6 myotomes?

Which muscle is NOT typically part of the C5–C6 myotomes?

• A. Biceps Brachii
• B. Triceps
• C. Brachialis
• D. Deltoid

Answer: (B)

Muscle groups mapped to nerve roots define myotomes, so testing patterns reflect the root levels that predominantly control each muscle. For the C5–C6 level, the common plays are shoulder abduction (deltoid) and elbow flexion (biceps brachii and brachialis). The triceps, which extends the elbow, is mainly driven by C7 (with some contribution from adjacent roots in some individuals), not by C5–C6. So, the triceps is not typically part of the C5–C6 myotomes. In practice, weakness in elbow extension suggests C7 involvement, while weakness in elbow flexion and shoulder abduction points to C5–C6. (Variations can exist, but this is the usual pattern.)

Muscle groups mapped to nerve roots define myotomes, so testing patterns reflect the root levels that predominantly control each muscle. For the C5–C6 level, the common plays are shoulder abduction (deltoid) and elbow flexion (biceps brachii and brachialis). The triceps, which extends the elbow, is mainly driven by C7 (with some contribution from adjacent roots in some individuals), not by C5–C6. So, the triceps is not typically part of the C5–C6 myotomes. In practice, weakness in elbow extension suggests C7 involvement, while weakness in elbow flexion and shoulder abduction points to C5–C6. (Variations can exist, but this is the usual pattern.)