Participants. This was a before-and-after, repeated-measures clinical study. A recruitment flyer was used to enlist students at the University of Illinois at Urbana-Champaign, Champaign, IL. The authors conducted this pilot study to explore whether these vibrations could affect plantar SBF responses. The study was conducted between September and November 2019. All participants were healthy and free from skin disease, cardiovascular disease, neuropathy, orthopedic disease, and other diseases. Those who were exposed to chronic vibration stimulus (arm-hand vibration syndrome) and those who were routinely exposed to whole body vibrations were excluded. The detailed procedures of the experiment were introduced to all recruited participants. The participants signed the informed consent documents before the experiment. The collected data were coded for data analyses. The code for each participant’s name was not available to the research team. This study (no. 20322) was approved by the Institutional Review Board of the University of Illinois at Urbana-Champaign.
Instrumentation. Local vibrations (2 g peak acceleration, 1 mm amplitude) at 35 Hz were applied with a vibrator. The vibrator consisted of a voice coil motor, a controller, and a power supply. The voice coil motor (YLM40-20; JDStek, CA) was chosen because of its structural stability with high positioning resolutions, fast acceleration/deceleration, and high-speed capacity. The voice coil motor could be controlled through applying voltage. The position sensor and controller (MS 15 TTLx20; RSF Elektronik, Tarsdorf, Austria) was used to control the voice coil motor with the resolution at 0.5 µm (1 mm for 2000 counts). The radius of the vibrator head was 1 cm.
The majority of studies in the literature applied low-intensity and low-frequency (below 50 Hz) local vibrations to participants and induced positive results such as improved SBF. It was demonstrated that intermittent vibrations at 35 Hz could protect aged muscle from prolonged mechanical compression in mice.19,24,25,30 According to the findings of these studies, we selected 35 Hz vibrations as an intervention in the current study. A sham control (0 Hz vibration) was used in this study.
To ensure equal weight-bearing between the right and left foot, each subject stood on the F-scan plantar pressure system (Tekscan; South Boston, MA). The real-time plantar pressure distributions were displayed on the computer screen to guide the user to place equal weight on both feet. Each F-scan sensor contained 960 sensing pixels, and the size of each pixel was 5.08 mm × 5.08 mm.
The SBF under the first metatarsal head of the right foot was measured noninvasively and continuously by laser Doppler flowmetry (LDF) (PeriFlux 5001; Perimed, Las Vegas, NV). The LDF was calibrated and warmed for 30 minutes before measuring SBF.
Procedures. All participants relaxed for at least 30 minutes in the laboratory to acclimate to the temperature of the room, which was 24 ± 2°C. During this period, participants were asked to complete the demographic and medical history form. The information was used to determine the eligibility of the participant. For example, if a participant checked a cardiovascular disease under medical history, he or she would be excluded from this study.
Participant height, weight, and blood pressure were measured. Participants were randomly assigned to 1 of 2 protocols (ie, standing with local vibrations and standing with sham vibration). The authors created 16 cards, including 8 vibration-first cards and 8 sham-first cards. Each time, researchers picked a card for the participant before the experiement. The LDF probe was taped to the first metatarsal head while the participant was sitting on a chair with hip and knee at 90-degree flexion and ankle in a neutral position. Participants were standing during the vibration procedure. Participants were informed that 2 types of vibrations were being tested in this study, but they were not aware which protocol was being applied. The experimental procedures were as follows: 10-minute baseline SBF measurement, 10-minute vibration intervention (35 Hz or sham control), and 10-minute recovery SBF. A 30-minute washout period was allowed between the 2 protocols (35 Hz vibration and sham control).
Data analysis. To understand the mechanisms responsible for the recovery SBF response after standing and standing combined with local vibrations (35 Hz), the authors performed wavelet analysis of the SBF data.33,34 The following calculation procedures were used.
For an SBF signal, the continuous wavelet transform is defined as:
where Ψ is the mother wavelet function, t is time, and s is the scale related to the central frequency of Ψs,t. In this study, the Morlet wavelet was used as the mother wavelet function, defined as:
By choosing ω0=2π, s is related to the central frequency of Ψs,t , f by s = 1/f . The mean wavelet amplitude of a characteristic frequency, denoted as A[f1, f2], was defined as the mean absolute value of the wavelet coefficients over time and over the frequency interval [f1, f2]. In the present study, because the signals lasted 10 minutes, the authors did not take the frequency component (0.005–0.0095 Hz) into account. Only 5 characteristic frequencies were investigated: metabolic (0.0095–0.02 Hz), neurogenic (0.02–0.05 Hz), myogenic (0.05–0.15 Hz), respiratory (0.15–0.4 Hz), and cardiac (0.4–2.0 Hz). Their relative amplitudes were computed as:
Wavelet analysis was performed using Matlab34 (MathWorks, Natick, MA).
Mean baseline SBF and recovery SBF were determined as an average over a 10-minute previbration and postvibration period, respectively. The SBF ratio was calculated to determine the changes of plantar SBF after each standing protocol to minimize the variations of SBF.34,35 The two-sample t test was used to compare the SBF ratio and ratio of wavelet amplitudes of the 2 protocols. All data were expressed as means ± SD. Confidence interval was 95%, and P < .05 was considered as statistical signiﬁcance. The statistical analyses were performed using SPSS Data Analysis Software (Version 25, Chicago, IL).