Physical demands at the workplace have been shown to be predictive of RTW in other words, those workers with more physically demanding work were slower to resume employment after a lowback injury. Physical demands of the workplace are often derived from the coding of occupations. These codes may, at fi rst, seem crude, but they have shown to be predictive more often than self-reported measures where the worker is asked about physical demands of the
job. Studies that used self-reported measures only provide moderate evidence for an effect of physical demands on RTW. Some studies in our review found an effect of what seemed extreme differences in physical demands that were present in the study population, for instance, when comparing rail maintenance workers to of fi ce workers in one company . However, most studies did not fi nd an effect of self-reported physical demands. These fi ndings suggest that physical demands classi fi ed through occupational codes and self-report of physical demands are not interchangeable. This may be because a workers’ perception of the physical demands of the job is biased by getting injured at work.