# Development of a Caterpillar-Type Walker for the Elderly People
## 1 Introduction  
The proportion of the global population aged 65 and over is increasing. Korea is an aging society, with the elderly exceeding 14% of the total population as of 2017. Currently, the proportion of the elderly population makes up 15.7% of the population, and it is expected that Korea will be entering a super-aging society by 2025 [1, 2]. Aging and its associated deterioration in physical function are directly related to changes in exercise capacity. As aging progresses, sensory functions and reflexes deteriorate, and overall muscle strength decreases. In the case of lower extremity muscle strength, it decreases by 40% compared to adults in their 30 s, causing problems with body balance function, which results in decreased walking ability [3]. This decline in gait function becomes noticeable after the age of 75-80. The gait pattern of these elderly tends to a slouched posture, a shorter gait distance, and a wide distance between the feet, which is called essential senile gait disorder [4, 5]. This decline in the walking ability not only causes many restrictions on the daily life of the elderly but also becomes a major cause of falls. Falls account for the second-highest percentage of elderly deaths and occur at the highest frequency among elderly injury factors. This phenomenon is recognized as a serious problem for many countries [6]. According to a 2017 survey concerning the condition of the elderly, 15.9%of all elderly had a fall accident in the past year [7]. Among the major causes of these falls, walking environment (31%), as well as gait and balance disorders (17%), was the biggest contributor. Moreover, the decline in mobility interfered with social life, leading to depression and possibly other adverse mental health outcomes [8; 9]. A walker is often used by the elderly to maintain a social life and physical activity. As they are light and inexpensive, most elderly use them throughout their daily activities. The walking assistance market for the elderly is expected to continue to grow [10; 11; 12; 13; 14]. The general circular wheel, which has been widely used in most walkers, is difficult to operate on external walking paths with rough or uneven surfaces. When crossing obstacles, the impact applied to the walker is transmitted to the user directly, and the user can fall if it collides with an obstacle. In addition, when using the walker on a downhill slope, the elderly with a reduced upper extremity muscle strength can lose the walker. On the other hand, on an uphill slope, considerable weight is applied to the rear handle making the center of gravity unaligned, which can cause the front side of the walker to lift, inducing a fall [15; 16; 17; 18]. To compensate for these shortcomings, a previous study has developed an algorithm that analyzes the amount of vibration when crossing obstacles for attenuating shock and diminishing the vibration generated on the handlebar [19]. In another previous study, a walker that can be used comfortably and safely was developed by reducing the vibration transmitted to the user based on the analysis of the mass ratio, damping ratio, and natural frequency occurring in various road environments [20]. [PERSON] et al. proposed an algorithm that can control a motor-based electric walker for safe walking by calculating the speed and the inclination of the ground from the center of pressure of the foot, using a force-sensing resistor (FSR) [21]. Additionally, based on the EMG experiment, a walker equipped with an active brake that could be used for various purposes was developed to assist the elderly in rural environments where roads are rough and bumpy [22]. However, there has been a lack of research on a device that can solve the problems associated with slipping on slopes, excessive use of muscles to overcome obstacles and wheel obstruction in uneven terrain. Since the caterpillar moves by traction force due to friction acting on a large contact area with the ground, it is suitable for use on uneven or sloped ground [23].