Motor Consequences of S1 Radiculopathy
S1 radiculopathy primarily causes foot plantarflexion weakness and diminished ankle reflexes, with sensory impairment in the lateral foot and posterior leg distribution. 1
Primary Motor Deficits
The hallmark motor findings of S1 nerve root dysfunction include:
- Foot plantarflexion weakness - This is the most characteristic motor deficit, affecting the gastrocnemius-soleus complex 1
- Diminished or absent ankle (Achilles) reflex - A key clinical finding that helps localize the lesion to S1 1
- Foot eversion weakness - May be present but typically less prominent than plantarflexion weakness 2
The neurologic examination should specifically test these motor functions as they directly correlate with S1 nerve root integrity. More than 90% of symptomatic lumbar disc herniations occur at L4/L5 and L5/S1 levels, making S1 radiculopathy one of the most common presentations 1.
Atypical Presentations
While rare, S1 radiculopathy can occasionally present with isolated hamstring (knee flexion) weakness as the sole initial motor manifestation 2. This atypical presentation can lead to diagnostic delays, so clinicians should maintain a high index of suspicion when evaluating posterior thigh and knee symptoms, even without classic foot weakness.
Natural History and Recovery
Most patients with motor deficits from S1 radiculopathy recover full strength within 3-12 months with conservative management alone 3. In a prospective study of 40 patients with radicular motor weakness, 82.5% did not require surgery, and among those followed for 12 months, there were statistically significant improvements in all areas of strength 3. Strength recovery typically occurs in the first 3 months, though ongoing improvement continues throughout the year 3.
Clinical Examination Approach
When evaluating suspected S1 radiculopathy, perform:
- Straight-leg raise test - Positive between 30-70 degrees with 91% sensitivity but only 26% specificity for herniated disc 1
- Crossed straight-leg raise - More specific (88%) but less sensitive (29%) 1
- Ankle reflex testing - Compare bilaterally for asymmetry
- Plantarflexion strength testing - Have patient stand on toes or push against resistance
- Sensory examination - Check lateral foot and posterior calf distribution
Associated Sensory Findings
Sensory loss in the painful dermatome is the most frequent finding at physical examination, occurring in 56% of cases 4. For S1 radiculopathy, this typically involves the lateral aspect of the foot and posterior leg 1.
Electrodiagnostic Considerations
EMG abnormalities are found in less than 50% of lumbosacral radiculopathy cases and can be predicted by clinical findings including myotomal weakness, abnormal deep reflexes, and paresthesias 4. The most frequent motor neurographic abnormality in S1 radiculopathy is decreased compound muscle action potential amplitude from the abductor hallucis muscle 4. F-wave abnormalities, particularly prolonged maximum-minimum latency range, are more characteristic of radiculopathy than distal axonopathies 5.
Red Flags Requiring Urgent Evaluation
Patients with neurological deficit (sensory or motor changes) should be referred to specialist services within 2 weeks of presentation 6. This is particularly important for severe or progressive motor weakness that could indicate cauda equina syndrome or significant nerve root compression requiring surgical consideration.