Reliability and validity of static knee strength measurements obtained with a chair-fixed dynamometer in subjects with hip or knee arthroplasty.

Abstract

To evaluate the reliability (intertrial, interevaluator) and the concurrent validity of strength measurements obtained with a chair-fixed dynamometer and to recommend a clinical protocol that minimizes standard error of measurement (SEM). Within-session repeated measures of maximal static strength of knee flexors and extensors at 30 degrees and 60 degrees of flexion on the chair-fixed and Cybex dynamometers. Ambulatory physiotherapy department of a rehabilitation hospital. Convenience sample of 50 subjects with total hip (n=25) or knee (n=25) arthroplasty. Not applicable. Reliability was quantified by indices of dependability and corresponding SEMs estimated with the generalizability theory, whereas coefficients of correlation were used to assess the validity. Indices of dependability confirmed excellent intertrial (0.98-1.00) and a very good interevaluator (.92-.99) reliability for the strength measures obtained for different movements and positions. An average of 3 trials minimized the magnitude of the SEMs (>2 Nm for all measurements). When comparing the strength values obtained with the Cybex dynamometer to those measured with the chair-fixed dynamometer, strongest relations were attained when the tested knee was positioned at 60 degrees compared with 30 degrees for subjects with a total knee (.78-.92 vs .87-.93) or hip (.73-.85 vs .86-.91) arthroplasty. A clinical protocol averaging 3 trials with the knee positioned at 60 degrees after a familiarization period, both for knee flexors and extensors, performed by a trained therapist is recommended to minimize measurement errors on strength values measured with the chair-fixed dynamometer.