Dynamic, semidynamic and static analyses of heavy lifting: three approaches applied to sack handling

The load on the lower back was calculated by means of biomechanical models of different degrees of complexity for twelve subjects lifting 30-kg sacks from three different heights. Motions and ground force reactions were registered by means of a video camera, an advanced optoelectronic measuring system, and a force plate. The results showed that static analyses underestimated the load on the back. However, fully dynamic analyses were not required but could be replaced by simpler ‘semidynamic’ analyses. The experiments were carried out during two periods with an interval of 5 months. The calculated loads on the back were larger during the first period. This could possibly be explained by differences in the experimental setup or by the fact that the subjects were very young, 16–17 years, and may have increased in growth and strength during the interval.