Trunk muscle control in response to (un)expected turns in cart pushing

Before altering the travel direction in normal gait, anticipatory activation in trunk muscles is observed, followed by a top-down sequence of rotation of body segments. Turning while pushing a cart is a more challenging task for the trunk because of its low stiffness in pushing while walking and the interaction with the high inertia of the cart. 12 healthy subjects pushed a 200 kg cart at shoulder and hip height while making turns (gradual, sharp and unexpected sharp). The normalized electromyogram amplitudes of left (right) lateral and anterior external oblique muscle, and right (left) internal oblique muscle were averaged to represent left (right) trunk rotator muscle activity. The baseline values of trunk rotator muscle activity before the turn and the peak values after the turn were determined. Additionally, peak values of hand forces, twisting moments and twisting motions were assessed. Before turning, higher trunk rotator muscle activity than in straight pushing without turning was only observed before making a turn in the gradual or sharp turn conditions. After the turn, clockwise twisting motion was associated with a clockwise twisting moment induced by the reaction forces at the left hand. Anticipatory activation was initially absent in the unexpected sharp turn, while bilateral trunk rotator muscle activity increased after the turn, indicating co-contraction. In the unexpected turn condition the combination of an uncontrolled twisting motion with delayed muscle activation may increase the potential risk of low-back injury.