The validity and reliability of the Functional Strength Measurement (FSM) in children with intellectual disabilities

Background There is lack of valid and reliable field‐based tests for assessing functional strength in young children with mild intellectual disabilities (IDs). Aim The aim of this study was to investigate the test–retest reliability and construct validity of the Functional Strength Measurement in children with ID (FSM‐ID). Method Fifty‐two children with mild ID (40 boys and 12 girls, mean age 8.48 years, SD = 1.48) were tested with the FSM. Test–retest reliability (n = 32) was examined by a two‐way interclass correlation coefficient for agreement (ICC 2.1A). Standard error of measurement and smallest detectable change were calculated. Construct validity was determined by calculating correlations between the FSM‐ID and handheld dynamometry (HHD) (convergent validity), FSM‐ID, FSM‐ID and subtest strength of the Bruininks‐Oseretsky test of motor proficiency – second edition (BOT‐2) (convergent validity) and the FSM‐ID and balance subtest of the BOT‐2 (discriminant validity). Results Test–retest reliability ICC ranged 0.89–0.98. Correlation between the items of the FSM‐ID and HHD ranged 0.39–0.79 and between FSM‐ID and BOT‐2 (strength items) 0.41–0.80. Correlation between items of the FSM‐ID and BOT‐2 (balance items) ranged 0.41–0.70. Conclusion The FSM‐ID showed good test–retest reliability and good convergent validity with the HHD and BOT‐2 subtest strength. The correlations assessing discriminant validity were higher than expected. Poor levels of postural control and core stability in children with mild IDs may be the underlying factor of those higher correlations.