# Scoring of Human Body-Balance Ability on Wobble Board Based on the Geometric Solution
## 1 Introduction  
Balance is essential to remain steady when walking, sitting up, standing, or playing sports. People utilize balance constantly in their daily routines, without even thinking about it. There is evidence that balance training produces moderate-to-large reductions in the risk of body imbalance, fall, or subsequent injuries, and contributes to the optimization of motor performance [1, 2, 3]. Various factors lead to decreased balance control, such as sensory feedback, muscle strength, cognitive function, and biomechanical constraints [4]. For example, our previous study, related to gait analysis for older adults, found that the symmetry between left and right feet differed in fast-walking speed because elderly patients with diabetes had a worse balance than healthy elderly adults [5, 6]. Therefore, balance training is practical for adults and is suitable for people of any age. Furthermore, in some studies, balance-ability training is used to improve postural control in many sports (and for many fitness levels), such as basketball, baseball, soccer, etc. [7, 8, 9]. Thus, people need to train and evaluate their balance ability to enhance their capacity and prevent falls, especially older patients. The authors have proposed a non-invasive and low-cost system for trainees who would like to improve their balance ability and have been studying methods to help improve health and quality of life by self-health management [10]. Home-training apps based on artificial intelligence (AI) are an effective solution to help users exercise with the correct posture, even if users exercise alone at home [11, 12]. Representative developed apps include Aaptiv Coach, Fitbod, VI Trainer, Kaia Personal Trainer, VAY Fitness Coach, Millie Fit, infiGro, and Gymfitty [13]. These apps can automatically calculate exercise frequency by estimating exercise posture with extracted key points through AI-based human-pose-estimation technology. Most people are getting busier, so they have no time to exercise regularly. Maintaining regular exercise is even more challenging. Thus, these kinds of apps are helpful. The primary role of a human coach is to manage the trainee to ensure they continue to exercise at first. Then, the other significant role of a coach is to evaluate the trainee's exercises with the correct pose. Unfortunately, these developed apps do not provide the function of the second important role, which is assessing the posture of the trainee [14; 15]. Examples of exercise-posture-evaluation systems include expensive motion-capture and force-plate systems [16; 17]. The marker-based motion-capture system is a system that attaches a marker to the subject's body, recognizes the attached marker by a plurality of cameras, reconstructs the subject's motion in a computer, and evaluates the posture of the body in various situations. It seems complicated to proceed from experimental preparation to analysis alone at home. Markerless motion capture uses standard video and often relies on deep-learning-based software (pose-estimation algorithms) to perform movement analysis with reduced data collection and processing time, compared to marker-based methods [18]. However, the hardware employs either depth cameras or standard video cameras and may be used in single- or multi-camera systems, and it is more expensive than an RGB camera. Although with the force-plate system is not necessary to attach the marker to the human body, this system needs professional knowledge to understand the analysis results of the center of pressure (COP) and the vertical-reaction forces (VRFs) during standing, after removing the visual information [19; 20; 21; 22]. Therefore, there is a need for a low-cost device and an evaluating method that anyone can easily understand. Thus, the current study aimed to propose and verify a new algorithm to score the human body-balance ability on the wobble board (HBBAWB), based on a geometric solution using a cheap and portable device.