Which movement is tested by the C5 myotome?

Which movement is tested by the C5 myotome?

• A. Ankle dorsiflexion
• B. Elbow extension and wrist flexion
• C. Shoulder elevation (C4)
• D. Shoulder abduction and external rotation

Answer: (D)

Understanding myotomes means linking a muscle action to its nerve root. The C5 myotome mainly controls shoulder abduction through the deltoid muscle, which is why lifting the arm sideways at the shoulder is a classic test for this level. External rotation is also influenced by C5 via the infraspinatus muscle, so including external rotation in the test makes it a more complete reflection of C5 motor function. That’s why the option describing shoulder abduction and external rotation best matches the C5 myotome. By contrast, ankle dorsiflexion maps to L4–L5, elbow extension to C7, and shoulder elevation (shrug) to C4, which is why they aren’t the correct representation for C5.

Understanding myotomes means linking a muscle action to its nerve root. The C5 myotome mainly controls shoulder abduction through the deltoid muscle, which is why lifting the arm sideways at the shoulder is a classic test for this level. External rotation is also influenced by C5 via the infraspinatus muscle, so including external rotation in the test makes it a more complete reflection of C5 motor function. That’s why the option describing shoulder abduction and external rotation best matches the C5 myotome. By contrast, ankle dorsiflexion maps to L4–L5, elbow extension to C7, and shoulder elevation (shrug) to C4, which is why they aren’t the correct representation for C5.