Effect of visual display terminal height on the trapezius muscle hardness: quantitative evaluation by a newly developed muscle hardness meter

Abstract

The aim of this study was to determine trapezius muscle hardness in 9 healthy volunteers before and after word processing tasks with a video display terminal (VDT) at three different heights. When using a desktop personal computer (PC), no change was observed in muscle hardness even after a 30-min task if a subject was in the reference posture with a declination angle formed by the Reid's line directed toward the upper edge of the PC screen and the horizontal plane within 5–10°. However, an increase in muscle hardness was observed after a 15-min task in a posture of looking up at the screen (angle of elevation: 15–20°) and after a 30-min task in a posture of looking down at the screen (angle of declination: 15–20°). When the same tasks were performed with a notebook PC, muscle hardness increased after 15 min. Fifteen minutes of relaxation exercise reduced the muscle hardness caused by VDT work.

Introduction

When using a video display terminal (VDT) and keyboard for data entry, the screen height and keyboard position have a strong influence on the angles of the operator's neck, shoulders, elbows and trunk inclination (Villanueva et al., 1996; Straker et al., 1997). It was reported that long-term VDT work might be a significant risk factor in the development of musculoskeletal complaints because of constrained postures of the neck and shoulders (Chaffin, 1973; Hagberg, 1984; Onishi et al., 1982). On the other hand, there is a report (Aarås et al., 1997) that the trapezius load showed no significant differences when comparing data entry with sightline to the centre of the screen at an angle of 15–30° below the horizontal. There is also an epidemiological report (Aarås, 1994) that the incidence of sick leave due to musculoskeletal illness in the neck and shoulder region is lower in workers who take flexion of the head and neck for the predominant work position. Like the above, there is a lot of discussion about a causal relationship between VDT work and musculoskeletal complaints and as well as the optimal gaze direction to the screen (Smith et al., 1981; Onishi et al., 1982; Arndt, 1983; Hermans and Spaepen, 1995; Fernström and Ericson, 1997; Jensen et al., 1998). However, there has been no study that quantitatively investigated muscular hardness as an important objective sign of musculoskeletal complaints such as shoulder stiffness and tension-type headache. There are many cases especially in Japan in which musculoskeletal complaints and muscle stiffness lead to tension-type (so-called muscle contraction) headache (Maeda, 1977). Sakai et al. (1995) reported the results of a study using a newly developed noninvasive muscle hardness meter for patients with tension-type headache. The study showed that the trapezius muscles were significantly harder in-patients who had a feeling of severe or painful stiffness. Therefore, it is important to study the relationship between VDT work and muscle stiffness.

From a bioengineering point of view, a living body is a viscoelastic material; therefore, its hardness (elasticity) can be expressed by Young's modulus, which is calculated by the ratio of stress (pressure) and strain (displacement). Therefore, in order to study musculoskeletal complaints it is important to measure subjective complaints of muscle stiffness as an objective, quantitative and rheological parameter of Young's modulus of muscular tissue.

In the present study, we used a muscle hardness meter developed by Horikawa et al. (1993) to objectively and quantitatively evaluate the effect of different screen heights on trapezius muscle hardness during VDT work.