Evaluation of physical workload affected by mass and center of mass of head-mounted display
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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2023, Journal of BiomechanicsCitation 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).
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