Author(s): Justyna Michalska 1,*, Anna Kamieniarz 1, Anna Brachman 1, Wojciech Marszalek 1, Joanna Cholewa 2, Grzegorz Juras 1, Kajetan J. Slomka 1
Unintentional falls among people over 65 yr. of age are an increasing problem and are the leading cause of injury-related deaths . According to the WHO, between 2015 and 2050, the proportion of the world's population over 60 years old will nearly double, from 12% to 22%. Currently, in developed countries, the percentage of elderly subjects (ESs) is more than 20%. ESs experience falls due to significant deterioration of balance ability associated with a decrease in muscle strength, sensory function and widespread degeneration in the central nervous system .
Moreover, with age, accompanying neurological diseases may appear. Parkinson's disease (PD) is the second most common neurodegenerative disorder . This chronic and progressive disease leads to rigidity, tremors, bradykinesia and thus impaired balance . Therefore, PD subjects are more exposed to the risk of falls than their peers .
The fear of falling (FOF) could appear well before and/or after an incident of a fall which creates a vicious cycle . Concomitant psychological symptoms of falls are the FOF, which is common among older adults regardless of whether they have sustained a fall. The FOF might also be a proper reaction to certain situations, leading elderly subject to be cautious, and can contribute to fall prevention through careful choice of movement activity . Nevertheless, ESs increasingly cease physical activity because of a FOF .
The FOF is assessed with many self-reported questionnaires, among which the Falls Efficacy Scale International (FES-I) test is widely used . It has become a useful instrument for predicting the risk of falls in elderly subjects due to its excellent reliability and validity across different populations . The questionnaire refers to most basic, everyday activities. Only people with a FOF may have difficulty performing these things.
Most daily activities require keeping the center of gravity (COG) within the confines of the base of support (BOS). Therefore, falls are quite frequent during everyday activities, especially when the COG is voluntarily shifted near or outside the limit of stability (LOS) . The LOS is depicted by the stability boundary and anatomically was assumed to be the foot envelope. The COG usually travels along a much smaller area of the BOS . It is suggested that the anterior mechanical stability boundary is located on the line connecting the extreme points located in the sagittal plane at the height of the first heads of the metatarsal bones . Moreover, from a biomechanical point of view, the rotation axis is located slightly further than the area of the first metatarsophalangeal joint . Crossing this mechanical limit inevitably involves changing the standing strategy (heel rise or toes co-contraction) or imposing gait initiation.
In older adults over 65 yr, 60% of falls occur in a forward direction . Additionally, Youn et al.  noticed that more than 70% of PD subjects had a history of falls in the forward direction....