Background/objective: The use of the American Spinal Injury Association (ASIA) motor score as an outcome measure requires metrological study. This paper tests the hypothesis that a more accurate representation of motor function is obtained using separate upper and lower extremity scales rather than combining all 20 key muscle ratings into a single ASIA motor score.
Methods: We analyzed archived data from 6116 ASIA motor scale records extracted from the National Spinal Cord Injury Statistical Center Database.
Results: The hypothesis that separate scales more accurately represent motor function than a single motor scale was supported (chi2(difference) = 2596; df=1; P < 0.0001). Two scales account for 87% of the variance, whereas a single scale accounts for only 82%. Lower extremity function is well represented in both solutions; however, upper extremity function is accurately represented only with the use of 2 separate scales.
Conclusions: The use of components of the ASIA standards for other than classification of spinal cord injury needs study. Several lines of study converge to provide strong support for the existence of 2 distinctive dimensions underlying the ASIA motor scale. The use of a single motor score in spinal cord injury research should be questioned and justified to the extent possible. The use of upper and lower extremity scales will lead to a reduction in measurement error when the motor score is used as an outcome measure. The confirmation of 2 separate dimensions underlying the ASIA motor score will enable more accurate representation of motor function in spinal cord injury research.