Force direction in pushing and pulling and musculo-skeletal load

In pushing and pulling wheeled objects, the effect of the exerted force on local musculo-skeletal loads depends on the direction of force exertion. Several questions about the direction of force exertion in pushing and pulling, the effects of handle height and force level on force direction, and the quantitative implications for musculoskeletal load, were addressed in the present study. Eight subjects pushed against or pulled on a stationary bar at various handle heights and horizontal force levels, while they were walking on a treadmill. Generally, a significant change towards a more horizontal force direction was observed at increasing handle heights and force levels. From the observed net joint torque values in the shoulder and lumbo-sacral joint it was deduced that local musculoskeletal loads can not be deduced unequivocally from the magnitude of the force required to perform a task. Therefore, knowledge of the direction of force exertion with respect to body postur, is essential for a proper evaluation of musculo-skeletal loads.