# Development of New Soft Wearable Balance Exercise Device Using Pneumatic Gel Muscles
## 1 Introduction  
Fall prevention is important for extending the healthy lifespan of older individuals. Aging often causes disability related to muscle weakness, balance dysfunction, decreased joint kinetics, and kinematics during gait [1; 2; 3]. A decrease in the ability to control posture is correlated with the risk of fall among older individuals [4]. A previous study reported that balance impairment increases the risk of fall by 1.2 to 2.4 times [5]. Thus, improvement of balance function is important for the extension of the healthy life span of older individuals. To address this issue, physical therapists often provide balance-focused exercises for patients or older individuals, promoting the improvement of balance function. Previous studies revealed that balance exercises can improved balance function of healthy older individuals as well as prevent falls [6; 7]. However, the improvement of balance function is difficult because balance function involves many components. [PERSON] et al. [8] reported that balance is composed of many factors, including sensory, strength, motor coordination, and alignment. [PERSON] et al. [9] proposed the following nine components of balance: functional stability limits, underlying motor systems, static stability, dynamic stability, verticality, anticipatory postural control,sensory integration, cognitive influences, and reactive postural control. Although balance exercises can reduce the risk of falls, appropriate exercise programs designed for older individuals' body functions and structure are required [10]. Balance exercises used for reactive postural control and stability are particularly important for fall prevention because reactive postural control is necessary for withstanding unexpected perturbations. Most older individuals exhibit decreased reactive postural control and dynamic stability. Compared with stable older individuals, unstable older individuals exhibit prolonged tibialis anterior muscle latency when experiencing unexpected fall conditions [11]. In addition, unstable older individuals tend to change their center of pressure (COP) trajectory in response to even small perturbations, compared with stable older individuals [12]. Exercises focused on reactive postural control and dynamic stability can improve postural control ability among older individuals. [PERSON] et al. [13] reported that older individuals perform feed-forward control to prevent unexpected slips by mimicking slippery conditions. [PERSON] et al. [14] reported that a balance assistance exoskeleton was able to help human single step balance strategies against perturbations. Another previous study reported that balance exercises using robotics to present perturbations can improve COP control and stability [15]. Moreover, systematic reviews also reported that perturbation-based balance exercises could reduce the risk of falls among older individuals [16]. However, balance exercises for reactive postural control have some limitations. First, these methods typically require large and expensive devices for generating perturbations. Second, these devices tend to be heavy, requiring metal frames and power sources. Finally, these methods can only be conducted in specific settings, such as laboratories or hospitals, because they require large devices and specialized environments. Thus, it is difficult for older individuals and patients to continue balance exercises for reactive postural control in their homes or other facilities. Recently, a number of wearable assistive devices have been developed for human health and rehabilitation. These devices can assist human movement and aid activities of daily living. In many cases, soft wearable assistive devices use lightweight materials as actuators. [PERSON] et al. [17] reported that a soft robotics suit using elastic bands improved gait kinematics and spatiotemporal parameters. Pneumatic artificial muscles are a useful actuator type that can be utilized in wearable assistive devices.