# Gait Biomechanics for Fall Prevention among Older Adults
## 1 Introduction  
Advancements in medical science and social security systems support longevity, and in our modern society, the life expectancies of developed nations are generally above 80 yrs, which has sharply increased since the middle of the last century [1]. While humans have pursued a longer lifespan, if their well-being and quality of life (QOL) are insufficient, not only senior individuals themselves but also the surrounding family could suffer due to the need for constant care and high medical costs. For a country with a severe trend of ageing, such as Japan, healthcare costs (i.e., medical, long-term nursing care, etc.) have already exceeded the total national revenue [2]. As taxation alone is not sufficient, national bonds have been issued as one of the few options to match these increasing financial demands [3, 4]. Without accommodating for the fundamental adaptations necessary for an ongoing ageing society, current unsustainable healthcare systems will soon face critical degradation. Instead of mere longevity, our attention should be shifted to the concept of 'healthy life expectancy', broadly defined as \"the time in which people can live with sufficient well-being and QOL\" [5]. To improve our 'healthy life expectancy', it is important to prevent or even reverse the onset of 'frailty' among older adults.  
'Frailty' in the context of ageing refers to general health declines, which can be divided into physical (e.g., sarcopenia), mental (e.g., depression) and social components (e.g., isolation) [6, 7]. As illustrated in Figure 1, falls are the devastating result of frailty and a trigger for other health issues [8]. The current review, therefore, focuses on 'falls among senior adults' due to the high frequency and serious consequences of falls (i.e., hospitalisation and injury-related death).  
Adequate exercise such as 'active walking' is vital and strongly recommended for older adults to maintain their physical, mental and social health, but falls risks paradoxically exist when they are walking [9]. Therefore, it is important to ensure safe walking, but until recently, limited techniques were available to quantify the risk of movement-related falls. As such, only general associations with actual falls episodes have been reported with respect to chronic health conditions, the history of falls, medication (polypharmacy), muscle atrophy (e.g., sarcopenia), mental health, cognitive functions, osteoarthritis, motor control systems, visual and auditory functions, joint range of motion, connective tissue conditions (e.g., tendons and ligaments), proprioception and bone health [10; 11; 12; 13; 14; 15; 16; 17; 18; 19; 20; 21; 22; 23]. These findings have been useful for general guidelines on the prediction of falls risks, but more specific approaches based on motion analysis are required to deepen our understanding of particular movement-related falls risks during active walking.  
Biomechanical approaches describe a fall as a two-fold scenario: perturbation to dynamic balance and subsequent failure in balance recovery [24; 25; 26]. Quantifiability is the strength of biomechanical approaches that can objectively evaluate individuals' falls risks and the effectiveness of intervention strategies (i.e., training, rehabilitation and assistive device) [27]. The current review aims to summarise the latest research findings on fall prevention from biomechanical aspects, but it is important to first highlight the key epidemiologic information about falls in the senior population.