In epidemiologic studies on musculoskeletal disorders, some risk factors, especially physical load, cannot be determined independently from the worker. Posture, movement and external load are the result both of physical work requirements forced on the worker and of the worker's capacity to adopt particular techniques. Risk factors are also adjusted in relation to the worker's health. This paper presents a dynamic model that links exposure to risk factors for back pain and disability. Its aim is to help identify core elements in exposure assessment strategies for epidemiologic studies on back disorders. In this dynamic model, risk factors are determined relative to health status in order to distinguish between etiological and prognostic factors. Measurement techniques for various risk factors are classified into self-reports, observations, and direct instrumentation. Features of commonly used techniques are discussed with respect to feasibility, accuracy, and precision. In addition, consideration is given to the optimum allocation of measurements taking into account the effects of random and systematic variation in exposure due to tasks, workplaces, and workers.