Knee flexion and base of support in asymmetrical handling: effects on the workerʹs dynamic stability and the moments of the L5/S1 and knee joints

Objective. To investigate the effects of knee flexion and of the variations of feet lateral spacing on dynamic stability and on the net joint moments distribution between the back and knees. Background. The width of the base of support and knee flexion effects on joint moments in asymmetric handling and especially on the workerʹs stability have rarely been studied. Methods. Fourteen healthy male subjects performed an asymmetrical lifting task, using four different techniques: two imposed lateral feet spacings (41 and 57 cm) and two knee flexion amplitudes (slightly and deeply flexed knees). A tridimensional dynamic rigid body model was used to estimate the triaxial net reaction moments at L5/S1 and at the knees, using two force platforms. New developments have been undertaken to characterize workersʹ stability while handling: the horizontal force required to destabilize the worker was calculated as a measure of dynamic stability. Results. The width of the base of support had little effect on L5/S1 and knee moments; however, the subjects were less stable with the narrow base of support. Using the slightly flexed knees technique, trunk maximal resultant moments were slightly smaller (202 vs. 216 Nm), and maximal resultant knee moments were larger (96 vs. 62 Nm). Furthermore, asymmetric moments at the trunk and the asymmetric position of the knees were reduced with this technique, but subjects were less stable. Conclusions. The use of a slightly flexed knees technique in asymmetrical handling of low-lying loads appears advantageous because it reduces L5/S1 moments while increasing the kneesʹ flexing moments, although this may compromise workersʹ stability.