The relation between relative force (i.e., the actual force relative to the maximal voluntary force) and endurance time (i.e., the time the force can be maintained) is employed in a number of methods for ergonomics research. The aim is to assess the development of localized muscular fatigue as a consequence of isometric contractions and to compare activities regarding both the intensity and the duration of the isometric contractions exerted. The main assumption underlying this approach is that the relation is identical for different muscles and in different situations. Several anatomical and physiological factors contributing to variation in the relation are discussed and the consequences of this variation for the validity of the methods employing the relation are quantified. From this analysis, it appears, that the use of the relation between relative force and endurance time with the assumption mentioned leads to unreliable results. Therefore assessment of isometric contractions in labour situations will have to be based on fundamentally different methods.