Exploring the relationship between local and global dynamic trunk stabilities during repetitive lifting tasks

Lifting is a major risk factor for low back injury. Lifters experience small continual perturbations, because moving a load provides a disturbance to the lifter׳s equilibrium. The goal of the present study was to examine the relationship between local and global trunk/spine stabilities during external perturbations introduced at the foot-floor interface. 12 healthy males were recruited to participate in this study. Participants completed a freestyle lifting protocol on a perturbation treadmill, under three randomized load conditions: ~0, 4, and 8 kg. Participants performed a total of 40 lifts under each load condition; no perturbations occurred during the first 20 lifts. During the last 20 lift cycles (in blocks of 5) the participants were randomly perturbed. Local dynamic trunk stability was quantified using the local divergence exponent (λmax) of the first 20 lifts. In addition, the distance traveled from the unperturbed lifting pattern (B), the time to max distance (Tau), the relaxation distance (A), and the rate of return toward the normal lifting pattern (Beta) were analyzed following each external perturbation. An increase in lifted load lead to significantly increased local trunk stability (p=0.046). Higher load also lead to decreased distance (B) traveled away from the unperturbed trajectory (p=0.023). Results agree with previous research that increasing load lifted significantly improves local trunk/spine stability during lifting. Here we have shown that altered local stability also translates into a greater ability to resist external global perturbations, which may reduce injury risk and should be explored in the future.