The rate of onset of muscular force development was also evaluated. The onset rate indicates how rapidly the trunk is loaded during exertion and may be related to the risk of back injury. Greater rates of onset were observed during the unexpected condition. The duration of force required to control the load was investigated. The duration of the muscle force also increased significantly when the weight of the load increased.
Results stress the importance of dynamic interpretations in biomechanical models.
Static models used for occupational workplace design consider only the mean muscle force activity, it appears that such models may underestimate the loading due to dynamic events. Peak components of muscular force were significantly affected by all independent variables in this experiment. Trunk muscles over react during unexpected loading by a factor of two compared with expected loading. The rate of muscular force onset is exaggerated in an unexpected exertion which may lead to a particularly dangerous situation since greater trunk forces are occurring very rapidly. Refinements of such a model could quantify the risks associated with unexpected events such as sudden load handling, slips and falls.