The L3 myotome is tested by knee extension, the ISNCSCI key movement for L3, through the quadriceps femoris. L3 also helps hip adduction. The patellar reflex tests L3 and L4.
| Nerve root | L3 |
|---|---|
| Area | Knee extension (ISNCSCI key muscles: knee extensors) |
| How to test | The patient straightens the knee against resistance at the lower shin, starting from a bent position. Strength is graded 0-5 on the Medical Research Council scale and compared side to side. |
| Muscles | |
| Nerve Supply | Femoral nerve; Obturator nerve |
Straightens the knee and helps pull the thigh inward.
Quadriceps weakness from L3 or L4 radiculopathy causes the knee to give way on stairs. Femoral neuropathy causes the same weakness, but hip adduction is spared because adductors are supplied by the obturator nerve. Myotome charts vary slightly between sources, and most muscles receive more than one root, so a single root lesion usually weakens rather than paralyses.
Compression of the L3 root causing quadriceps weakness, anterior thigh numbness and a reduced patellar reflex.
Femoral nerve damage causing quadriceps weakness and knee buckling, with intact hip adduction.
A painful diabetic disorder of the lumbosacral plexus that causes thigh pain followed by wasting and weakness of the quadriceps.
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