# Understanding Balance Control in the Context of Riding a Personal Mobility Device
## 4 Discussion
### Application to the SPMD  
In this study, the contexts of riding the SPMD to move at a standstill and to maintain a constant speed were considered by the balance control using the whole-body movements. It was investigated with the body movement direction and distance. The experimental factors of the direction and distance were controlled using the COP and examined for both the weight-shifting ability and the postural steadiness in order to consider the riding contexts. Although the COP movements on the flat surface may not be directly applied to the COP movements on the SPMD due to its various design characteristics of the SPMD, the weight-shifting ability and the maintaining postural equilibrium of a rider are expected to yield similar patterns based on the leaning direction and distance of a rider on the SPMD. The measures derived in both weight-shifting and balance task analysis can be also utilized to assess a rider's performance on the SPMD while the ranges of the target distance and direction have to be modified further based on riding speed and control types of the SPMD. The rider's motion of leaning on an SPMD is directly related to accelerating the speed of the SPMD. The target distance of the COP was used for controlling conditions of the experiment in this study. It can be also applied that achieving a certain level of riding speed is controlled to examine the COP movements for analyzing the performance of a rider's balance control. For example, the MT and MT\({}_{\text{PSM}}\) can be used to assess a rider's weight-shifting performance on how fast and accurately the rider controls the Segway mini within a moving forward direction while achieving certain levels of speed. As aforementioned, design characteristics of the SPMD can differ a lot based on each type of the SPMD. The type can be classified by design factors such as the number of wheels, direction control methods, riding postures, control sensitivity and etc. For example, Segway drift is a self-balancing electric skate that is two separate skates with one small wheel for each and limited in performances such as speed and climbing. Since the combination of the design factors can increase the difficulty of an experiment dramatically, those factors are difficult to be incorporated into the research design all in one in an experimental environment. Furthermore, riding the SPMD in the experiment demands recruiting participants of fully skilled riders and ensuring safety concerns. In this point of view, the experiment requires simplified conditions to generalize the concept of the balance control in the context of riding the SPMD as a preliminary study before developing the study in line with each type of the SPMD. There are tremendously a lot of studies in the field of human balance research. Many of those used the COP as an objective measure for the evaluation of balance. Although they are quite relevant, this study deeply and comprehensively studied the factors of target direction and distance using both objective and subjective measures. Further, the objective measures in the weight-shifting task were derived from two specific submovements and reorganized into three types to describe the COP movement of a goal-directed whole-body movement. These measures can also be utilized in other similar studies of postural movement. The method of measuring the postural equilibrium with specific postures controlled by the COP was conducted only for 3 s of recording the COP in the balance task. This is because participants were given enough time of practice and they located their COP at target locations with their own face before the recording started. In addition, the real-time COP feedback was used as a means of performing tasks during both weight-shifting and balance tasks but also as visual feedback for participants constantly receiving their instant status and interacting with it for their balance control. It works as if SPMD riders receive feedbacks while riding such as vision, vibration, and others.