Background and purpose: The EK scale comprises ten categories (EK 1-10), each contributing to an overall picture of function in the non-ambulatory stage of Duchenne muscular dystrophy (DMD). The purpose of the present study was to investigate content and construct validity of the EK scale as a tool to discriminate between levels of functional ability in individuals with DMD or spinal muscular atrophy (SMA) who were non-ambulatory.
Method: Data from a sample of 56 subjects with DMD and 38 with SMA, who were non-ambulatory, were obtained from four separate studies. The relationship of functional ability by use of the EK scale and (1) muscle strength, (2) contractures, (3) forced vital capacity and (4) years of wheelchair dependency were assessed. All items of the EK scale were used except the one representing severe hypoventilation.
Results: Regression analyses showed that the EK sum was the most significant explanatory variable (p < 0.05) of all variables measured to explain muscle strength in both DMD and SMA subjects. The individual categories of EK (1-10) all contributed as significant explanatory variables (p < 0.05) to the other variables measured.
Conclusions: The categories and items of the EK scale were relevant and valid as means of discriminating between levels of functional performance in the population studied which was evidence of content and construct validity.