# Understanding Balance Control in the Context of Riding a Personal Mobility Device
## 1 Introduction
Unexpected and rapid weight shifting may be required depending on the riding circumstances such as sudden stops and obstacle avoidance. The balance control and pattern in such conditions can be studied in relation to the design of the SPMD control interface for the safety of riders. As a type of vehicle, the safety of SPMD riders is a very critical aspect. Hence, it is necessary to study human characteristics as in automobile research. Balance is a generic term used to describe the dynamics of body posture to prevent falling [9]. When a person's center of mass is projected vertically to the ground, the point of contact with the ground is called the center of gravity, and the balance can be stabilized when the point is in the base of support (BOS) [10; 11; 12]. Previous studies related to human balance control have evaluated the static and dynamic postural control in humans. In those studies, the center of pressure (COP) measured through a force plate is one of the most widely used parameters for assessing human balance control. The COP is calculated from the ground force measured on a force plate, which is located at the center of the distribution of the total force applied to the support surface, and represents the weighted average of all pressures on the area in contact with the support surface [13; 14]. The COP can quantitatively evaluate the balance of human static and dynamic postures [15; 16]. Consequently, it can be used to assess human balance in various fields including human characteristics [17; 18; 19; 20; 21; 22], work and environment [23; 24; 25; 26], and clinical and rehabilitation [27; 28; 29]. Although there are various balance control studies using the COP, the postural controls in the context of SPMD riding need to be further investigated. SPMD riders often have to shift their weights rapidly and accurately to move the vehicle or to avoid an obstacle. They also need to maintain balance with a leaning posture to control the speed of the vehicle. In this regard, there are studies that examined the weight-shifting ability in discrete COP movements and the postural equilibrium in different leaning postures using the COP. [PERSON] et al. [18; 19] identified the effects of age on the performance of the COP movement for weight shifting in terms of speed and accuracy with different target distances and compared the anterior and posterior COP movements. They found different control strategies between young and older women in the COP movement speed and the number of submovements. In addition, the movement directions affected the COP movement speed and accuracy. [PERSON] et al. [30] also investigated the differences between young and older adults by task difficulty in the control of weight shifting in terms of time,fluency, and accuracy of the COP movement. In their study, the task difficulty such as target distance and target size affected the weight-shifting performance but there were differences between the two age groups. [PERSON] and [PERSON] [31] investigated the effects of visual information on the maintenance of postural equilibrium in different leaning postures. It was found that balance maintenance was affected by vision and visual feedback in relation to the time when vision was removed, but there were no differences between the vision and visual feedback on the equilibrium maintenance. From the abovementioned studies, it is evident that the target distance and direction have significant effects on the control performance and strategy in the weight shifting and maintenance of postural equilibrium. Therefore, this study aimed to investigate the balance control of SPMD riders in a simplified and restricted movement simulation. The two objectives of this study were to determine (1) the effects of target distance and direction on the control of weight shifting and (2) the effects of target distance and direction on the balance control. The target distance and direction were specified as visual targets in the limit of stability (LOS) of the subjects using real-time COP feedback.