Elsevier

Journal of Biomechanics

Volume 44, Issue 1, 4 January 2011, Pages 121-127
Journal of Biomechanics

Control of trunk motion following sudden stop perturbations during cart pushing

https://doi.org/10.1016/j.jbiomech.2010.08.036Get rights and content

Abstract

External perturbations during pushing tasks have been suggested to be a risk factor for low-back symptoms. An experiment was designed to investigate whether self-induced and externally induced sudden stops while pushing a high inertia cart influence trunk motions, and how flexor and extensor muscles counteract these perturbations. Twelve healthy male participants pushed a 200 kg cart at shoulder height and hip height. Pushing while walking was compared to situations in which participants had to stop the cart suddenly (self-induced stop) or in which the wheels of the cart were unexpectedly blocked (externally induced stop). For the perturbed conditions, the peak values and the maximum changes from the reference condition (pushing while walking) of the external moment at L5/S1, trunk inclination and electromyographic amplitudes of trunk muscles were determined. In the self-induced stop, a voluntary trunk extension occurred. Initial responses in both stops consisted of flexor and extensor muscle cocontraction. In self-induced stops this was followed by sustained extensor activity. In the externally induced stops, an external extension moment caused a decrease in trunk inclination. The opposite directions of the internal moment and trunk motion in the externally induced stop while pushing at shoulder height may indicate insufficient active control of trunk posture. Consequently, sudden blocking of the wheels in pushing at shoulder height may put the low back at risk of mechanical injury.

Introduction

Pushing has been associated with the risk of low-back pain (Damkot et al., 1984, Harber et al., 1987, Hoozemans, 2001, Plouvier et al., 2008). This is remarkable since in pushing, joint moments around the lumbar spine are low (Hoozemans et al., 2004). However, these low moments probably coincide with a relatively low trunk stiffness (Stokes et al., 2000, Chiang and Potvin, 2001), which may put the spine at risk of mechanical injury when trunk perturbations occur (Cholewicki and McGill, 1996), especially given the high inertia of objects handled in industrial pushing tasks (Chaffin et al., 1999, Nussbaum et al., 2000).

When pushing a cart, perturbations of the trunk may occur because of sudden stops. One may, for example, be required to suddenly stop the cart to avoid a collision. The high inertia of the transported object may in this case impose a sudden, yet self-induced, flexion perturbation of the trunk similar to that when lifting an unexpectedly heavy object (van der Burg et al., 2000). Alternatively, sudden stops may occur due to an external event, for example when an obstacle blocks the wheels. In contrast to the self-induced stop, this may impose an external trunk extension moment due to high reaction forces at the hands. Both situations may perturb trunk movement, which could be a cause of injury.

When experiencing unpredictable continuous perturbations during pushing while walking (Lee et al., 2010) and lifting (van DieenDieën et al., 2003), participants respond by stiffening the trunk using cocontraction. The objective of the present study was to investigate how trunk motion and trunk muscle activity are controlled in relation to unexpected sudden stops during pushing. We hypothesized that both types of sudden stops (self-induced and externally induced) could lead to uncontrolled trunk motions, i.e. an increase in trunk inclination due to an external flexion moment during self-induced stops and a decrease in trunk inclination due to an external extension moment during externally induced stops. Additionally, we hypothesized that trunk inclination would be more affected when sudden stops occur during pushing at shoulder height than at hip height. As higher trunk moments and hence higher muscle activity would be present prior to the perturbation when pushing at hip height (Hoozemans et al., 2007). Furthermore, we hypothesized that trunk flexor and extensor muscles would cocontract in response to perturbations in both types of sudden stops.

Section snippets

Methods

Twelve healthy male volunteers (age 30.2 (SD 5.4) years, height 1.86 m (SD 0.06 m) and weight 79.4 kg (SD 8.1 kg)) participated in the experiment after signing an informed consent. Participants reported no history of low-back pain or other musculoskeletal disorders within the past 12 months. The ethics committee of the Faculty of Human Movement Sciences approved the experiment.

Results

The mean self-selected walking speed during all pushing tasks was 1.17 ms−1 (SD 0.15). The walking speed during the reference condition was not different from the walking speed just before the perturbations (1.16±0.13 ms−1 vs. 1.18±0.16 ms−1; F (1,11)=0.52, p=0.604).

ANOVA for repeated measures on the log transformed data showed significant effects of condition (reference and two sudden stops) on the peak values of external moment, trunk inclination and trunk muscle EMG (Table 1). Compared to the

Discussion

The present study was designed to investigate how trunk inclination and trunk muscle activity are controlled after sudden stops while pushing a cart at walking speed. In the self-induced stop, an external flexion (internal extensor) moment coincided with a decrease in trunk inclination. In contrast, the externally induced stop appeared to cause an involuntary trunk motion, a decrease in trunk inclination due to an external extension (internal flexor) moment. Smaller changes in trunk inclination

Conflict of interest

The authors declare that no financial and personal relationships with other people or organisations have inappropriately influenced the content of the work reported in this paper.

Acknowledgements

We thank Sjoerd M. Bruijn for his crazy and explosive shouting played in the experiment, Marit Balder for her assistance in data acquisition and Gert S. Faber for his assistance in data analysis.

References (18)

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