Is the trunk movement more perturbed after an asymmetric than after a symmetric perturbation during lifting?

Low back injury is associated with sudden movements and loading. Trunk motion after sudden loading depends on the stability of the spine prior to loading and on the trunk muscle activity in response to the loading. Both factors are not axis-symmetric. Therefore, it was hypothesized that the effects on trunk dynamics would be larger after an asymmetric than after a symmetric perturbation. Ten subjects lifted a crate in which, prior to lifting, a mass was displaced to the front or to the side without the subjects being aware of this. Crate and subject movements, crate reaction forces and muscle activity were recorded. From this, the stability prior to the perturbation was estimated, and the trunk angular kinematics and moments at the lumbo-sacral joint were calculated. Both perturbations only minimally affected the trunk kinematics, although the stability of the spine prior to the lifting movement was higher in the sagittal plane than in the frontal plane. In both conditions the stability appeared to be sufficient to absorb the applied perturbation.