Vibrotactile Feedback for Improving Standing Balance

Abstract

Maintaining balance standing upright is an active process that complements the stabilizing properties of muscle stiffness with feedback control driven by independent sensory channels: proprioceptive, visual, and vestibular. Considering that the contribution of these channels is additive, we investigated to what extent providing an additional channel, based on vibrotactile stimulation, may improve balance control. This study focused only on healthy young participants for evaluating the effects of different encoding methods and the importance of the informational content. We built a device that provides a vibrotactile feedback using two vibration motors placed on the anterior and posterior part of the body, at the L5 level. The vibration was synchronized with an accelerometric measurement encoding a combination of the position and acceleration of the body center of mass in the anterior-posterior direction. The goal was to investigate the efficacy of the information encoded by this feedback in modifying postural patterns, comparing, in particular, two different encoding methods: vibration always on and vibration with a dead zone, i.e., silent in a region around the natural stance posture. We also studied if after the exposure, the participants modified their normal oscillation patterns, i.e., if there were after effects. Finally, we investigated if these effects depended on the informational content of the feedback, introducing trials with vibration unrelated to the actual postural oscillations (sham feedback). Twenty-four participants were asked to stand still with their eyes closed, alternating trials with and without vibrotactile feedback: nine were tested with vibration always on and sham feedback, fifteen with dead zone feedback. The results show that synchronized vibrotactile feedback reduces significantly the sway amplitude while increasing the frequency in anterior-posterior and medial-lateral directions. The two encoding methods had no different effects of reducing the amount of postural sway during exposure to vibration, however only the dead-zone feedback led to short-term after effects. The presence of sham vibration, instead, increased the sway amplitude, highlighting the importance of the encoded information.

Keywords: balance; biofeedback; postural control; somatosensory integration; vibrotactile feedback.

Figure 1

Experimental set-up. The participant was asked to stay still in the standing position, wearing headphones and our portable device composed by: (A) a sensor (IMU) placed on the back at L3 level; (B) the microprocessor unity connected to the PC via Wi-Fi; (C) two vibration motors attached to the skin of the participant: on the back and on the abdomen at L5 level. The IMU recorded the accelerometric signal and sent it to a microprocessor (WiPy) that saved them on a microSD card. The accelerometric measurements were used for controlling the vibration motors.

Figure 2

Relation between the Anterior-Posterior (AP) acceleration and the vibration frequency. The black line represents, for the always on method (V₋T_+AO), the relation between the amplitude of the acceleration signal measured by the IMU sensor on the AP direction in absolute unit (m/s²) and the vibration frequency (in Hz) applied to one motor or the other: the motor on the abdomen, for positive acceleration respect to the natural stance, and the motor on the back, for negative acceleration. The standard deviation of the acceleration measurement recorded during the initial trial with eyes open (std_V+) was used for defining the limit of the dead zone, i.e., the region where the vibration was silent for the DZ method (V₋T_+DZ): this region is represented in the figure by the two dotted lines. Outside that region the vibration was controlled in in the same way for both methods (AO and DZ).

Figure 3

Protocol adopted for group 1 (upper row) and group 2 (bottom row). Trials were either with the visual feedback (i.e., eyes open: V₊), or without it (i.e., eyes close: V₋). The vibrotactile feedback was either off (T₋), or on (T₊). There were three types of vibrotactile feedback: Dead Zone (DZ), Always On (AO), or Sham (S). Ti (where i goes from 1 to 9 for group 1 and to 11 for group 2) indicates the trial numbers.

Figure 4

Examples of the accelerometer signal (low-pass filtered, cut off frequency 3.5 Hz) in absence (V₋T₋) and presence (V₋T₊) of supplemental vibrotactile feedback. Each panel compares, for one typical participant, the accelerometric signal in the (V₋T₋) condition with the same signal measured in the three conditions with vibration on: the dead-zone method (V₋T_+DZ) in (A) (note that the dead zone is delimited by the threshold (Thr), i.e., the two dashed lines); the always on method (V₋T_+AO) in (B); the sham feedback (V₋T_+S) in (C).

Figure 5

RMS and F95 parameters in the AP direction for group 1 (DZ method) in (A,B), and for group 2 (AO method) in (C,D), respectively. The error-bars represent the standard error of the mean obtained for all the participants. ^*significant differences of rm-ANOVA: ^***p < 0.001.

Figure 6

RMS and F95 parameters in the ML direction for group 1 (DZ method) in (A,B), and for group 2 (AO method) in (C,D), respectively. The error-bars represent the standard error of the mean obtained for all the participants. ^*significant differences of rm-ANOVA: ^**p < 0.01.

Figure 7

Effects of the sham feedback (V₋T_+S; T10) in comparison with the performance in the last trial V₋T_+AO (T8) and in the two no vibration trials before and after the sham trial (V₋T₋; T9, T11). RMS and F95 for the AP direction are reported in (A,B), respectively. RMS and F95 for the ML direction are reported in (C,D), respectively. The error-bars represent the standard error of the mean obtained for all the participants. ^*significant differences of rm-ANOVA: ^*p < 0.05, ^**p < 0.01.