Validity and reliability of an electromyography-based similarity index to quantify lower extremity selective voluntary motor control in children with cerebral palsy

Objective: To quantify selective voluntary motor control (SVMC) objectively and more precisely, we combined the "Selective Control Assessment of the Lower Extremity" (SCALE) with surface electromyography. The resulting Similarity Index (SI) measures the similarity of muscle activation patterns. This study evaluated the preliminary validity and reliability of this novel SISCALE measure in children with cerebral palsy (CP). Methods: We investigated concurrent validity by correlating the SISCALE of 24 children with CP (median age 10.6 years) with comparator assessments. For discriminative validity, the patients' SISCALE scores were compared to 31 neurologically intact age-matched peers. Test-retest reliability was quantified using intraclass correlation coefficients (ICC) and minimal detectable change (MDC) values. Results: The SISCALE correlated strongly with the SCALE (ρ = 0.90, p < .001) and the Gross Motor Function Classification System (ρ = -0.74, p < .001). SISCALE scores were significantly lower in children with CP compared to healthy peers. Test-retest reliability appeared good (for the more and less affected leg, ICC ≥ 0.84, and MDC ≤ 0.17). Conclusions: Validity and reliability of the SISCALE leg and total scores lay within clinically acceptable ranges. Further clinimetric analyses should include responsiveness. Significance: A neurophysiology-based assessment could contribute to a more refined assessment of SVMC impairments. Keywords: Cerebral palsy; Clinimetric properties; Motor control; Outcome measure; Surface EM