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Spasticity — a condition characterized by involuntary muscle stiffness and tightness — is a common complication after stroke and can significantly limit mobility, independence and quality of life.
Today, two of the most commonly used measures of spasticity in patients after stroke are the Modified Ashworth Scale (MAS) and Tardieu Scale. However, as subjective assessments, the MAS and Tardieu Scale have a limitation: They may not capture the sort of subtle differences in how easily each patient’s stretch reflex is triggered, limiting their ability to support individualized rehabilitation planning. A new study at Shirley Ryan AbilityLab, recently published in IEEE Transactions on Neural Systems and Rehabilitation Engineering, could lead to more targeted patient treatments. The study was co-authored by Seongyeon Yang, researcher, Harris Family Foundation Arms + Hands Lab; Sungjin Bae, PhD, postdoctoral researcher, Arms + Hands Lab; and W. Zev Rymer, MD, PhD, director, research planning, Arms + Hands Lab, and principal investigator, Sensory Motor Performance Program.
Its findings indicate that an ultrasound-based assessment method for estimating the stretch reflex threshold (SRT), a quantitative measure of reflex excitability, could reveal differences in reflex behavior that are not represented by conventional clinical-grading scales or by external force and muscle-activity measurements alone.
For this study, the researchers recruited 10 subjects with chronic post-stroke spasticity. As a mechanical tapper gently tapped the patients’ distal biceps tendons at increased depths, an ultrasound probe integrated in the tapper measured changes in the distance between the tendon and the underlying bone, while a force sensor and surface electromyography recorded reflex force and muscle activity.
As indentation depth increased, the ultrasound response showed a clear change in pattern that allowed the researchers to estimate each participant’s SRT. The ultrasound findings were strongly correlated with the force and muscle-activity measurements, and the resulting SRT estimates showed good repeatability when the test was repeated during the same visit.
The researchers also found that the starting distance between the tendon and bone was related to SRT. This suggests that each person’s initial muscle-tendon configuration may provide useful context for understanding differences in reflex thresholds.
Notably, while all participants had the same MAS score, their ultrasound-derived SRTs showed variation. According to the authors, this suggests that the method may capture quantitative variability not represented by a single clinical grade while providing structural context for interpreting that variability.
“Clinical scales will always be useful, but patients with the same score can show substantially different quantitative SRT estimates,” said Seongyeon. “Our goal with this study was to use ultrasound imaging — a tool already familiar to many clinicians — to provide a more objective measure of reflex sensitivity while showing how the tendon moves relative to the underlying bone. We hope this approach can eventually support more individualized spasticity assessment and rehabilitation planning.”
