A Vibrotactile Feedback Device for Seated Balance Assessment and Training

Many people suffer from seated instability, which can compromise functional independence and lead to secondary health complications. Our protocol holds the potential to assess, challenge, and improve balance during sitting. Our technique combines elements of existing balance research tools into one novel device that is optimized for clinical use and accessibility.

Individuals who suffer under a newer muscular impairment may struggle to maintain seated balance. Our protocol provides access to assessment and training techniques that are known to benefit balance rehabilitation outcomes. This protocol can be applied to investigate balance control mechanisms and to optimize sensory feedback methods.

New users of this protocol should be sure to use our supplemental drawing and solid model files to produce a working replica of the device. At the beginning, weld a base nut to a steel plate to construct an attachment interface for interchangeable, hemispherical bases. Using a computer numerical controlled, or CNC milling machine, construct a cylindrical chassis, lid, and base from polyethylene, then bolt the base plate to the base, and place the chassis on the base.

Use the milling machine to construct a 37mm-long, 32mm outer diameter cylindrical polyvinyl chloride sleeve that fits onto a threaded rod. After welding steel flanges to each side of a steel hitch, bolt the hitch to the front of the base. Use a CNC turning machine to construct five identical 63mm-high, 152mm diameter polyethylene cylinders.

In the center of the top surface of each cylinder, cut a 32mm hole to a depth of 38mm so that it fits the cylindrical sleeve with some interference. On the bottom surface of each cylinder, use the CNC turning machine to cut a uniformly curved base with the unique radius of curvature for each of the five cylinders, maintaining the overall height of 63mm. To construct the leg support attachment, first weld a 70mm steel hitch insert perpendicularly to one end of a 575mm steel extrusion.

At the other end, clamp a 300mm cylindrical steel footrest to the extrusion. Use a bandsaw to cut a rectangular 29 by 100mm steel bar to a length of approximately 160mm so that it weighs 3.6kg. Insert the steel bar at the back of the chassis to counterbalance the leg support attachment, and assemble the device.

Insert the clevis pins through the hitch and the hitch insert to connect the leg support. Then adjust the location of the clamp to the desired footrest height. Thread the rod into the base stud, such that approximately 35mm of the rod protrudes from the base, and insert the protruding rod into the desired curved base.

Apply the grip tape to the lid, and cover the device with the lid. To instrument the device, connect an inertial measurement unit and eight vibrating tachters to a microcontroller. Program the microcontroller such that it reads anteroposterior and mediolateral tilt angles from the inertial measurement unit, and turns the vibrating tachters on or off based on the tilt angles.

Secure the inertial measurement unit in the center of the chassis and arrange the vibrating tachters in a regular octagon with a radius of 10cm, centered 8cm anterior of the center of the chassis, so that they will lie under the seat of an average sized person. Then connect the microcontroller to a computer, and open the software user interface. To conduct the balance experiments, recruit consenting participants who are free of neurological and musculoskeletal disorders, and acute or chronic back pain, and record each participant's age, weight, and height.

Next, open the user interface. The compass graph shows the tilt angle of the device, plus half the tilt velocity of the device in the anteroposterior and mediolateral directions. Prior to each balance trial, instruct the participant to don noise-canceling headphones, fold their arms across their chest, maintain an upright posture as much as possible, and verbally cue when they are ready.

Use the dropdown menus in the trial parameter section of the user interface to label the current difficulty and eye condition, and click record to start the trial. For trials with eyes open, instruct the participant to focus on a fixed point straight ahead to help maintain balance. For trials with eyes closed, use a blindfold to ensure that the participant is completely deprived of visual feedback.

Perform 20 30 second seated balance trials in series, taking breaks as warranted to avoid fatigue, and stopping at any time if necessary. An algorithm will compute which anteroposterior and mediolateral feedback thresholds to use and display the thresholds in the Q3 column of the user interface. The vibrotactile feedback thresholds can be optimized to provide feedback cues on direction and timing that are tailored toward a given task or goal.

After four familiarization trials, copy the values in the Q3 column into the right column and click refresh to update the feedback thresholds shown on the compass graph based on the fourth familiarization trial. As the anteroposterior and mediolateral tilt angles are automatically stored in real time in a text file for analysis, analyze the anteroposterior and mediolateral signals to characterize the sitting performance for each of the experimental conditions. This table shows the postero-graphic measures derived from anteroposterior and mediolateral support surface tilts averaged for 144 balance trials, and performed by 12 participants under each experimental condition.

Observations of anteroposterior tilt were significantly different between the eye open and eye closed balance conditions for the root-mean-square, centroidal frequency, and frequency dispersion. Consistent with other reports, these postero-graphic measures can discriminate between balance tasks during trials in which the vibrotactile feedback system was active. The centroidal frequency of the anteroposterior tilt observations was significantly higher than during the control trials.

Consistent with other reports, this vibrotactile feedback protocol has a measurable effect on balance performance. All structural components have a corresponding solid model and drawing that are available for download and can be used to replicate the construction process. The procedure can be used to test hypotheses regarding the fundamental nature of dynamic upright sitting and the effectiveness of vibrotactile feedback as a balance training technique.

This research provides a critical foundation for future work on clinical assessment and training tools for populations with impaired seated balance to improve their quality of life. The power tools used to construct this device may cause bodily harm;please follow all safety protocols.