# Effects of the Weight and Balance of Head-Mounted Displays on Physical Load
## 1 Introduction  
For positive experiences using virtual reality (VR) systems, user comfort when wearing head-mounted displays (HMDs) is essential. Recently, rapid advances in VR technology has led to the increased availability of VR experiences [1, 2]. In addition to significant commercial interest from the entertainment industry, VR technology is used as an image display device in various fields such as design and engineering. However, few studies have investigated the fatigue induced by wearing commercially available HMDs. The weight and balance of HMDs are key factors impacting user comfort and affecting the head movements and physical loading experienced by HMD users. Therefore, these factors must be considered when seeking to improve the comfortability of HMDs. The authors in [3, 4] suggested that increasing the mass of an HMD and the position of its center of mass (COM) may affect the physical workload. There is a relationship between the moment and the workload on the neck and lumbar joints. In the field of aviation, the authors in [5, 6, 7] reported the relationship between the physical features of helmet-mounted displays and the workload on the neck. In particular, the authors in [7] discussed the maximum torque that can be tolerated by the neck. For long-term use, it is necessary to reduce the weight of the HMD or optimize the COM position. Therefore, it is worth exploring whether similar trends apply to HMDs. The authors in [8] evaluated the biomedical stress in the neck and shoulders due to wearing an augmented reality (AR) HMD, including electromyography (EMG) measurements of the flexion angle, moment, and muscle activity, while also considering the user-reported discomfort. Their results clarified the differences in biomechanical stresses and usability for different target distances and sizes in AR systems. Similarly, the authors in [9] focused on vertical target location with VR HMDs. They concluded that targets in the VR interface should be displayed between eye height and \(15^{\circ}\) below eye height. Elsewhere, [PERSON] and [PERSON] [10] investigated the effects of HMD weight and balance on the physical workload using a professional grade HMD (weighing 800 g with a head-worn attachment made of plastic resin). They established ranges of the mass and COM for four postures. Many lighter consumer grade HMDs are becoming available, such as the Oculus Rift CV1 (468 g) [11] and various smartphone-based HMDs [12]. These lightweight HMDs have elastic belts as the head attachment. Therefore, they should also be investigated as their weight and balance might result in more comfortable HMDs. The authors in [13] studied three different types of HMD and increased their weight to evaluate user discomfort, reporting differences between soft-belt headsets and integrated headsets. However, they only applied additional weight to the front side of the HMD. Alternatively, the authors in [14] focused on the product structures and attachment modes of AR glasses. Their results suggested that comfort varies for different attachment modes for identically weighted devices. Some studies focused on the comfort of HMDs, but few studies comprehensively investigated weight and balance. The consumer HMDs are already made lighter in weight, but different weight balances should be considered. [PERSON] and [PERSON] investigated the relationship among physical workload, weight, and balance [10]. However, the participants held the instructed postures for five seconds with professional heavy HMD. If we assume a natural HMD usage situation, the participants should get the spontaneous postures for at least a few minutes. Therefore, we decided to vary the weight and balance of the lightweight HMD to investigate the physical workload by the participant's spontaneous posture change during game play. This study aims to clarify the relationship between the physical loads, weight, and balance of HMDs.