Applied Ergonomics

Volume 84, April 2020, 103010
Applied Ergonomics

The effects of target location on musculoskeletal load, task performance, and subjective discomfort during virtual reality interactions

https://doi.org/10.1016/j.apergo.2019.103010Get rights and content

Highlights

  • •
    Effect of target location on biomechanical exposures in virtual reality was tested.
  • •
    Target locations affected biomechanical exposures of the neck and shoulder.
  • •
    Excessive target locations at 15° above and 30° below eye height should be avoided.

Abstract

The objective of this study was to evaluate the effect of different target locations on musculoskeletal loading and task performance during virtual reality (VR) interactions. A repeated-measures laboratory study with 20 participants (24.2 ± 1.5 years; 10 males) was conducted to compare biomechanical exposures (joint angle, moment, and muscle activity in the neck and shoulder), subjective discomfort, and task performance (speed and accuracy) during two VR tasks (omni-directional pointing and painting tasks) among different vertical target locations (ranged from 15° above to 30° below eye height). The results showed that neck flexion/extension angle and moment, shoulder flexion angle and moment, shoulder abduction angle, muscle activities of neck and shoulder muscles, and subjective discomfort in the neck and shoulder significantly varied by target locations (p's < 0.001). The target locations at 15° above and 30° below eye height demonstrated greater shoulder flexion (up to 52°), neck flexion moment (up to 2.7Nm), anterior deltoid muscle activity, and subjective discomfort in the neck and shoulder as compared to the other locations. This result indicates that excessive vertical target locations should be avoided to reduce musculoskeletal discomfort and injury risks during VR interactions. Based on relatively lower biomechanical exposures and trade-off between neck and shoulder postures, vertical target location between eye height and 15° below eye height could be recommended for VR use.

Introduction

As virtual reality (VR) technology has rapidly been advanced over the past decade, VR is widely used in many fields including education, professional training and gaming (Miller et al., 2013; Nakamura et al., 2016). VR interface provides great potential benefits as a new computer-human interface including its natural and intuitive interactions, immersive three-dimensional surroundings, and cost-effective simulation (Chen and Or, 2017; Lin et al., 2017). However, this new interface poses potential physical ergonomic risk factors which are associated with musculoskeletal discomfort and injuries especially in the neck and shoulder regions.
Poorly designed hand-held input devices and head-mounted displays (HMDs) are known to increase risks for musculoskeletal discomfort during VR interactions (Baber et al., 1999; Chihara and Seo, 2018; Nichols, 1999). For example, non-intuitive design (poor cognitive mapping) of a hand-held input device can increase the difficulty in use and lead to awkward postures. In addition, previous studies showed that the center of mass (COM) position and the mass of HMDs significantly affected user's neck joint torque, and the optimal COM position of HMDs varied depending on users' postures (Baber et al., 1999; Chihara and Seo, 2018). Another study showed that the use of HMD increased the head rotation, neck muscle activities, and subjective discomfort compared to using the external display during document-editing tasks (Kim and Shin, 2018).
Moreover, free gestural movements and prolonged static postures of upper extremities without the arm support during VR interactions are associated with shoulder discomfort and fatigue (Kar et al., 2015; Ram et al., 2017; Samani et al., 2015). As the upper extremities account for 6% of the whole body mass (Plagenhoef et al., 1983), repetitive and sustained arm postures during VR interactions could lead to a substantial shoulder fatigue (Ram et al., 2017; Zeng et al., 2012).
Previous studies in a conventional computer-human interface showed that the biomechanical exposures in the neck and shoulder can be reduced by adjusting display heights and objects on a computer screen (ANSI/HFES 100, 2007; Straker et al., 2009, 2008). These previous studies provided important recommendations for ergonomics guidelines and standards for computer workstation set up. However, there is no sufficient information available for appropriate target locations in VR interface. Therefore, the study aim was to evaluate the effect of different vertical target locations on the biomechanical exposures and subjective discomfort in the neck and shoulder, and task performance during VR tasks. We hypothesized that different vertical target locations in a VR interface would affect VR users’ biomechanical exposures in the neck and shoulder and task performance with a longer-term goal of identifying more ergonomic object displacements in a VR interface to reduce biomechanical loading in the neck and shoulder.

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Section snippets

Participants

Twenty adult participants (10 males and 10 females) were recruited via e-mail solicitations in a university community. The mean and standard deviation of age, height, and weight were 24.2 ± 1.5 years old, 168.7 ± 9.1 cm, and 69.6 ± 11.1 kg, respectively. Eighteen participants were right-handed and two participants were ambidextrous. All participants had no current (past 7 days) musculoskeletal pain and no history of musculoskeletal disorders in the neck and upper extremities. The experimental

Neck and shoulder postures

The mean self-selected target locations of VR interaction tasks were slightly below eye height (−0.5° to −6.9°). Neck flexion/extension, shoulder flexion and abduction angles were significantly affected by target locations during both VR tasks (p's < 0.001) where a clear inverse relationship between neck and shoulder flexion was found (Fig. 4). Neck was extended when targets were located at 15° above eye height. As target locations moved from neutral (eye height) to 30° below eye height, neck

Discussion

This study evaluated the effects of five different vertical target locations (15° above eye height, eye height, 15° below eye height, 30° below eye height, and self-selected) on biomechanical exposures (muscle activity, posture, and joint moment) in the neck and shoulders, subjective discomfort, and task performance during VR tasks. The results showed that target locations significantly affected neck flexion/extension angle and moment, shoulder flexion angle and moment, shoulder abduction

Conclusion

Given higher amount of shoulder flexion angle, neck flexion moment, muscle activities of the neck and shoulder, excessive vertical target locations (15° above and 30° below eye height) should be avoided to reduce musculoskeletal loading and discomfort during VR interactions. Based on lower biomechanical measures and subjective discomfort, targets (e.g. icons and objects) in VR interface should be displayed between eye height and 15° below eye height, especially more frequently used targets.

Declaration of competing interest

None.

Acknowledgement

The authors would like to thank Jitesh Mulchandani for developing virtual reality apps.

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