Comparison of acceleration signals of simulated and real-world backward falls

https://doi.org/10.1016/j.medengphy.2010.11.003Get rights and content

Abstract

Most of the knowledge on falls of older persons has been obtained from oral reports that might be biased in many ways. Fall simulations are widely used to gain insight into circumstances of falls, but the results, at least concerning fall detection, are not convincing. Variation of acceleration and maximum jerk of 5 real-world backward falls of 4 older persons (mean age 68.8 years) were compared to the corresponding signals of simulated backward falls by 18 healthy students. Students were instructed to “fall to the back as if you were a frail old person” during experiment 1. In experiment 2, students were instructed not to fall, if possible, when released from a backward lean. Data acquisition was performed using a tri-axial acceleration sensor. In experiment 1, there was significantly more variation within the acceleration signals and maximum jerk was higher in the real-world falls, compared to the fall simulation. Conversely, all values of acceleration and jerk were higher for the fall simulations, compared to real-world falls in experiment 2.

The present findings demonstrate differences between real-world falls and fall simulations. If fall simulations are used, their limitations should be noted and the protocol should be adapted to better match real-world falls.

Introduction

Over the past several decades, falls among older adults have gained increasing attention worldwide, with many studies focusing on fall prevention [1]. Facing the demographic shift and the observed high incidence rates of falls that often leads to injuries, fractures and even death, this common problem has become a very important public healthcare issue. Although much research has advanced the field, the understanding of falls is still limited.

Most of the knowledge on falls to date has been obtained in large-scale epidemiological studies. These have been performed using questionnaire-based fall reports which rely on oral reports by the subjects themselves or by proxies and are biased in many ways [1], [2]. False-reporting or not reporting of falls can be related to the cognitive status of the subjects, the shame of reporting, the fear of consequences, or simply difficulties in defining a fall. Zecevic et al. showed that fallers and caregivers may have different definitions of falling [2]. For example, falling to the ground without an injury might not be interpreted as a fall by every person. There is evidence that 75–80% of all falls without injury are not reported at all [3].

Objectively measured data of falls in a real-life environment are missing and, therefore, many aspects of fall events remain unclear. Body fixed sensor technology, optoelectronic approaches, exo-sensors, and even video material might enhance our understanding of falls and thereby lead to more effective methods for fall prevention and fall detection. However, due to access problems to the target population and other difficulties (e.g. cost, adherence), the number of recorded, documented and published real-world fall data of older people is minimal. Most studies report that approximately one out of three older person falls unintentionally each year. Although this incidence rate is quite high in terms of a disease, in relationship to the feasible observation periods of existing sensor devices the events are relatively rare. The number of measurements needed strongly depend on the particular hypothesis. As an example, to capture 100 real-world falls it would be necessary to record approximately 100,000 days of physical activity (300 person years). Given a sensor observation period of 7 days this means about 15,600 observation intervals each with recharging and data download. In the absence of sufficient real-life fall recordings, researchers focus on data derived from fall simulation studies to develop biomechanical models of falls [4]. Most of the work is done in the field of fall detection, mainly using devices based on accelerometer technology [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19]. There is a large consensus that sensor-based fall detection could prove to be very useful for patients. Primarily, it could automatically provide help and shorten rescue time if a faller is unable to send an alarm. Approximately 3% of all fallers lie for more than 20 min without external support [20]. In a cohort of people aged 90 years or older, 80% of the fallers were unable to get up by themselves and 30% remained on the floor for an hour or more [3]. Analysis of the pre-impact sensor signals could help to understand fall mechanisms, such as potential protective movements during the fall. Thus, specific exercise regiments could be developed. Furthermore, information on the impact intensity could lead to new preventive approaches like designing or choosing adequate absorbing floor surfaces. These approaches so far are derived from simulation models [21]. Unfortunately, most of the simulation approaches have failed so far to come up with satisfactory results when they are applied to real-world situations. For example, although fall detection algorithms achieved high sensitivity and high specificity in most simulation studies, the devices had a poor performance in real-life, resulting in high rates of either false positive or false negative alarms and therefore poor user compliance of the patients [22].

To our understanding, the observed gap between experimental and real-life performance of fall detection devices might be due to random and/or systematic differences between simulated and real-world falls for many reasons. For example, the experimental simulation implies consented information to the volunteer. Most experimental designs allow self-initiation of the fall leading to anticipation that may change postural control and response mechanisms, whereas an actual fall generally is unanticipated [23]. Most of the simulation approaches focused on the impact phase of the fall and neglect the phase before the fall. Only a few studies considered other phases of the fall event [16], [24], [25], [26]. The second ethical dilemma is linked to this impact phase. While many (injurious) falls occur on hard materials, such as tiles, ethical constraints require soft landing materials or protective clothing, both absorbing the impact, to avoid injuries to the participants. Thus, acceleration values of a simulated impact phase will never reflect a real-world fall. Basically, real-world falls in contrast to simulated falls are uncontrolled and not planned; recording a real-world fall means recording the intuitive reaction of a person. To perform meaningful simulations, real-world fall events and possible differences between real-world and simulated falls have to be considered first. Subsequently, it might be possible to create an experimental protocol for more realistic fall simulations.

Although the recording of real-world falls is difficult, we were able to collect a number of real-world fall acceleration data in a recent European project (www.sensaction-aal.eu) within a high risk population. Based on these data, we tested two hypotheses in the corresponding experiments 1 and 2: (1) there are systematic differences in acceleration values between real-world and simulated, self-initiated backward falls; (2) a sudden release from a backwards lean during the simulation and the instruction to avoid falling will reduce the differences between real-world and simulated falls.

Section snippets

Subjects and design

Acceleration signals of 5 falls of 4 patients (mean age 68.8 years, SD 4.5, all women) suffering from progressive supranuclear palsy (PSP) were used to describe real-world backward falls. The data were taken from a cross-sectional study to describe clinical aspects of PSP patients [27] and from an intervention study to investigate the feasibility of audio-feedback to improve balance, which was offered to the participants after the cross-sectional study 3 times per week [28]. PSP is an atypical

Results

Fig. 2 shows the acceleration pattern of a real-world fall (a), a simulated fall of experiment 1 (b), and a simulated fall of experiment 2 (c). In experiment 1, when the participants tried to fall like an older person, there was significantly more variation within the acceleration signal during the fall phase in the real-world falls compared to the fall simulation. This observation is demonstrated by the fact that all median values of variance and maximum jerk along all axes were higher in

Discussion

This study, to the best of our knowledge, compared for the first time real-world falls of older persons with low level of functional performance with simulated falls of younger persons, while falls were measured with accelerometers. The real-world falls were well documented falls in the backwards direction; these falls were compared to simulated backwards falls. The acceleration signals of the fall phase (pre-impact) were used to describe the backwards fall. As shown in this study, the fall

Conflict of interest

RC van Lummel is the owner of McRoberts BV, the provider of the DynaPort® MiniMod.

Acknowledgements

Menno Zuidema from McRoberts BV helped to prepare acceleration signals for further analysis. The study was funded by the Robert Bosch Foundation and by the European Commission (FP6 project SENSACTION-AAL, IST-045622). The sponsors had no influence on the design and conduction of the study, or on the writing of the manuscript and the decision to submit the manuscript for publication.

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