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Aim. To investigate measurement properties of a practical test of cervical flexor endurance (CFE) in whiplash patients including inter-rater reliability, sensitivity to clinical change, criterion related validity against the Neck Disability Index (NDI), and discriminant validity for injured versus uninjured populations.Methods. Two samples were recruited, 81 whiplash patients, and a convenience sample of 160 subjects who were not seeking treatment and met criteria for normal pain and range of motion. CFE was measured using a stopwatch while the subject, in crook lying, held their head against gravity to fatigue.Results. Inter-rater reliability in whiplash patients was in a range considered 'almost perfect' (Intraclass Correlation = 0.96). CFE had greater inter-subject variability than the NDI or range of motion in any of three planes. However, the effect size for improvement in CFE over treatment was as large as the effect sizes for all of those measures. In multivariate regression, CFE changes accounted for changes on the NDI better than the three ranges of motion. CFE discriminated whiplash patients who were within six months of injury (n = 71) from age and gender matched normals with high effect size (ES = 1.5).Conclusions. These findings provide evidence of reliability and validity for CFE measurement, and demonstrate that CFE detects clinical improvements. Variance on CFE emphasizes the need to consider inter-, and intra-subject standard deviations to interpret scores. [ABSTRACT FROM AUTHOR]