A Vibrotactile Feedback Device for Seated Balance Assessment and Training

All methods described here have been approved by the Health Research Ethics Board of the University of Alberta.

1. Construction and Assembly of Structural Components

1. Construct an attachment interface for interchangeable hemispherical bases: weld a base nut to a steel weld plate. 
2. Use a computer numerical controlled (CNC) milling machine to construct a cylindrical chassis, lid, and base from polyethylene as shown in Figure 1. Bolt the base plate to the base and the base to the chassis.
   NOTE: The mill features for attachment of bolts and other parts are according to the drawing files and 3D solid model files provided (see Supplementary Files 1 and 2). All structural components have a corresponding solid model and drawing that are available for download and can be used to replicate the construction process.
3. Use a milling machine to construct a cylindrical polyvinyl chloride sleeve that fits onto a threaded rod, as shown in Figure 1. Make the sleeve 37 mm long, with an outer diameter of 32 mm.
4. Weld steel flanges to each side of a steel hitch, as shown in Figure 1. Bolt the hitch to the front of the base.
5. Use a CNC turning machine to construct 5 identical cylinders from polyethylene, each with a height of 63 mm and a diameter of 152 mm. In the center of the top surface of each cylinder, cut a 32 mm hole to a depth of 38 mm so that it fits the cylindrical sleeve (see Step 1.3. above) with some interference. 
6. On the bottom surface of each cylinder, use a CNC turning machine to cut a uniformly curved base with a unique radius of curvature for each of the 5 cylinders, maintaining the overall height of 63 mm, as shown in Figure 2. 
   NOTE: The radius of curvature and height of the base determine the stability of the device. The suggested radii of curvature for this height are between 110 mm (very unstable) and 250 mm (slightly unstable), as shown in Table 1.
7. Construct a leg support attachment as shown in Figure 3, by first welding a 70 mm steel hitch insert perpendicularly to one end of a 575 mm steel extrusion. At the other end, clamp a 300 mm cylindrical steel footrest to the extrusion. 
   NOTE: For detailed part dimensions, see Supplementary File 1 (drawings) and Supplementary File 2 (3D solid models).
8. Use a bandsaw to cut a rectangular steel bar (29 mm by 100 mm) to a length of approximately 160 mm so that it weighs 3.6 kg. Insert the steel bar at the back of the chassis to counterbalance the leg support attachment, as shown in Figure 1.
9. Assemble the device as shown in Figure 4. Connect the leg support by inserting clevis pins through the hitch and hitch insert. Adjust the location of the clamp to the desired foot rest height. Thread the rod into the base stud such that approximately 35 mm of the rod protrudes from the base.  Insert the protruding rod into the desired curved base. 
10. Apply grip tape or another suitable upholstery to the lid. Put on the lid. 

2. Instrumenting the Device

1. Acquire a microcontroller (see the Table of Materials), an inertial measurement unit and eight vibrating tactors. Connect the inertial measurement unit and vibrating tactors to the microcontroller.
2. Program the microcontroller such that it reads antero-posterior (AP) and medio-lateral (ML) tilt angles from the inertial measurement unit and turns the vibrating tactors on or off based on the tilt angles. See Supplementary File 3 (exemplary microcontroller script) and Step 2.2.1.
   NOTE: Inertial measurement units that utilize accelerometers and gyroscopes are prone to error. Perform a positional calibration of the sensors: rest the device on a level surface and use this position as a baseline for all following measurements. Use a motion capture system or similar approach to validate the tilt angle measurements and ensure that they are sufficiently accurate throughout the expected range of use (spatial and temporal). Ensure the vibrating tactors operate at a frequency of no more than 200 Hz, so as to induce a one-to-one response of sensory receptors in human skin or muscle⁹.
   1. Upload the microcontroller script that generates vibrotactile cues based on a feedback control signal that represents a weighted sum of AP (or ML) tilt angle and velocity.
      NOTE: The computer activates three tactors closest to the left, right, front, or back of the surface when the control signal exceeds a threshold in that direction; or five tactors if an AP and ML threshold are surpassed simultaneously; none of the tactors are active when the control signal is below the threshold in both directions (i.e., in the no-feedback zone).
3. Secure the inertial measurement unit in the center of the chassis. Arrange the vibrating tactors on a regular octagon with a radius of 10 cm, centered 8 cm anterior of the center of the chassis so that they will lie under the seat of an average-sized person¹⁰. A photograph of one potential arrangement is shown in Figure 4.
   NOTE: If the vibrating tactors are not powerful enough to vibrate the user, improve the interface between tactor and skin by cutting holes into the lid and fixating the vibrators to rest flush with the surface. If the method used to secure the vibrators in place causes dampening of the vibration, consider using a two-part mounting enclosure with a loose-fit locating pin, as shown in Figure 5.
4. Connect the microcontroller to a laptop or desktop computer via a universal serial bus (USB) or other suitable communication method. Open the user interface, shown in Figure 6.
   NOTE: Alternatively, connect the microcontroller to a battery or other power source. This improves the portability of the device, but precludes a user interface.

3. Exemplary Assessment and Training Protocol

1. Recruit consenting participants who are free of neurological or musculoskeletal disorders and acute or chronic back pain. Record each participant’s age, weight, and height. Then, for each participant, carry out the following procedure.
2. Open the user interface (Figure 6). The compass graph shows the device’s tilt angle plus half its tilt velocity in the AP direction (vertical axis) and ML direction (horizontal axis).
3. Prior to each balance trial, instruct the participant to don noise-cancelling headphones, fold his or her arms across the chest, maintain an upright posture as much as possible, and verbally cue the experimenter of being ready.
4. Perform twenty 30 second seated balance trials in series¹¹, taking breaks as warranted to avoid fatigue, stopping at any time if necessary.
   1. Sequence the trials as follows (example only): randomly select one of two “base stability level/eye condition” combinations, hereafter called balance conditions (more difficult base and eyes open; or less difficult base and eyes closed)¹². Perform four trials of the first balance condition to familiarize the participant with the task and to identify appropriate control signal thresholds for the vibrating tactors in the seat (see Step 3.4.5 below).
      NOTE: It is more difficult to maintain balance on a base with a small radius of curvature than on a base with a large radius of curvature (Table 1 shows the relative stability of all five interchangeable bases). Four trials have been found to be sufficient to achieve a stable performance of the balance task².
   2. Randomly select three of the next six trials to be control trials: switch the vibrating tactors off for the duration of these trials. To turn the vibrational feedback on or off, toggle the Feedback slider to the desired setting in the user interface. Repeat this sequence of ten trials for the second balance condition.
   3. Label the current difficulty and eye condition by selecting from the drop-down menus in the Trial Parameters section of the user interface. Click Record to start the trial.
      NOTE: The participants’ safety is paramount. The experimenter should supervise all balance activities and be prepared to assist in the event of balance loss. Clear the area of any potential hazards and be aware of local emergency protocols.
   4. 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.
      NOTE: For balance paradigms where the movement of the feet should be restricted, attach the foot support and insert the counterbalance beneath the lid.
   5. An algorithm computes which AP and ML feedback thresholds to use and displays them in the Q3 column of the user interface. After four familiarization trials, copy the values shown in the Q3 column into the Write Column, and then click Refresh to update the feedback thresholds shown on the compass graph (pink) based on the fourth familiarization trial.
      NOTE: The computed threshold values displayed in the Q3 column of the interface are equal to the third quartile for each tilt direction (AP, ML) during the previous trial. This feedback scheme is based on the notion that balance function is improved when feedback is optimized for each individual^13,14, while providing too much feedback may detriment learning¹⁵. Once the two threshold values have been selected for a given individual, they can be kept constant for that individual to be able to assess improvements over time or with an intervention.
5. As the AP and ML tilt angles are automatically stored, in real-time, in a text file for analysis, analyze the AP and ML signals to characterize sitting performance for each of the experimental conditions.
   1. In time domain, calculate the following posturographic measures from each time series⁸: root-mean-square (a measure of the variance of the motion) and the mean velocity (a measure of the average angular speed of the motion).
   2. In frequency domain, calculate the following posturographic measures from each time series⁸: centroidal frequency (a measure of the motion’s overall frequency) and frequency dispersion (a measure of the variance in the motion’s frequency)⁸.
6. Use a linear mixed model to estimate and characterize the effects of two fixed-effects factors, (1) the balance condition (stability level and eye condition combined) and (2) vibrotactile feedback, on each of the posturographic measures (dependent variables), considering the correlation of repeated measurements from each participant¹⁶ (one random-effects factor).
   1. Test for significance of the fixed effects by computing the ratio of the variance between the group means to the variance of the residuals, and comparing the result to an F-distribution.