Effect of table top slope and height on body posture and muscular activity pattern

Abstract

The objective of this study was to assess the effect of table top slope and height on body posture and muscular activity pattern. Twelve asymptomatic participants performed a 5-min reading task while sitting, in six experimental conditions manipulating the table top slope (20° backward slope, no slope) and its height (low, medium, up). EMGs recordings were taken on 9 superficial muscles located at the trunk and shoulder level, and the angular positions of the head, trunk and pelvis were assessed using an inertial orientation system. Results revealed that the sloping table top was associated with a higher activity of deltoideus pars clavicularis ( P < 0.05) and a smaller flexion angle of the head ( P < 0.05). A tentative conclusion is that a sloping table top induces a more erect posture of the head and the neck, but entails an overload of the shoulder, which might be harmful on the long run.

Résumé

L’objectif de cette étude a été d’évaluer l’effet de l’inclinaison et de la hauteur de la table sur la posture et le patron d’activité musculaire chez l’être humain. Douze sujets asymptomatiques ont réalisé une tâche de lecture de 5-min en posture assise, au cours de six conditions expérimentales faisant varier l’inclinaison (pas d’inclinaison, inclinaison en arrière) et la hauteur (basse, moyenne, élevée) du plateau de la table. Les signaux EMG ont été enregistrés sur neuf muscles superficiels du tronc et de l’épaule, tandis que les positions angulaires de la tête, du tronc et du bassin étaient évaluées au moyen d’une centrale inertielle. Les résultats ont montré que lorsque le plateau était incliné, la flexion de la tête était réduite ( p < 0,05) et l’activité du deltoïde antérieur plus élevée ( p < 0,05). Il en est conclu que les tables avec plateau incliné favorisent une posture plus droite de la tête et du cou, mais génèrent au niveau de l’épaule une charge additionnelle qui pourrait être nocive sur le long terme.

1 Introduction

During the last decades, the growing use of biomechanical tools (EMG, video analysis, force plates, pressure measurement) in the field of human movement analysis has fostered many studies exploring the effect of work station design on the biomechanical strain sustained by the body . However, few studies were devoted to an in-depth exploration of the influence of table height and slope on body posture and muscular activity. Using inclinometers (statometric method) and EMG recordings of trapezius pars descendens, Bendix and Hagberg found that the head and trunk moved to a more upright posture with sloping desks (22° and 45°). However, no concurrent variation was found in the EMG data. In another study using inclinometers , similar results were found for body posture, with a more erect position of the head (+6°) and the trunk (+7°) when sitting with a 10° sloping desk. Still based on inclinometer recordings, another experiment confirmed this phenomenon, with a more upright posture of the head (+8.9°) and the trunk (+7.8°) when using a 10° sloping desk. These findings are in line with a host of results gathered through a less accurate technique, which consists of video recording with adhesive markers placed on the trunk and the lower limbs . Indeed, Bridger depicted a lower trunk and neck flexion, and Mandal reported that the neck angle was reduced with a 15° sloping table. However, besides Mandal’s experiment , none explored the effect of table height.

To our knowledge, no extensive EMG measurements has been performed so far under varying conditions of table slope or height, with the exception of a single study that only focused on trapezius pars descendens . As a consequence, little is known about the adaptation of the postural muscles pattern under these different conditions, about its relation with posture, or about the risk of muscular fatigue. The issue of the sliding effect of a sloping desk on the forearms and its consequences on muscular activity at the shoulder level also remains to be explored. Such findings would be quite relevant to determine the extent to which a sloping table top, advocated by many authors , is instrumental in reducing the biomechanical load sustained by the postural chain.

The general purpose of the present study was to assess the effect of table height and slope on body posture and on muscular activity along the neck, trunk and shoulders. To this aim, EMG recordings and angular position measurements were taken while sitting with two slopes and three heights of the table top.

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