Assessment of physical exposure in relation to work-related musculoskeletal disorders--what information can be obtained from systematic observations?

Establishing an association between physical work load and work-related musculoskeletal disorders requires a quantitative assessment of exposure to the main risk factors. Commonly used methods are questionnaires, diaries, interviews, systematic observations, and direct measurements. While questionnaires provide subjective information and have low reliability, measurements are technically sophisticated, expensive, and unable to identify some important risk factors. Systematic observations offer a compromise. The observation methods devised during the past 15 years are described, together with their reliability and validity. Most of them use duration or frequency of certain postures or events as the main measure of exposure. Their ability to quantify the main risk factors posture, manual handling, and repetitive work differs. All available methods have drawbacks and have been used to a limited extent in epidemiologic studies. There have been improvements however, and it is concluded that observation methods will have a place in epidemiologic studies on musculoskeletal disorders.