Question

In L5 root involvement, which among the following is not affected?

• A. Thigh adduction
• B. Knee flexion
• C. Knee extension
• D. Toe extension

Hint:

Thigh adductors are obturator-supplied (L2 to L4), outside the L5 myotome.

Answer 1

The Correct Option is A

Step 1: Map the L5 myotome. The L5 nerve root contributes to hip extension and abduction, knee flexion, ankle dorsiflexion, and especially big toe and toe extension (extensor hallucis longus is the signature L5 muscle).
Step 2: Check each option against L5. Knee flexion (hamstrings) is partly L5 and S1, so it can be affected. Knee extension (quadriceps) is mainly L3 and L4 but receives an L5 contribution, so it may be mildly affected. Toe extension (extensor digitorum longus and extensor hallucis longus) is a core L5 function and is clearly affected.
Step 3: Examine thigh adduction. The adductor muscles of the thigh are supplied by the obturator nerve from roots L2, L3 and L4. This action does not involve L5.
Step 4: Conclusion. Thigh adduction is preserved in an L5 lesion.
Hence the correct answer is option A, Thigh adduction.