Reliability of Isometric Lower-Extremity Muscle Strength Measurements in Children With Cerebral Palsy: Implications for Measurement Design

Background Children with cerebral palsy (CP) typically show muscle weakness of the lower extremities, which can be measured with the use of handheld dynamometry (HHD).

Objective The purposes of this study were: (1) to determine test-retest reliability and measurement error of isometric lower-extremity strength measurements in children with CP with the use of HHD and (2) to assess implications for measurement design.

Design A test-retest design was used.

Methods Fourteen children with hemiplegic (n=6) or diplegic (n=8) spastic CP (Gross Motor Function Classification System levels I–III), ages 7 to 13 years, were assessed for isometric strength on 2 separate days (occasions) with the use of HHD, with 3 trials per muscle group. The intraclass correlation coefficient, standard error of measurement, and smallest detectable difference (SDD) were calculated for different measurement designs.

Results Intraclass correlation coefficient values of single measurements for all muscle groups ranged from .70 to .90, and the SDD was large (>30%). Regarding measurement error, the largest source of variability was found for occasion. A 2-occasion mean decreased the SDD by 9% to 14%. For trials, a greater improvement in SDD was found when 2 trials were averaged instead of 3. A measurement design of 2 trials–2 occasions was superior to the often-used approach of 3 trials–1 occasion.

Limitations The small sample size was the major study limitation.

Conclusions Handheld dynamometry is reliable and can be used to detect changes in isometric muscle strength in children with CP when using the mean of at least 2 trials. To further improve reliability, taking the average of 2 occasions on separate days is recommended, depending on group size and muscle group.