## 1 Introduction  
The frequency of falls and fall-related injuries increases with age. As the population ages, falls become one of the major health problems, not only for those with some degree of balance or mobility impairment, but also among healthy active seniors [1; 2; 3]. Previous studies in the USA have revealed that falls were the leading cause of external injury among the elderly, accounting for 24 percent of emergency visits [4] and responsible for 70 percent of accidental deaths in persons aged 75 years and older [1]. The estimated expenditure for treatments of fall-related injuries amounts to 20.2 billion US dollars annually [5]. Further, previous studies also revealed that one-third of people 65 years and older experience at least one fall each year [3; 6; 7]. This severe problem has a significant impact on personal health and medical care costs in older adults. As half of the baby-boom generation crosses into its 60s, falls, and associated costs, are becoming a major problem worldwide. Hip bones are among the most frequently fractured body parts resulting from falls. They are a frequent cause of early death, functional dependence, and high medical care costs [8]. At the moment, wearable sensor systems tend to collect information such as velocity, acceleration, vibration, and tilt signals to discriminate falls [9; 10], however, they are fall detectors, and cannot prevent injuries resulting from falls. Obviously, these fall-detectors can significantly shorten the time for an emergency treatment, but hardly protect seniors during falls. The current prevention method is to use hip protectors (Figure 1). The pads are held in position by being incorporated into a garment that is supposed to keep the pad in the right position with respect to the hip area. Unfortunately, it often fails to do so because the pocket containing the pad can move away from the area during falls [11]. Additionally, some seniors refuse to use hip protectors because they find them constraining their daily activities [12]. Hence, a new type of protector that is only activated during a fall is desired. Such an idea can be realized by using a wearable-sensor system to provide a fall-triggered air-pad unit (like an air-bag in cars). The unit could be placed within a vest connected to shorts (the air-bag). The core of such a unit is a pyrotechnic gas generator. Such a generator for cars made by SEVA Technologies is only \(\sim\)100 mm long with a diameter of \(\sim\)30 mm, weighs \(\sim\)300 g and is less than \(\upxi\)10 [13]. It should be noted that the desired air-pad requires about 1/20 the amount of air that a car airbag does. Technically, such a mini air-pad unit would not change the appearance or weight of a vest significantly. There are several existing studies exploring the possibility of wearable systems for injury protection during falls. Most of the previous studies started prototyping without characterization of or comparison between senior daily activities and their possible fall movements [14; 15]. Their developments were related to the hardware development, i.e., the focuses were not to explore the reliability of fall detection methods. One potential risk of such an approach would be that the airbag may be activated occasionally during daily activities. There was one study that kinematically compared daily activities and falls [16], but involved only two realistic activities, i.e., walking and sitting down, (the other tested conditions--lying down on a bed and getting out of the bed--would not be practical, i.e., sleeping with airbag). There are too many unknowns which have not been investigated. Biomechanically, for developing such a wearable-sensor system to prevent hip fractures, studies are first required to distinguish between routine daily activities and potential fall situations.