The effects of actuator selection on non-volitional postural responses to torso-based vibrotactile stimulation

Abstract

Background: Torso-based vibrotactile feedback may significantly reduce postural sway in balance-compromised adults during quiet standing or in response to perturbations. However, natural non-volitional postural responses to vibrotactile stimulation applied to the torso remain unknown.

Methods: The primary goal of this study was to determine, for two types of actuators (tactors) and in the absence of instruction, whether vibrotactile stimulation induces a directional postural shift as a function of stimulation location. Eleven healthy young adults (20-29 years old) were asked to maintain an upright erect posture with feet hip-width apart and eyes closed. Two types of tactors, Tactaid and C2, which differ in design and stimulation strength, were placed on the skin over the right and left external oblique, internal oblique, and erector spinae muscles in a horizontal plane corresponding approximately to the L4/L5 level. Each tactor of the same type was activated twice randomly for each individual location and twice simultaneously for all locations at a frequency of 250 Hz for a period of 5 s.

Results: Vibration applied over the internal oblique and erector spinae muscle locations induced a postural shift in the direction of the stimulation regardless of the tactor type. For the aforementioned four locations, the root-mean-square (RMS) and power spectral density (PSD) of the body sway in both the A/P and M/L directions were also significantly greater during the vibration than before or after, and were greater for the C2 tactors than for the Tactaid tactors. However, simultaneous activation of all tactors or those over the external oblique muscle locations did not produce significant postural responses regardless of the tactor type.

Conclusion: The results suggest that the use of a torso-based vibrotactile sensory augmentation display should carefully consider the tactor type as well as the instruction of corrective movements. Attractive instructional cues ("move in the direction of the vibration") are compatible with the observed non-volitional response to stimulation and may facilitate postural adjustments during vibrotactile biofeedback balance applications.

Figure 1

(a) Inertial measurement unit (IMU). (b) C2 and Tactaid tactors. (c) Elastic tactor belt with Tactaid tactors and IMU. (d) Stimulation locations.

Figure 2

(a) Illustrative postural trajectories and 95% confidence interval elliptical fits for each vibration period when the tactor was placed over the left internal oblique. Positive values are defined as movements in the anterior and lateral right directions. (b) Illustrative A/P postural trajectories. Positive values are defined as movements in the anterior direction. (c) Illustrative M/L postural trajectories. Green, red, and blue lines represent pre-, per-, and post-vibration periods, respectively.

Figure 3

(a) Average A/P postural trajectories. Positive values correspond to movements in the anterior direction. (b) Average M/L postural trajectories. Positive values correspond to movements in the lateral right direction. Red and blue lines represent average postural trajectories for the C2 and Tactaid tactors, respectively. Shaded areas indicate standard error of the corresponding average postural trajectories.

Figure 4

Average postural shift vectors during vibration as a function of tactor location. Red and blue vectors correspond to shifts induced with the C2 and Tactaid tactors, respectively. Dashed lines indicate standard error of the corresponding mean vector direction.

Figure 5

Average magnitude of the postural shift vector for the C2 (circles) and Tactaid (squares) tactors during vibration as a function of tactor location. Error bars indicate standard error of the mean (*p < 0.05, **p < 0.01, ***p < 0.0001). Bird’s-eye view drawings illustrate vibration locations.

Figure 6

Average A/P and M/L RMS sway values for the C2 (circles) and Tactaid (squares) tactors during vibration as a function of tactor location. Red and blue symbols represent the A/P and M/L RMS sway values, respectively. Error bars indicate standard error of the mean (*p < 0.05, **p < 0.01, ***p < 0.0001). Bird’s-eye view drawings illustrate vibration locations.

Figure 7

Average A/P (a) and M/L (b) PSD magnitudes (frequencies less than 0.6 Hz) for the C2 (circles) and Tactaid (squares) tactors during vibration as a function of tactor location. Error bars indicate standard error of the mean (***p < 0.0001). Bird’s-eye view drawings illustrate vibration locations. Note that the scale in (a) is ten times greater than that in (b).