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The L3 spinal level is primarily associated with knee extension. This occurs because the L3 nerve root contributes to the innervation of the quadriceps muscle, which is the primary muscle responsible for extending the knee joint. The quadriceps is crucial for activities such as standing up and walking.

In contrast, the other options focus on functions that are not primarily controlled by the L3 nerve root. Hip flexion is mainly influenced by the L2 spinal level and also involves L3, but it is not the most direct association. Big toe extension is primarily controlled by the L5 nerve root, and plantar flexion is associated with the S1 nerve root, as it involves the gastrocnemius and soleus muscles. Therefore, knee extension remains the most accurate function linked specifically to the L3 spinal level.