Evaluation of S1 Nerve Root, Motor, Sensory and Reflex

Courtesy: Prof Nabil Ebraheim, University of Toledo, Ohio, USA

Examination of the S1 Nerve Root

Overview

S1 nerve-root examination into three parts:

1. Sensory examination
   2. Motor examination
   3. Reflex examination

It also compares S1 with L5 and explains how different types of disc pathology at the L5–S1 level may affect different roots.

1. Sensory Examination

For S1, examine sensation over the:

Lateral side of the foot

gives a useful comparison across the foot:

• L4 – medial side of foot
• L5 – dorsum/top of foot
• S1  l-ateral side of foot

Quick recall

Medial – L4
Top – L5
Lateral – S1

2. Reflex Examination

The important reflex for S1 is:

Ankle reflex

So:

S1 – ankle jerk

3. Motor Examination of S1

S1 motor examination is more complex because several muscles are supplied by the S1 root.

The muscles mentioned are:

• Gluteus maximus
• Lateral hamstrings
• Gastrocnemius
• Flexor hallucis longus
• Peroneus longus
• Peroneus brevis

4. S1 Muscles From Proximal to Distal

from proximal to distal:

1. Gluteus maximus
2. Lateral hamstring
   • Biceps femoris
3. Gastrocnemius
4. Flexor hallucis longus
5. Peroneus longus and brevis

This gives a useful examination sequence from hip to foot.

5. Important S1 Motor Functions

From the listed muscles, the main functions that can be assessed are:

Muscle	Function relevant to examination
Gluteus maximus	Hip extension
Biceps femoris	Lateral hamstring function
Gastrocnemius	Plantar flexion
Flexor hallucis longus	Great-toe flexion
Peroneus longus/brevis	Foot eversion

The   does not provide detailed techniques for testing each individual muscle.

6. EMG Findings in S1 Radiculopathy

A patient with:

L5–S1 disc herniation affecting the S1 root

may show EMG abnormalities in the S1-supplied muscles listed above.

The   specifically mentions:

• Sharp waves
• Fibrillations

7. L5–S1 Disc Herniation and Root Involvement

This is an important distinction in the  .

Posterolateral L5–S1 Disc Herniation

Posterolateral L5–S1 disc herniation – S1 nerve-root irritation

This is the typical pattern emphasized by  .

Foraminal L5–S1 Disc Herniation

In contrast:

Foraminal L5–S1 disc herniation – L5 radiculopathy

So the same disc level may affect a different root depending on the location of the herniation.

Key concept

Posterolateral L5–S1 – S1
Foraminal L5–S1 – L5

8. L5–S1 Isthmic Spondylolisthesis

L5–S1 isthmic spondylolisthesis may cause L5 radiculopathy

This is contrasted with posterolateral L5–S1 disc herniation, which affects S1.

9. Comparison With L5 Nerve Root

The lecture then contrasts S1 with L5.

L5 Motor Muscles Mentioned

• Extensor hallucis longus
• Extensor digitorum longus
• Medial hamstrings
• Gluteus medius

L5 Sensory Area

Dorsum/top of the foot

L5 Reflex

No specific reflex

Typical Disc Level

Posterolateral L4–L5 disc herniation ? L5 nerve-root involvement

S1 vs L5 — High-Yield Table

Feature	L5	S1
Typical posterolateral disc level	L4–L5	L5–S1
Sensory area	Dorsum/top of foot	Lateral side of foot
Important distal muscle	EHL	Gastrocnemius
Additional muscles	EDL, medial hamstrings, gluteus medius	FHL, peronei, lateral hamstrings, gluteus maximus
Reflex	No specific reflex	Ankle reflex
Foraminal L5–S1 disc	Can affect L5	—

Disc Position and Root Affected

This is probably the most useful conceptual point in the  :

At L5–S1

Posterolateral herniation
affects the traversing S1 root

Foraminal herniation
affects the L5 root

also associates:

L5–S1 isthmic spondylolisthesis – L5 radiculopathy

Practical S1 Examination Sequence

Sensory

Test:

Lateral side of foot

Motor

Examine S1-supplied muscles from proximal to distal:

Gluteus maximus – lateral hamstrings – gastrocnemius – FHL – peronei

Reflex

Check:

Ankle jerk

Then correlate with the suspected level and location of disc pathology.

Key Take-Home Points

• S1 examination consists of sensory + motor + reflex assessment.
• S1 sensory area – lateral foot.
• S1 reflex – ankle jerk.
• Important S1 muscles include:
  • Gluteus maximus
  • Biceps femoris
  • Gastrocnemius
  • Flexor hallucis longus
  • Peroneus longus and brevis
• EMG in S1 radiculopathy may show fibrillation potentials/sharp waves in affected muscles.
• Posterolateral L5–S1 disc – S1 radiculopathy.
• Foraminal L5–S1 disc – L5 radiculopathy.
• L5–S1 isthmic spondylolisthesis – L5 radiculopathy according to  .
• L5 has no specific reflex emphasized in this  .

Exam Pearls

L4 – medial foot
L5 – dorsum of foot
S1 – lateral foot

S1 – plantar flexion + ankle jerk

L5–S1 posterolateral disc – S1

L5–S1 foraminal disc – L5

L4–L5 posterolateral disc – L5