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

Abstract

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.

Introduction

Turning during walking challenges the dynamic equilibrium of the human body because of the requirement to translate and rotate the body towards the new direction of travel. When initiating a turn in normal gait, a top-down temporal sequence is shown in initiation of rotation of body segments [1], [2], [3], [4]. In planned turns, foot placement in the preceding steps is modified to initiate the shift of the body center of mass [4], [5], [6], while when having to make a turn unexpectedly, subjects may stop first and only then change direction [3], [6].

Pushing while turning in transporting patients has been studied from the perspective of trunk loading in relation to the association between pushing tasks and low-back pain [7]. Pushing tasks frequently entail handling objects with a high inertia and unpredictable mechanical interactions with this object and the environment. At the same time, low trunk loading and consequently low trunk muscle activity and trunk stiffness may render control over trunk posture and movement problematic [8], [9], [10], [11]. Trunk control in pushing while turning has to our knowledge not been studied previously.

In turning during normal walking early activation of trunk extensor muscles was found [3]. This suggests that anticipatory activation is used to control the trunk as in postural perturbations in upright standing [12], [13], [14]. When performing voluntary movements such as rapid arm movements, anticipatory activation of trunk muscles is directionally specific [15], [16], which was also observed in dealing with an external object, such as when stopping a moving object [12], [13] or lifting a box with known inertial properties [17], [18]. Also, anticipatory activation was scaled with respect to object properties in lifting [19]. In turning while pushing, the perturbing moment on the trunk will mainly be a twisting moment. We, therefore, hypothesize that trunk rotator muscles are activated directionally specific prior to making a preplanned turn while pushing a cart, i.e. left trunk rotator muscles are more active when turning to the left, and scaled with respect to task demands, i.e. activity is higher when making a sharp turn than a gradual turn.

In cart pushing, sudden turns may be required to avoid a collision. This may not allow sufficient time for anticipatory trunk muscle activation. This could lead to loss of control over trunk movement, which in other types of perturbations appeared to be counteracted by a subsequent, co-contraction of trunk muscles [14], [20]. Therefore, our hypothesis was that trunk rotator muscles would cocontract during performance of a sudden turn.