Objective: To expand the construct validity of the Four Square Step Test (FSST) in people with multiple sclerosis (PWMS).

Design: Cross-sectional study. Setting: Multiple sclerosis center. Participants: PWMS (N=218; 133 women, 85 men; mean age, 43.2±13.5y; mean disease duration, 7.5±7.7y since diagnosis) were enrolled in the study. The Expanded Disability Status Scale score was 3.1±1.3, indicating minimal to moderate neurologic disability. Interventions: Not applicable. Main outcome measures: The FSST, posturography measures, 2-minute walk test, timed Up & Go test, timed 25-foot walk, fall status, Falls Efficacy Scale International, Modified Fatigue Impact Scale, instrumented cognitive assessment, and 12-item Multiple Sclerosis Walking Scale.

Results: The FSST score of the total sample was 11.0±4.9. Significant differences were observed between the very mild, mild, and moderate disability groups: 8.8±3.4, 11.1±4.9, and 14.1±5.3, respectively. In terms of fall status, the fallers demonstrated a significant slower FSST score than the nonfallers (12.5±5.7 vs 9.0±2.6, respectively). Modest significant correlation scores were found between the FSST and the timed Up & Go test and 2-minute walk test (Pearson ρ=.652 and ρ=-.575, respectively). In terms of posturography, all measures were significantly associated with the FSST scores. A significant positive relation was observed with the visual spatial cognitive domain (Pearson ρ=-.207).

Conclusions: The current study supports and broadens the construct validity of the FSST in PWMS.