Evaluation of physical workload affected by mass and center of mass of head-mounted display

doi:10.1016/j.apergo.2017.11.016

Applied Ergonomics

Volume 68, April 2018, Pages 204-212

https://doi.org/10.1016/j.apergo.2017.11.016 Get rights and content

Highlights

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A gazing task while wearing head mounted display (HMD) was conducted.
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The HMD had different levels of the mass and center of mass (COM) positions.
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The joint torques of neck and lumbar region were calculated during the task.
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The neck joint torque was significantly affected by the mass and COM of HMD.
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The proper COM position varied depending on the position of the target object.

Abstract

A head-mounted display (HMD) with inappropriate mass and center of mass (COM) increases the physical workload of HMD users. The aim of this study was to investigate the effects of mass and COM of HMD on physical workload. Twelve subjects participated in this study. The mass and posteroanterior COM position were 0.8, 1.2, or 1.6 kg and −7.0, 0.0, or 7.0 cm, respectively. The subjects gazed at the target objects in four test postures: the neutral, look-up, body-bending, and look-down postures. The normalized joint torques for the neck and the lumbar region were calculated based on the measured segment angles. The results showed that the neck joint torque was significantly affected by mass and COM and it increased as the HMD mass increased for all test postures. The COM position that minimized the neck joint torque varied depending on the test postures, and the recommended ranges of COM were identified.

Introduction

A head-mounted display (HMD) is a head-mounted equipment with an image display monitor, and applied to several fields such as engineering design and aviation. In the engineering design process, for example, HMD is used for providing virtual reality (VR) environment. The use of VR helps the designers in all stages of the design process (i.e., from upstream to downstream) by sharing the detailed images of the final product in the early stage itself (Aromaa and Väänänen, 2016). HMD users move their necks and trunks to secure the field of view to see the virtual objects in various postures. While viewing with an HMD, excessive physical workload may occur in different body parts including the cervical segment due to awkward posture and static posture maintenance (Nichols, 1999). An increase in the mass of HMD and a change in the position of the center of mass (COM) may affect the physical workload of HMD users because they may increase the moment acting on the neck and lumbar joints (Baber et al., 1999, McCauley-Bell, 2002). In the aviation field, Äng and Harms-Ringdahl (2006) and Van den Oord et al. (2012) reported that helicopter pilots experience neck pain because they routinely wear head-mounted equipments, such as a helmet with night vision goggles. Therefore, the mass and COM of HMD should be determined to consider their effects on the physical workload of HMD users.

Knight and Baber (2007) evaluated the physical workload during a simulated patient treatment with HMD by using the rapid upper limb assessment (RULA; McAtamney and Corlett, 1993), which is an observational method to determine physical workload. A higher RULA score represents a higher physical workload. Their study results showed that wearing an HMD resulted in a significantly high RULA score. Forde et al. (2011) investigated the effects of wearing night vision goggles (NVG) equipped in front of the helmet, on cervical spine load, which was found to increase significantly due to the NVG use. Thuresson et al. (2003) measured the electromyogram (EMG) activity of neck muscles during NVG use in different trunk inclinations and neck flexion angles. The average EMG activity in all posture conditions significantly increased on wearing an NVG. Murray et al. (2016) also reported that the use of NVG increased EMG activity in neck muscles. These studies revealed that a head-mounted equipment increased the physical workload in users; however, they cannot determine recommended ranges of mass and COM of HMD because multiple levels were not provided for the mass and COM in their experiments. Knight and Baber (2004) measured EMG activities of the neck muscles while wearing HMD for gazing at the target object. Their HMD had three levels of weight and counterweight for each weight condition (i.e., six conditions in total). However, the ‘with’ and ‘without’ counterweight conditions were implemented in different subject groups, and the quantitative evaluation for the appropriate COM range was not provided.

It is necessary to determine the proper mass and COM of HMD to reduce the physical workload and musculoskeletal disorder risk. Therefore, the objective of this study was to quantitatively evaluate the effects of mass and COM of HMD on the physical workload of HMD users. Approximate functions of the physical workload were predicted as a function of the mass and COM, and their proper ranges were discussed.

Section snippets

Subjects

Twelve healthy Japanese subjects (six men and six women) aged between 21 and 24 years participated in this experiment. All subjects were university students, and none had a musculoskeletal disorder. Their mean ± standard deviation stature and body mass were 162.6 ± 9.1 cm and 56.7 ± 10.8 kg, respectively. This experiment was approved by the research safety and ethics committee of the Hino campus of Tokyo Metropolitan University (ID: 109). A written informed consent was obtained from all

Results

In this chapter, all figures represent the average values of 12 subjects, and the error bars indicate the standard deviations.

Neutral posture

The effect of COM was significant for the left rotation angle of the head. However, the difference of the angle from the ‘without HMD’ condition was only 4° at maximum. Therefore, the posture during gazing was almost constant, irrespective of the mass and COM of HMD. The effect of eye dominant may be one possible reason for that the left rotation angle showed negative value (or rotate to the right). Pradham et al. (2001) reported that the head rotates to the dominant eye side to make the

Conclusions

The body segment postures of the neck and lumbar region while gazing with an HMD at the target objects were evaluated for four test postures: the neutral, look-up, body-bending, and look-down postures. The physical workloads of the neck and lumbar were calculated and evaluated in order to investigate the effects of the mass and COM of the HMD on the workload. The major findings are as follows:

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In the case of look-up posture, the subjects extended their neck joint when the COM position moved

Acknowledgements

We received a generous support from Canon, Inc. with respect to the use of the HMD prototype and instructions on the setting of experimental conditions.

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Citation Excerpt :
The obvious example from this investigation is military personnel who often don helmets with night-vision goggles, where the prevalence of acute bouts of neck pain during or shortly after flight exceed 75 % (Harrison et al., 2007; Lange et al., 2011; van den Oord et al., 2010). Similarly, lengthy bouts with a VR headset are also associated with neck and shoulder discomfort (Kim and Shin, 2021), and the magnitude of HSM and location of the COM may be potential risk factors (Baber et al., 1999; Chihara and Seo, 2018; Penumudi et al., 2020). Together, these results highlight the sensitivity of the head-neck system since the 0.56 kg VR headset examined by Kim and Shin (2021) would only translate to between 10 and 22 N of added compression to the cervical spine (<10 % increase).
Occupations or activities where donning head-supported mass (HSM) is commonplace put operators at an elevated risk of chronic neck pain. Yet, there is no consensus about what features of HSM influence the relative contributions to neck loads. Therefore, we tested four hypotheses that could increase neck loads: (i) HSM increases gravitational moments; (ii) more muscle activation is required to stabilize the head with HSM; (iii) the position of the HSM centre of mass (COM) induces gravitational moments; and (iv) the added moment of inertia (MOI) from HSM increases neck loads during head repositioning tasks.
We performed a sensitivity analysis on the C5-C6 compression evaluated from a 24-degree freedom cervical spine model in OpenSim for static and dynamic movement trials. For static trials, we varied the magnitude of HSM, the position of its COM, and developed a novel stability constraint for static optimization. In dynamic trials, we varied HSM and the three principle MOIs.
HSM magnitude and compression were linearly related to one another for both static and dynamic trials, with amplification factors varying between 1.9 and 3.9. Similar relationships were found for the COM position, although the relationship between C5-C6 peak compression and MOI in dynamic trials was generally nonlinear.
This sensitivity analysis uncovered evidence in favour of hypotheses (i), (ii) and (iii). However, the model’s prediction of C5-C6 compression was not overly sensitive to the magnitude of MOI. Therefore, the HSM mass properties may be more influential on neck compression than MOI properties, even during dynamic tasks.
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View full text

Evaluation of physical workload affected by mass and center of mass of head-mounted display

Highlights

Abstract

Introduction

Section snippets

Subjects

Results

Neutral posture

Conclusions

Acknowledgements

Appl. Ergon.

Int. J. Ind. Ergon.

Neurosci. Lett.

Appl. Ergon.

J. Electromyogr. Kinesiol

Appl. Ergon.

Int. J. Ind. Ergon.

Man. Ther.

Estimation of inertia properties of the body segments in Japanese athletes

Biomechanism

Neck pain and related disability in helicopter pilots: a survey of prevalence and risk factors

Aviat. Space Environ. Med.