Validity of self-reports of knee-straining activities at work: a field study with 6-month follow-up.

Objectives: To measure short-term and long-term validity of self-reported duration of kneeling and squatting at work and to examine the possibility of differential misclassification due to knee complaints.

Methods: Work-related kneeling and squatting were analysed for 190 male subjects (mean age, 35.0 and SD, 11.5) in field by both measurements and questionnaires. Posture capturing was performed with an ambulatory measuring system (CUELA). Immediately after the measurement (t(0)), each participant was asked to estimate frequency and duration of five specific knee postures taken during the measurement period. After 6 months (t(1)), the survey was repeated (n = 125). Health status of all subjects was recorded by Nordic questionnaire. Statistical analysis was performed by using nonparametric tests, correlations, and Bland-Altman plots.

Results: At both time points, subjects were able to recall the occurrence of knee postures rather well (100.0-57.6% agreement) but many of them failed in quantifying their knee load. We found poor-to-moderate correlations between measurements and self-reports for all examined postures in both surveys (0.23 < ρ < 0.63). The durations of knee postures were both over- and underestimated but overestimations predominated (t(0), 74.7% and t(1), 87.2% overestimations). High-exposed subjects seemed to misjudge their exposure to a greater extent than low-exposed ones, while knee complaints seemed to have no impact on the assessment behaviour.

Conclusions: As our study showed, self-reported knee loading may deviate widely from measured exposure. These limitations of self-reporting emphasise the arguments in favour of using objective data whenever possible, for example by complementing self-reported occurrence of knee postures with quantitative measurement data.