Time interval affects physical activity scores: a comparison of the Marx Activity Rating Scale and the Hospital for Special Surgery Pediatric Functional Activity Brief Scale

Purpose: The Marx Activity Rating Scale (Marx Scale) is a commonly used activity-related patient-reported outcome which evaluates the highest activity level within the last year, whereas the Hospital for Special Surgery Pediatric Functional Activity Brief Scale (HSS Pedi-FABS) assesses highest activity level within the last month. This study aims to determine whether the different timeframes used for the common items of Marx Scale and HSS Pedi-FABS affect scores, and if so, to determine whether age or injury status affect this difference. Methods: The Marx Scale and four analogous items on the HSS Pedi-FABS were administered in random order to patients being evaluated for knee injuries and in addition to healthy controls to enroll an uninjured comparison group. Responses to each question were scored from 0 to 4 for a maximum overall score of 16. Paired and independent-sample t tests were used to determine mean differences between groups. Results: The final cohort included 88 participants of which 47% were children (ages 10–17) and 51% had a knee injury. All participants except for healthy adults scored significantly lower on the HSS Pedi-FABS than the Marx Scale (p < 0.05). On the HSS Pedi-FABS activity scale, healthy participants scored significantly higher than injured participants (p < 0.01), but there were no significant differences based on age. Conversely, on the Marx Scale, children scored higher than adults (p ≤ 0.001), but there were no significant differences based on injury. Conclusion: Physical activity level differs when evaluated with the Marx Scale or the analogue part of HSS Pedi-FABS with timeframe being the only difference between the two. The lower scores on the HSS Pedi-FABS are likely due to seasonal changes in activity which do not affect the Marx Scale. HSS Pedi-FABS analyzes a shorter window and it is more likely to capture changes in physical activity due to a recent injury than the Marx Scale which is better suited for assessing general physical activity level unaffected by seasonality or recent injury. Understating the differences between these physical activity scales can better guide clinicians when using them and interpreting scores. Level of evidence: II.