A Perturbation Platform System for Balance Testing and Rehabilitation Interventions

Falling is a leading cause of injury and death in the United States. Researchers and clinicians strive to identify and rehabilitate those at risk of falling in order to mitigate the impact of fall events and prevent future falls. Recently, perturbation-based balance testing and interventions have received increased attention, partly because reactive postural control responses triggered by perturbations are important for balance recovery during actual falls. However, current systems are limited by the need for multiple individuals to operate the device, downtime between trials to reset the perturbation and/or single mode functionality. To this end, we have developed a Perturbation Platform System that can induce perturbations in both vertical and angled directions. The system consists of two box platforms that can individually perform straight (25.4 mm, 50.8 mm and 76.2 mm) and angled (5°, 10°, and 15°) height changes while an individual is standing or walking overground. In addition, the system can automatically reset to the original position following perturbation. The platform achieves peak downward accelerations of 5.41 m/s2 during drop events that simulate sudden changes in foot-contact surfaces. and control

. Similarly, the amputee community experience major health risks as a result of falling with more than half of lower limb amputees reporting falls over the previous year and 75% of those reporting multiple falls [7,8]. In addition to the potential for significant injuries, falling can have negative consequences related to psychological and social aspects of daily living with individuals who experience a fall often limiting their physical activity and social interactions due to fear of falling [9].
With falling being a critical clinical problem, researchers and clinicians often seek to develop assessment and intervention methods to both identify those at risk of falling and help reduce the risk of future falls. Studies have shown promising results in identifying mechanisms underlying balance deficits in several populations utilizing primarily horizontal perturbations. For example, a motor-driven cable system [10] was used to apply a lateral pull to the pelvis to study reactive stepping in older adults [11] and individuals post-stroke [12]. Others have used unexpected forward and backward platform translations to study postural reflexes in individuals post-stroke [13]. Treadmill acceleration/deceleration and sideways translations have also been used to study balance control in young and older adults [14]. These early systems focused primarily on horizontal perturbations, however, falls often involve vertical displacements. As a result, more recent work has utilized vertical perturbations to assess startle reactions and balance control [15]. This system had subjects standing on a platform over an in-ground force plate. With computer control, electromagnets would release the standing surface and induce a drop perturbation. However, a limitation of the system was that resetting required the subject to step off the platform while it was manually reset. Other studies have used a pneumatically actuated platform to facilitate weight transfer during step initiation in individuals with Parkinson's disease [16,17]. However, this system could only perform unilateral perturbations and was limited to 15 mm of vertical displacement.
In addition to platform translations, studies have used angled surfaces to simulate uneven terrain. A static device that could vary the surface angle up to 15° in inversion or eversion was designed to assess balance recovery mechanisms on uneven terrain in individuals with lower-limb amputations [18]. This device was designed to sit atop a standard in-ground force plate with the individual walking unilaterally over the uneven surface on an elevated walkway. The angle of the surface was modulated manually, with two individuals being needed to manipulate the top plate into the orientations of interest.
Current devices are often limited by the need for multiple individuals to operate the device, downtime between trials to reset the perturbation, singular functionality, and/or the capability to only test unilateral or bilateral perturbations. To overcome these limitations, the purpose of this paper was to present a novel Perturbation Platform System that was designed to perform multiple perturbation types and automatically reset itself between trials without the test subject leaving the platform.

Methods Design
The design of the Perturbation Platform System was guided primarily by the goal to operate the device remotely by a single individual and reset to the start position on command while supporting the participant's body weight. The system was designed to provide a sudden vertical displacement of the standing support surface up to 76.2 mm and provide either unilateral or bilateral perturbations.
This distance was selected because it is sufficient to trigger a rapid postural reaction for balance recovery and the impact force is reasonable (approximately 1 bodyweight) for safe, repeated exposures [15]. The platform can also tilt up to 15° in both directions providing the ability to perform ankle inversion/eversion or plantarflexion/dorsiflexion perturbations depending on orientation. These angle magnitudes have been established to provide a maximum perturbation without risk of injury [18]. In addition, the platform was designed to be triggered on foot contact from the user, allowing the direction and distance of the drop to be concealed to simulate unexpected perturbations. Lastly, the system is relatively portable such that it can be moved by a single individual for setup in a gait laboratory or used in a clinical setting. To this end, the main components of the system were constructed from ABS plastic with 2 platform units each weighing 25 kgs, measuring 406 mm wide x 508 mm long x 236 mm tall and include handles for ease of transport ( Fig. 1). Final implementation in the laboratory setting includes in-ground force plates directly beneath each platform and an elevated stage surrounding the system to facilitate overground walking trials as well as helping to alleviate potential anxiety due to the sensation of height when standing on the platforms. The system is described in detail below.
The Perturbation Platform System consists of three main components: 1) two movable standing surfaces, 2) a high-pressure air source, and 3) a remote control interface (Fig. 1). The operation of each platform utilizes four, double acting, pneumatic pistons (Space Saver Low Profile, SS-150, Mead Fluid Dynamics, Chicago, IL, USA) ( Fig. 2A). Each piston features 76.2 mm of travel with the capability to lift 80 kg at a working pressure of 6.9 Bar. With four pistons per platform, each platform can lift a 320 kg person or 640 kg when two systems (i.e., bilateral perturbations) are used together. The system is controlled via a 5/3 directional air valve (IMI Norgren, K81EA00KV0KV02W1, Littleton, CO, USA) (Fig. 2B). The valve is controlled with two 12-volt solenoids that can be activated or deactivated to raise or lower the pistons. The use of the 5/3 valve allows for directional control of the pistons along with a third mode where airflow is closed, which is activated when no electrical signal is applied. This provides a safety feature in the event of power loss that will lock the pistons in place without dropping the subject unexpectedly. A single, high-pressure air source is provided from an external compressor (Fig. 2C). Currently, the compressor source is in a separate room with the air line routed through the floor to provide quiet and discreet operation of the platforms. Air is supplied and exhausted from the pistons evenly via two distribution blocks (Fig. 2D). An exhaust muffler helps to quiet exiting air to prevent startling participants or alerting them to the timing of upcoming perturbations (Fig. 2E). Lastly, each piston is fitted with a one-way variable flow control valve at the inlet/outlet to allow for speed control of the piston drop and return.  (Fig. 6), which allows a wide range of unilateral or bilateral testing scenarios.

Performance Testing
Testing was performed to verify the function and capability of the Perturbation Platform system. A single subject (male, 1.66 meters, 73 kgs, 36 years old) provided informed consent to the testing as approved by the University of Texas Internal Review Board. While standing with one foot on each platform, the system was run through the nine configurations available (Fig. 6) and returned to the original position with the subject remaining on the platform.
In order to characterize the motion of the system, a Vicon motion capture system (Vicon, Centennial, Results from the three drop heights were then compared using a one-way ANOVA and multiple comparison test to determine overall and pairwise differences (p < 0.05) between the drop heights, respectively. All statistical testing performed in Matlab (MathWorks, Natick, MA, USA).

Results
Results from testing of the various configurations showed that in each of the nine scenarios the platforms were able to achieve the intended final orientation and then return to the initial starting position. The motion capture data verified the intended drop heights to be 26.9 ± 1.1, 51.1 ± 1.4 and 73.8 ± 1.6 mm for the Min, Mid and Max levels, respectively (Table 1)   acceleration of 4.9 m/s 2 , which is also less than our peak value of 5.41 m/s 2 . This indicates that our device will be able to produce comparable or improved perturbation responses as previous studies.
The inability of the platform to reach full free fall acceleration may be due to friction from the guide rollers applying an upward force to the falling plate. It can also be seen in the instantaneous acceleration results that the Max and Mid acceleration curves have a plateau of zero acceleration (constant velocity) around mid-fall while the Min curve has a smooth transition from negative to positive (Fig. 7). This may be due to the return profile of the pneumatic pistons. While viewing highspeed video, we noticed that the pistons initially move quickly from under the platform, but near midstroke, they slow momentarily and contact the platform before fully retracting. However, the platform motion is uninterrupted during the fall, as seen in the position plot (Fig. 7), and the perception of falling (peak acceleration) is consistent across fall heights and sufficient to illicit a perturbation response. In the future, pistons with faster retraction speeds could easily be retrofit if this behavior is found to inhibit testing.
The Perturbation Platform System is designed for both laboratory research settings and clinical training. Similar perturbation testing has been used to understand the physical and neurological implications of advanced age, stroke, lower limb amputation and Parkinson's disease influence the response to sudden, unexpected perturbations [15,16,18]. This device could also be used in a clinical rehabilitation setting to give individuals safe practice when encountering an unexpected perturbation. Such practice can train their neuromuscular systems to help reduce or prevent future falling [4,16,19]. We plan future testing with the device to identify neuromuscular and biomechanical abnormalities during balance perturbations in older adults and will assess adaptive changes in balance control following bouts of perturbation exercises.
One limitation of the current system is the hard-wired controller. However, the Teensy microcontroller is capable of Bluetooth wireless communication. In the future, versatility can be added to make the system control wireless. In addition, the control could be integrated into an existing gait lab computer system to allow the control of the platforms in unison with other lab components such as the motion capture and/or force plates. This would allow for more advanced testing conditions that could

Ethics Approval and Consent to Participate
The subject in this study provided informed consent to all activities. This study was approved by the University of Texas at Austin's Institutional Review Board.

Consent for publication
Not applicable

Availability of data and materials
The dataset used and/or analyzed as a part of this study is available upon request from the corresponding author.

Competing interests
The authors declare that they have no competing interests.

Author Contributions
CP and RN participated in the design and fabrication of the Perturbation Platform System. SS and HH collected experimental data. All authors contributed to the analysis of data and writing of the manuscript.   Detail of spring pin support system. Spring pin hardware outlined.