Rationalization efforts in industry, both in the Scientific Management tradition and also based on recent rationalization theories, have as one of their main aims to increase the utilization of workers. Clearly, there is a limit to the amount of physical work each employee can perform without developing musculoskeletal disorders. Such limits are generally set by guidelines for acceptable work load. This paper reviews the physical work load concept, the historical development of guidelines, and current guidelines as found in ergonomics textbooks. The focus is on the change in the aim of the guidelines over time: increased productivity, reduced fatigue and finally improved musculoskeletal health. Current guidelines for physical work load mostly emphazise a reduction in the level of work load, while there are few guidelines that consider the repetitiveness and duration of work load. As the guidelines in general only consider one exposure variable, this is a particular problem in rationalization where all three exposure dimensions may be changed simultaneously. Present guidelines are mainly based on laboratory studies aiming to eliminate short-term physiological or psychological responses. These guidelines are clearly inadequate and may be misleading in view of recent research regarding the relationship between physical work load exposure and the development of musculoskeletal complaints at the work place.