Which nerve root is responsible for dorsal extension of the foot?

From the Guidelines

The nerve root primarily responsible for plantar dorsiflexion is L5, as indicated by the evaluation of great toe and foot dorsiflexion strength in a neurologic examination. Plantar dorsiflexion, which is the movement of lifting the foot upward toward the shin, is primarily controlled by the muscles innervated by the L5 nerve root. According to the evidence provided by 1 and 1, a focused neurologic examination should include evaluation of great toe and foot dorsiflexion strength to assess the L5 nerve root function. This is crucial because more than 90% of symptomatic lumbar disc herniations occur at the L4/L5 and L5/S1 levels, and damage to the L5 nerve root can result in weakness of dorsiflexion. Key points to consider in the examination include:

• Evaluation of knee strength and reflexes for L4 nerve root function
• Assessment of great toe and foot dorsiflexion strength for L5 nerve root function
• Examination of foot plantarflexion and ankle reflexes for S1 nerve root function
• Distribution of sensory symptoms to determine the affected nerve root. In clinical practice, identifying the specific nerve root responsible for plantar dorsiflexion is essential for diagnosing and treating conditions such as herniated discs or lumbar spinal stenosis, as mentioned in 1 and 1.

From the Research

Nerve Root Responsible for Plantar Dorsiflexion

• The nerve root responsible for plantar dorsiflexion is associated with the L3 and L4 roots, as stimulation of these roots can produce dorsiflexion torque 2.
• The L5 roots are primarily associated with plantar flexion, and the S1 roots are associated with plantar flexion, lateral leg and foot rotation, and inversion 2.
• There is no direct evidence in the provided studies to suggest that the S1 roots are responsible for plantar dorsiflexion, as they are primarily associated with plantar flexion and other movements 2, 3.
• The recovery of ankle dorsiflexion weakness following lumbar decompressive surgery is possible, with significant improvements seen at 6 weeks post-surgery, and further small improvements thereafter 4.

Associated Movements and Nerve Roots

• The L3 and L4 nerve roots are associated with dorsiflexion torque, while the L5 roots are associated with plantar flexion 2.
• The S1 roots are associated with plantar flexion, lateral leg and foot rotation, and inversion, but not with dorsiflexion 2.
• Foot dystonia can be caused by lumbar canal stenosis and/or nerve root compression, particularly of the L5 and S1 nerve roots 3.

Clinical Implications

• Decompressive surgery can lead to significant improvements in ankle dorsiflexion weakness, with the degree of deficit at presentation being predictive of the extent of recovery 4.
• Younger patients tend to make a better recovery from ankle dorsiflexion weakness following decompressive surgery 4.