At the L5 level, which muscle groups show 3/5 strength?

At the L5 level, which muscle groups show 3/5 strength?

• A. Lateral knee flexors and ankle invertors
• B. Hip abductors
• C. Hip adductors
• D. Long toe extensors

Answer: (A)

At the L5 motor level, the muscles most closely tied to the L5 nerve root tend to show this level of weakness. A 3/5 grade means the person can move the muscle against gravity but cannot resist further manual pressure. The lateral knee flexors and ankle invertors are classic examples of L5-innervated groups that often present at this grade. Lateral knee flexion involves the short head of the biceps femoris (among others) and is primarily supplied by L5–S1 roots, so weakness here tends to show as the ability to flex the knee against gravity but not against resistance. Ankle inversion is mainly powered by tibialis posterior (with contribution from tibialis anterior), both receiving L4–L5 input; weakness manifests as inability to invert the foot when resistance is applied, while gravity-assisted movement remains possible. Other muscle groups listed involve different nerve roots (for example, hip abductors are largely L4–S1) and thus don’t map as cleanly to a 3/5 strength at L5.

At the L5 motor level, the muscles most closely tied to the L5 nerve root tend to show this level of weakness. A 3/5 grade means the person can move the muscle against gravity but cannot resist further manual pressure. The lateral knee flexors and ankle invertors are classic examples of L5-innervated groups that often present at this grade.

Lateral knee flexion involves the short head of the biceps femoris (among others) and is primarily supplied by L5–S1 roots, so weakness here tends to show as the ability to flex the knee against gravity but not against resistance. Ankle inversion is mainly powered by tibialis posterior (with contribution from tibialis anterior), both receiving L4–L5 input; weakness manifests as inability to invert the foot when resistance is applied, while gravity-assisted movement remains possible.

Other muscle groups listed involve different nerve roots (for example, hip abductors are largely L4–S1) and thus don’t map as cleanly to a 3/5 strength at L5.