Question:medium
In L5 root involvement, which among the following is not affected?
In L5 root involvement, which among the following is not affected?
Show Hint
Thigh adductors are obturator-supplied (L2 to L4), outside the L5 myotome.
Updated On: Jun 24, 2026
- Thigh adduction
- Knee flexion
- Knee extension
- Toe extension
Show Solution
The Correct Option is A
Solution and Explanation
Sort the listed actions by their root supply and then find the one that has no L5 fibres. The L5 root is famous for powering dorsiflexion and toe and big toe extension, and it also feeds hip and knee movements lower down the limb.
Toe extension depends on extensor hallucis longus and extensor digitorum longus, both flagship L5 muscles, so weakness here is expected with an L5 lesion. Knee flexion uses the hamstrings, which carry L5 and S1 fibres, so it can suffer. Knee extension through the quadriceps is chiefly L3 and L4 but has some L5 input, so it is not wholly spared. Thigh adduction, by contrast, is driven by the obturator-supplied adductor group rooted in L2, L3 and L4, with no L5 component at all. Because L5 contributes nothing to adduction, that movement is the one left intact.
\[\boxed{\text{Thigh adduction}}\]
Toe extension depends on extensor hallucis longus and extensor digitorum longus, both flagship L5 muscles, so weakness here is expected with an L5 lesion. Knee flexion uses the hamstrings, which carry L5 and S1 fibres, so it can suffer. Knee extension through the quadriceps is chiefly L3 and L4 but has some L5 input, so it is not wholly spared. Thigh adduction, by contrast, is driven by the obturator-supplied adductor group rooted in L2, L3 and L4, with no L5 component at all. Because L5 contributes nothing to adduction, that movement is the one left intact.
\[\boxed{\text{Thigh adduction}}\]
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