Moments at the L5/S1 joint during asymmetrical lifting: effects of different load trajectories and initial load positions

The purpose of this study was to examine the reaction moments at L5/S1 about the three orthogonal orthopaedic axes on the trunk when performing asymmetrical lifting tasks under six experimental conditions. The first three conditions were three lifting modes referring to different load trajectories (diagonal, extension then twisting, twisting then extension). The next three conditions were three lifts referring to different initial load positions (90 °, 45 °, 0 °). Ten male volunteers (mean age 23.4 years) participated in the study. The mass of the load was 11.6 kg and two shelves were used: a low shelf of 22 cm from which the load was removed and a high shelf of 80 cm onto which the load was laid down following the manoeuvre. A dynamic 3D multisegment model was used to compute spinal loadings. The maximum reaction moments generated at the joint for the three load trajectories and for the three initial load positions were approximately of the same magnitude. On the other hand, significant differences were observed in the trunk orientation when the maximal moment was generated. Symmetrical trunk positions appeared to be more advantageous because of the reduction in the capability of maximal extension force in asymmetric trunk posture. A possible alternative for avoiding excessive stress on the back in asymmetric trunk posture may be to keep the shoulders parallel to the ground.