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Fall inducing movable platform (FIMP) for overground trips and slips
Journal of NeuroEngineering and Rehabilitation volume 17, Article number: 161 (2020)
Abstract
Background
The study of falls and fall prevention/intervention devices requires the recording of true falls incidence. However, true falls are rare, random, and difficult to collect in real world settings. A system capable of producing falls in an ecologically valid manner will be very helpful in collecting the data necessary to advance our understanding of the neuro and musculoskeletal mechanisms underpinning real-world falls events.
Methods
A fall inducing movable platform (FIMP) was designed to arrest or accelerate a subject’s ankle to induce a trip or slip. The ankle was arrested posteriorly with an electromagnetic brake and accelerated anteriorly with a motor. A power spring was connected in series between the ankle and the brake/motor to allow freedom of movement (system transparency) when a fall is not being induced. A gait phase detection algorithm was also created to enable precise activation of the fall inducing mechanisms. Statistical Parametric Mapping (SPM1D) and one-way repeated measure ANOVA were used to evaluate the ability of the FIMP to induce a trip or slip.
Results
During FIMP induced trips, the brake activates at the terminal swing or mid swing gait phase to induce the lowering or skipping strategies, respectively. For the lowering strategy, the characteristic leg lowering and subsequent contralateral leg swing was seen in all subjects. Likewise, for the skipping strategy, all subjects skipped forward on the perturbed leg. Slip was induced by FIMP by using a motor to impart unwanted forward acceleration to the ankle with the help of friction-reducing ground sliding sheets. Joint stiffening was observed during the slips, and subjects universally adopted the surfing strategy after the initial slip.
Conclusion
The results indicate that FIMP can induce ecologically valid falls under controlled laboratory conditions. The use of SPM1D in conjunction with FIMP allows for the time varying statistical quantification of trip and slip reactive kinematics events. With future research, fall recovery anomalies in subjects can now also be systematically evaluated through the assessment of other neuromuscular variables such as joint forces, muscle activation and muscle forces.
Background
Global fall incidence in elderly population(s) (age
65) has an annual mean rate of approximately 30% [1], with the rate doubling for individuals above 75 years old [2]. The importance of fall related solutions increases as the world population ages. However, the rarity and variability of real-world falls greatly impedes the progression of falls related research. It is impracticable to request the elderly to wear motion capture sensors all year round only to capture one instance of fall. Hence, systems capable of inducing falls in safe controlled environments are essential to advancing our understanding of the neuro and musculoskeletal mechanisms underpinning falls events.
Trips and slips are the focus of this work as they represent the majority of externally induced falls in real world settings [3,4,5]. Trips induce different recovery strategies depending on when in the gait phase an individual is perturbed. An elevating strategy is utilised when the swing leg encounters an easy to overcome perturbation during the early to mid swing gait phase [6, 7]. If the obstacle or perturbation is sufficiently large, a skipping strategy is utilised [8]. This is due to the perturbed leg being arrested from forward motion, requiring the contralateral leg to skip forward to reinstate a suitable base of support and regain stability. A lowering strategy is used during late-swing trips, where the perturbed leg lowers immediately after perturbation and an additional step is taken to clear the obstacle [6, 7].
Backwards falls are normally caused by slips that occur during the initial stance phase. Normally, the stance leg acts as a resistance force (foot to ground friction) during the initial stance phase that converts the forward momentum of the body into angular momentum of the upper body relative to the lower body. This conversion is possible because of the ankle rotating joint and the superiorly located body Center of Mass (CoM) to the resistive force. When there is a lack of friction, the resistive force is no longer sufficient to generate upper body angular momentum, and the entire body slides forward, creating a slip. Recovery strategies following a slip sees the slipped foot immediately adopting a flat-footed configuration relative to the ground and the contralateral leg is placed behind the CoM to provide a recovery moment. Subsequent walking gait following the initial slip response will follow the surfing strategy, with the swing foot sliding forward instead of stepping off quickly during the swing phase [9]. This is done to increase the contact area between the foot and the ground, which is though to increase frictional forces.
Trips and slips have often been studied and induced separately as their fall and recovery mechanisms are vastly different. Trips are commonly induced by an obstacle while walking on an instrumented treadmill [10,11,12]. These treadmill systems allow for precise and accurate velocity control that conventional overground walking systems are generally not able to replicate. Obstacles and perturbations can also be rendered easily as many mechanisms can be hidden around and under the instrumented treadmill systems. Though there are obvious benefits for the use of instrumented treadmill systems within the falls literature, it is widely known that an individual’s gait pattern changes when walking on a treadmill versus overground. Differences in an individual’s kinematics [13,14,15], joint moments and muscular activation [15,16,17] have been well documented. Additionally, control of the treadmill after fall onset is critical to replicate true fall dynamics. The treadmill must travel exactly to the speed of the recovery limb to prevent artificially widening or narrowing their base of support (BoS).
Another type of trip induction system uses overground walking to generate more realistic real-world type falls. This type of systems need to account for the subject’s changing linear position during walking gait. Multiple hidden obstacles are built to induce trips along a fixed pathway [6, 18,19,20]. Since different gait phases induce different recovery strategies, these obstacles have to be densely packed to synchronise the simulated trip with the correct gait phase [6, 9]. The number and size of these fall inducing mechanisms makes this an expensive experimental technique which may not be practical for many laboratories globally. A more cost-effective approach is to develop a localised brake and motor system in the place of multiple ground-based obstacles to induce falls over a distance [21]. The primary drawback of this system is the need for an overhanging railing harness system for safety, limiting its use to designated locations. The overhanging railing harness system also has high inertia which can alter the gait mechanics of the subject under investigation.
Slip experiments commonly depend on a split-belt treadmill [22, 23] or a motorised floor plate [24] to provide the sudden gain in acceleration during a slip. However, the limited actuation distance of these devices means that slip only occurs over a short distance. Once the motorised plate or treadmill stops, the subject can generally regain stability immediately, unlike real-world slips in which velocity decreases slowly over a slippery surface. Even if the deceleration of the motorised plate or treadmill is controlled, it is difficult to match the intended joint kinematics. Furthermore, as previously mentioned, treadmill-walking can change the gait mechanics of the subject, arguably preventing the observation of a true transition from walking to slipping. A sliding plate [25] is better at replicating true slip scenarios, but its limited sliding distance is an important constrain. Another method of inducing slip relies on sliding over a slippery surface [26,27,28]. This method replicates true slip scenarios, but similar to a trip, they are also constrained by a high inertia overhanging harness which can prevent the observation of a true transition from walking to slipping.
To the best of our knowledge, there exists one fall inducing robot for overground walking that allows for changes in heading angle, does not impose constraints on the walking path and is not constrained by a high inertia overhanging harness [29]. This robot induces fall-like imbalance through perturbation to the pelvis. However, this method of fall induction bypasses the lower limbs’ reactive responses that are present in real-world fall scenarios. The unwanted dynamics of the lower limbs caused by obstacles and slippery surfaces are disregarded, preventing the reproduction of ecologically valid fall recovery strategies. For example, a leg that encountered an obstacle during a real trip will experience sudden deceleration and the user will require time to overcome the unwanted dynamics and widen their BoS. Instead of leg deceleration during a trip, the forward pelvis perturbation from the robot may unintentionally assist the subject to widen their BoS, resulting in improved stability.
The purpose of this research is to develop a Fall Inducing Movable Platform (FIMP) for realistic fall induction (Fig. 1). The FIMP should have the following characteristics:
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Usable on relatively level ground without space constraints.
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Allows changes in heading angle, velocity and gait patterns.
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Minimises mechanical inertia from the safety harness system worn by the subject.
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Induces ecologically valid falls via ankle perturbations.
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Capable of inducing both a trip and slip.
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Capable of producing random, unexpected perturbations.
FIMP acts as a platform for the mounting and integration of sensors, actuators and processing units required to perform ecologically valid falls.
Another shortcoming of prior research in this fall-related field is in the analysis of the time varying human motion data (i.e., kinematic, kinetics, muscle force). Time varying or continuum human movement data are not analysed as a time series, but as numerous discrete, or zero-dimensional (0D), data points, such that only 0D statistics such as the maxima, minima, mean, and median can be analysed.
Such methods fail to take into account the shapes of the waveforms and predisposes the analysis to both type 1 and type 2 errors. Instead, a topological method for detecting statistically significant field changes in n-dimensional continua called Statistical Parametric Mapping (SPM) [30] was employed to overcome these shortcomings. SPM allows for the time-normalised analysis of a waveform in its entirety, such as flexion joint angles, forming a statistical parametric map. Significance is reached only when the value of the test statistic exceeds the test statistic threshold. SPM applicability to that analysis of joint kinematics [31] and clinical gait [32] has been established . In this paper, the SPM analysis toolbox, 1-Dimensional Statistical Parametric Mapping (SPM1D) [33], is applied to falls analyses. The usage of SPM1D with FIMP’s ecologically valid falls allows for the detailed time varying analysis of an individual’s or group of individual’s fall reactive kinematics performance.
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