Development of an implicit method for directing weight shifting to the affected side in patients with stroke: a proof of concept study

The system consists of a Wii balance board (Wiiboard, Nintendo, Japan) for capturing center of foot pressure (CoP) data. The subject was fitted with a vibration unit, with four vibrators worn around the pelvic girdle, which displayed CoP motion, coupled with a personal computer (PC) (Fig. 1). We measured CoP using a Wii balance board (WBB), with signals relayed to a PC by Bluetooth. CoP measurements (sampling) were taken every 20 ms, and the transmission period was 20 ms. The software screen displayed CoP and target weight bearing in real time (Fig. 2). The target weight-shift length can be set at any position from 0 to 1vibration motors (Z7AL2B169208200% of body weight by inputting the subject’s weight. Vibrations were transmitted to a vibration unit (i.e., pelvic belt) using a USB connection wireless module (nRF24LE1-F16Q32, Nordic, Norway). The communication cycle is 20 ms. The frequency bandwidth of the module ranges from 2400 to 2525 MHz. The vibration unit includes four built-in , KOTL, China). Vibration motors are mapped on a bony ridge (i.e., the bilateral anterior and posterior superior iliac spines) to efficiently convey vibrotactile stimulation (Fig. 3b). The rate of the vibration motor speed is 12,000 ± 2500 rpm. The intensity of vibrations can be changed by PC according to the degree of patient paralysis. Until the CoP position reaches the target weight-shift length, the device will continue to vibrate. Vibrations cease once the subject reaches the target weight-shift length.

Weight-shifting length is automatically determined by an implicit weight-shift length-extension. To reduce excessive voluntary movements and fear of falling, we applied Weber’s Law to the sense of weight, increasing the length of weight shifting [40]. This law enables calculation of the just-noticeable difference (JND), which is the minimum amount of change that can be perceived. In the perceptual literature, JND represents the smallest detectable change by which a subject can reliably discriminate between a comparator and an original stimulus. For example, in a size-discrimination task a participant might be asked to verbally report whether the length of a visually presented line (i.e., the comparator) differs from the length of a reference line. In this context, JNDs are defined statistically with the appropriate detection of a magnitude change based on an experimentally determined criterion [41]. Equation (1) represents Weber’s Law, where W is the current amount of weight bearing, K is the Weber ratio, and ΔW is the JND

Because the Weber ratio for sense of weight bearing has not been used previously, we conducted an experiment with 12 young, healthy subjects to derive the Weber ratio. Subjects included 12 healthy adults who provided written consent for participation (mean age 21.25 years, SD 0.72, male 9 participants, female 3 participants). Body mass index (BMI) for each participant is shown in Table 1. The present study was performed with approval of the Ethical Review Board of Waseda University.

Subjects stood with their feet 30 cm apart, and the experimental task was repeated weight-shift task to both left and right leg (Fig. 3a). First, we instructed subjects to shift 60% of their body weight to the left side by using a vibratory cue on the left side waist (i.e., L1/2; Fig. 2). When the weight shift reached 60% of the body weight, vibrations for the left side stopped, and the vibrator activated on the right side; this process was repeated. After weight shifting once to the left and once to the right, we increased the percentage of weight shifting, based on the set Weber ratio (initial value 0.05), without communicating this change to the participants (Fig. 3c). The initial value of 0.05 was based on preliminary experiments that found a somatic sensory Weber ratio of 0.03–0.3 [42]. This procedure was repeated at 90-second intervals, and after the experiment subjects reported if they felt an increase in the amount of weight shifting, according to two conditions (“Yes” = it increased; “No” = unchanged). After performing the weight-shift task using the initial Weber ratio, the Weber ratio was either increased or decreased according to participants’ responses (decreased when the participant felt an increase in the amount of weight shifting; increased if the participant did not). Once we established the point where a change of 0.01 in the Weber ratio caused a change in response, decreases were made in 0.002 increments from the noticeable value to find the Weber ratio where increases for weight shifting were unnoticeable for each participant (Fig. 4).

As a preliminary step toward a large-scale clinical trial, we applied the device to one patient with stroke to examine method feasibility. In this examination, we conducted simulated weight-shift training using the derived Weber ratio, and examined the following variables: (1) perception of vibratory stimuli: whether or not vibration cues could be perceived on the paretic side, (2) perception of increased weight bearing: whether or not the patient could perceive increases in weight bearing, (3) anxiety: anxiety during the task, and referent data on the influence of training, (4) weight-bearing dose on the paretic side was measured before and after intervention, and comparisons made.

Table 3 shows patient characteristics. Our patient was a 73-year-old man with left hemiplegia (Brunstrom’s recovery stage; see Appendix [43]), caused by right cerebral infarction. The patient was able to walk independently, and had no cognitive or mental disorders (i.e., Mini-Mental State Examination score above 23 [44]; no dementia).

We familiarized the patient with the weight-bearing guidance by vibration device. (At this point, the patient was not informed that the amount of weight bearing would increase.) For the weight-shift task, the patient assumed a left–right symmetrical standing position, with feet 30 cm apart on the WBB, with eyes open. In these conditions, the patient focused on a marker placed approximately 2 m in front of him. In the pretest, the patient performed the task of weight shifting to the paretic limb five times in order to evaluate baseline weight-shift ability. After the pre-testing, a continuous weight-shift tasks of both left and right sides was repeated over 70 s × 4 sessions with one-min rest intervals. During the weight-shift task, when the vibrations for one side stopped, he shifted his weight to the other side. The target weight-bearing dose was set at pre-test value (baseline) × 0.8. In doing so, the amount of weight bearing was increased for each session based on the Weber ratio. Lastly, we performed a post-test of the weight-shift task identical to the pre-test (i.e., weight shifting to the paretic limb five times to evaluate weight-shift ability).

The patient was questioned about his perception of vibratory stimuli applied from the left and right, and his perception of increased weight bearing in each session after completion of the post-test. We used a visual analog scale (VAS) to measure anxiety after the pre-test, and after the weight-shift training [45]. The VAS is frequently used to measure pain intensity. The pain VAS is a continuous scale comprised of a horizontal (HVAS) or vertical (VVAS) line, usually 10 cm (100 mm) in length, anchored by two verbal descriptors, one for each symptom extreme. VAS is a valid measure of fall anxiety, according to a recent study [46]. We asked the patient to indicate his fear of falling, on a scale from 1 to 10. The amount of weight bearing in the pre- and post-test was calculated using the data obtained from the WBB. For weight bearing, variables were compared pre-and post-testing. Statistical analyses were performed using a Wilcoxon signed-rank test as a non-parametric test because of the small sample size [47]. Significance level was set at p < 0.05.