The Quality Function Measure: reliability and discriminant validity of a new measure of quality of gross motor movement in ambulatory children with cerebral palsy

AIM: Optimizing movement quality is a common rehabilitation goal for children with cerebral palsy (CP). The new Quality Function Measure (QFM)--a revision of the Gross Motor Performance Measure (GMPM)--evaluates five attributes: Alignment, Co-ordination, Dissociated movement, Stability, and Weight-shift, for the Gross Motor Function Measure (GMFM) Stand and Walk/Run/Jump items. This study evaluated the reliability and discriminant validity of the QFM. METHOD: Thirty-three children with CP (17 females, 16 males; mean age 8y 11mo, SD 3y 1mo; Gross Motor Function Classification System [GMFCS] levels I [n=17], II [n=7], III [n=9]) participated in reliability testing. Each did a GMFM Stand/Walk assessment, repeated 2 weeks later. Both GMFM assessments were videotaped. A physiotherapist assessor pair independently scored the QFM from an assigned child's GMFM video. GMFM data from 112 children. That is, (GMFCS I [n=38], II [n=27], III [n=47]) were used for discriminant validity evaluation. RESULTS: QFM mean scores varied from 45.0% (SD 27.2; Stability) to 56.2% (SD 27.5; Alignment). Reliability was excellent across all attributes: intraclass correlation coefficients (ICCs) ≥0.97 (95% confidence intervals [CI] 0.95-0.99), interrater ICCs ≥0.89 (95% CI 0.80-0.98), and test-retest ICCs ≥0.90 (95% CI 0.79-0.99). QFM discriminated qualitative attributes of motor function among GMFCS levels (maximum p<0.05). INTERPRETATION: The QFM is reliable and valid, making it possible to assess how well young people with CP move and what areas of function to target to enhance quality of motor control.