EMG Abnormalities in Spasticity
In patients with spasticity, EMG typically shows normal insertional activity with exaggerated stretch reflexes during passive movement, but the key distinguishing feature is the absence of spontaneous activity at rest (no fibrillations or positive sharp waves) since the lower motor neuron remains intact.
Primary EMG Characteristics
Spasticity represents an upper motor neuron disorder, which fundamentally differs from lower motor neuron pathology on EMG. The critical findings include:
During Passive Stretch (Quick Stretch Maneuvers)
- High-amplitude motor unit action potentials (MUAPs) in response to rapid passive muscle lengthening 1
- Multiple muscles activate simultaneously with exaggerated responses to stretch 1
- This hyperactive stretch reflex is the electrophysiological hallmark of spasticity
At Rest
- Normal insertional activity (lasting <300 ms after needle movement)
- Absence of spontaneous activity - no fibrillation potentials or positive sharp waves 2
- This distinguishes spasticity from denervation injuries where spontaneous activity would be prominent
During Voluntary Movement
- Minimal or normal muscle activation patterns during volitional tasks 1
- Preserved motor unit recruitment (unlike denervation)
- May show impaired reciprocal inhibition in severe cases (mAS = 4), where antagonist muscles fail to relax appropriately 3
Distinguishing Spasticity from Spastic Dystonia
Recent evidence demonstrates that EMG can differentiate between two patterns of hypertonia 4, 1:
Spasticity pattern:
- Multiple muscles respond to quick stretch only
- Minimal activation during volitional movement
- Does NOT worsen with fatigue 4
Spastic dystonia pattern:
- Multiple muscles respond to volitional movement activities
- Minimal response to quick stretch
- Worsens significantly with fatigue 4
This distinction has clinical implications, as spastic dystonia may negatively impact performance during prolonged activity.
Quantitative EMG Findings
Surface EMG studies correlate with clinical spasticity scales 5:
- Reflex electromyographic threshold correlates with Modified Ashworth Scale scores
- Increased muscle activation during passive stretch correlates with spasticity severity
- The non-affected side in hemiplegic patients may also show abnormal patterns compared to healthy controls 3
Recent Observational Data
A 2026 study found associations between muscle architectural changes and EMG 6:
- Muscles with increased echointensity (Modified Heckmatt Scale 3-4) showed abnormal insertional activity in 53.6% of cases
- Of those with abnormal insertional activity, 67.3% had reduced or absent insertional activity
- This suggests chronic muscle architectural changes may develop in longstanding spasticity
Critical Clinical Pitfalls
- Do not expect fibrillations or positive sharp waves - their presence suggests concurrent lower motor neuron pathology, not spasticity
- Examine both affected and unaffected sides - the "normal" side in hemiplegic patients often shows subtle abnormalities 3
- Test during both rest and passive stretch - spasticity manifests primarily during stretch, not at rest
- Consider fatigue effects - if hypertonia worsens with fatigue, suspect spastic dystonia rather than pure spasticity 4
The EMG in spasticity fundamentally reflects intact but hyperexcitable motor neurons responding excessively to stretch stimuli, without the denervation changes seen in peripheral nerve or anterior horn cell disorders.