Metric properties of the Tonic Stretch Reflex Threshold (TSRT) as a measure of spasticity: a systematic review with meta-analysis

Introduction: Spasticity is a common symptom after brain injury, often interfering with functional recovery and rehabilitation. The Tonic Stretch Reflex Threshold (TSRT) was proposed as an objective neurophysiological assessment of spasticity that could overcome the limitations of clinical scales. This systematic review aimed to appraise the current evidence on the metric properties of TSRT. Methods: Electronic databases (MEDLINE, CINAHL, Scopus, Web of Science, and EMBASE) were screened from inception to June 30, 2025, for studies reporting data on reliability, validity, and/or responsiveness of TSRT in adults with stroke. Two reviewers independently selected the studies, assessed the methodological quality, and extracted relevant data. When possible, pooled estimates for each property were computed. Results: Of the 9804 titles retrieved, 17 were eventually included, to which 2 articles from cross-references were added. We found insufficient values for both intra-rater (two studies, ICC = 0.548, 0.330-0.710) and inter-rater (three studies, ICC = 0.687, 0.511-0.808) reliability, with high measurement error. Data on validity were found in 14 articles, with conflicting results on the association of TSRT with clinical scales of spasticity and motricity, but good ability to discriminate among relevant groups. Only one study investigated responsiveness with an external anchor, finding that TSRT measurements failed to accurately detect improved participants. Conclusion: Despite the potential of TSRT as a measure of spasticity, its metric properties, particularly reliability, are not fully supported. Future research should prioritize improving its reliability and investigating its validity and responsiveness with neurophysiological measures rather than relying solely on clinical scales.