Abstract
To determine whether the transition from static to dynamic friction in pushing causes a trunk perturbation, participants pushed a four-wheeled cart at shoulder height with and without expectation of cart movement. As hypothesized, the transition from static to dynamic friction causes a sudden unloading perturbation of the trunk. Increases in back muscle activity and trunk inclination were observed in response to the sudden drop in the contact forces between hands and cart. Furthermore, higher trunk muscle activity and higher trunk stiffness prior to the cart movement and a smaller change in trunk inclination after the cart movement were observed in the self-initiated condition (with expectation) compared to the unexpected, externally triggered release condition. These findings suggest that the sudden unloading in pushing tasks may put the spine at the risk of low-back injuries, however, preparatory activation of trunk muscles serves to increase trunk stiffness may reduce the impact of the perturbation.