A biomechanical investigation into the validity of the modified Ashworth Scale as a measure of elbow spasticity

Objective: To investigate the criterion validity of the modified Ashworth Scale. Population: Volunteers from a stroke population admitted to a district general hospital stroke unit diagnosed with a first ever stroke less than 26 weeks previously. Outcome measures: Resistance to passive movement about the elbow was simultaneously quantified (biomechanically) and graded (modified Ashworth Scale). Passive range of movement and peak instantaneous velocity during passive movement were also measured. Analysis: Criterion validity was investigated as convergent construct validity (using the Spearman's correlation coefficient) and concurrent validity (using analysis of variance). Results: One hundred measurements were taken on 63 subjects. Correlation between the modified Ashworth Scale and resistance to passive movement was 0.511. Resistance to passive movement and velocity showed significant differences between the modified Ashworth score of '0' and a modified Ashworth score greater than '0' (p<0.01). There were no significant differences between MAS '1', '1+' and '2'. Resistance to passive movement in the impaired arm was significantly higher than in the nonimpaired arm (p<0.01). Conclusion: The modified Ashworth Scale does not provide a valid measure of spasticity at lower grades but it may provide a measure of resistance to passive movement.