Effect of lifting two different weights for female workers in a transfer of boxes during a palletizing task

Numerous literature reviews indicate a moderate to strong association between manual material handling (MMH) and back injury. A few biomechanical studies contrasted the work techniques of female and male workers during MMH. Recently, Plamondon et al. (2014) found that one important biomechanical variable that characterized experienced females relative to male experts, during lifting of 15-kg boxes from the floor, was their patterns of interjoint coordination. Female workers showed a sequential motion initiated by the knees, followed by the hip and the back, while expert males showed a more simultaneous motion between segments. This had the consequence of putting the females at greater risk of back injuries due to the excessive back flexion that this type of coordination yields. An important question is why females used a more sequential coordination pattern? Strength could be one possibility considering that the same “absolute load” (15-kg) was studied. Interestingly, several studies reported that with an increase of the load, the interjoint coordination became more sequential, which delays lumbar spine extension (Burgess-Limerick et al.,1995). The objective of this study was to test whether female workers would repeat the same coordination pattern with a load adjusted for their overall back strength (females 10-kg; males: 15-kg), which can be considered as a “relative load” considering that the overall back strength of females is 2/3 of male’s strength (Mital et al.,1997).