Background Whole-body vibration (WBV) is associated with back and neck pain in military personnel and civilians. However, the role of vibration frequency and the physiological mechanisms involved in pain symptoms are unknown. Questions/purposes This study asked the following questions: (1) What is the resonance frequency of the rat spine for WBV along the spinal axis, and how does frequency of WBV alter the extent of spinal compression/ extension? (2) Does a single WBV exposure at resonance induce pain that is sustained? (3) Does WBV at resonance alter the protein kinase C epsilon (PKCe) response in the dorsal root ganglia (DRG)? (4) Does WBV at resonance alter expression of calcitonin gene-related peptide (CGRP) in the spinal dorsal horn? (5) Does WBV at resonance alter the spinal neuroimmune responses that regulate pain? Methods Resonance of the rat (410 ± 34 g, n = 9) was measured by imposing WBV at frequencies from 3 to 15 Hz. Separate groups (317 ± 20 g, n = 10/treatment) underwent WBV at resonance (8 Hz) or at a nonresonant frequency (15 Hz). Behavioral sensitivity was assessed throughout to measure pain, and PKCe in the DRG was quantified as well as spinal CGRP, glial activation, and cytokine levels at Day 14. Results Accelerometer-based thoracic transmissibility peaks at 8 Hz (1.86 ± 0.19) and 9 Hz (1.95 ± 0.19, mean difference [MD] 0.290 ± 0.266, p \ 0.03), whereas the video-based thoracic transmissibility peaks at 8 Hz (1.90 ± 0.27), 9 Hz (2.07 ± 0.20), and 10 Hz (1.80 ± 0.25, MD 0.359 ± 0.284, p \ 0.01). WBV at 8 Hz produces more cervical extension (0.745 ± 0.582 mm, MD 0.242 ± 0.214, p \ 0.03) and compression (0.870 ± 0.676 mm, MD 0.326 ± 0.261, p \ 0.02) than 15 Hz (extension, 0.503 ± 0.279 mm; compression, 0.544 ± 0.400 mm). Pain is longer lasting (through Day 14) and more robust (p \ 0.01) after WBV at the resonant frequency (8 Hz) compared with 15 Hz WBV. PKCe in the nociceptors of the DRG increases according to the severity of WBV with greatest increases after 8 Hz WBV (p \ 0.03). However, spinal CGRP, cytokines, and glial activation are only evident after painful WBV at resonance. Conclusions WBV at resonance produces long-lasting pain and widespread activation of a host of nociceptive and neuroimmune responses as compared with WBV at a nonresonance condition. Based on this work, future investigations into the temporal and regional neuroimmune response to resonant WBV in both genders would be useful.

Whole body vibration (WBV) has been linked to neck and back pain, but the biomechanical and physiological mechanisms responsible for its development and maintenance are unknown. A rodent model of WBV was developed in which rats were exposed to different WBV paradigms, either daily for 7 consecutive days (repeated WBV) or two single exposures at Day 0 and 7 (intermittent WBV). Each WBV session lasted for 30 min and was imposed at a frequency of 15 Hz and RMS platform acceleration of 0.56 AE 0.07 g. Changes in the withdrawal response of the forepaw and hind paw were measured, and were used to characterize the onset and maintenance of behavioral sensitivity. Accelerations and displacements of the rat and deformations in the cervical and lumbar spines were measured during WBV to provide mechanical context for the exposures. A decrease in withdrawal threshold was induced at 1 day after the first exposure in both the hind paw and forepaw. Repeated WBV exhibited a sustained reduction in withdrawal threshold in both paws and intermittent WBV induced a sustained response only in the forepaw. Cervical deformations were significantly elevated which may explain the more robust forepaw response. Findings suggest that a WBV exposure leads to behavioral sensitivity. Keywords: whole body vibration; spine; pain; injury Several epidemiological studies have linked exposure to whole body vibration (WBV) with neck and back pain, [1][2][3][4] suggesting that vibration can lead to the onset of both pain syndromes. American male workers operating vibrating vehicles, such as industrial trucks and tractors, have been reported to have a higher prevalence of low back pain and are three-times more susceptible to acute herniated lumbar discs than workers whose occupations do not involve such exposures.3,5 Also, military helicopter aviators report increased pain during deployment compared to their pre-deployment reports of pain, with between 22-37% reporting neck and 39-70% reporting low back pain. 4 Further, the frequency of pain was significantly higher for aviators who experienced substantially increased flight hours during deployment compared to those who did not, 4 suggesting that the amount of exposure to WBV may affect the pain. 4 Despite the strong suggestive evidence of these epidemiological studies that pain can develop from WBV and may be influenced by the nature and frequency of the exposure, there is still little known about how these factors relate to the onset, maintenance, and resolution of pain.A limited number of studies have defined the biomechanical response to vibration and related resonance and vibration frequency to physiological responses known to be involved in pain-related injuries. The resonant frequency of the seated human undergoing vertical vibration has been reported to be 4.5 Hz from a series of studies using accelerometers on the first and third lumbar vertebrae (L1, L3) and the sacrum of volunteers exposed to vertical vibrations, ranging in frequencies from 2 to 15 Hz. 6 A later study using similarl...

Structured Abstract Study Design In vivo study defining expression of the neurotrophins, brain-derived neurotrophic factor (BDNF) and nerve growth factor (NGF), in cervical intervertebral discs following painful whole body vibration. Objective The goal of this study is to determine if BDNF and NGF are expressed in cervical discs after painful whole body vibration in a rat model. Summary of Background Data Whole body vibration is a possible source of neck pain and has been implicated as increasing the risk for disc disorders. Typically anneural regions of painful human lumbar discs exhibit hyper-innervation, suggesting nerve in-growth as potentially contributing to disc degeneration and pain. BDNF and NGF are upregulated in painfully degenerate lumbar discs and hypothesized to contribute to this pathology. Methods Male Holtzman rats underwent seven days of repeated whole body vibration (15Hz, 30 minutes/day) or sham exposures, followed by seven days of rest. Cervical discs were collected for analysis of BDNF and NGF expression through RT-qPCR and western blot analysis. Immunohistochemistry also evaluated their regional expression in the disc. Results Vibration significantly increases BDNF mRNA levels (p=0.036), as well as total NGF mRNA (p=0.035). Protein expression of both BDNF (p=0.006) and the 75kDa NGF (p=0.045) increase by nearly 4- and 10-fold, respectively. Both BDNF mRNA (R2=0.396 p=0.012) and protein (R2=0.280; p=0.035) levels are significantly correlated with the degree of behavioral sensitivity (i.e. pain) at day 14. Total-NGF mRNA is also significantly correlated with the extent of behavioral sensitivity (p=0.044, R2=0.276). Both neurotrophins are most increased in the inner annulus fibrosus and nucleus pulposus. Conclusion The increases in BDNF and NGF in the cervical discs after painful vibration are observed in typically anneural regions of the disc, consistent with reports of its hyper-innervation. Yet, the induction of nerve in-growth into the disc was not explicitly investigated. Neurotrophin expression also correlates with behavioral sensitivity, suggesting a role for both neurotrophins in the development of disc pain.

Whole body vibration has been postulated to contribute to the onset of back pain. However, little is known about the relationship between vibration exposure, the biomechanical response, and the physiological responses of the seated human. The aim of this study was to measure the frequency and corresponding muscle responses of seated male volunteers during whole body vibration exposures along the vertical and anteroposterior directions to define the transmissibility and associated muscle activation responses for relevant whole body vibration exposures. Seated human male volunteers underwent separate whole body vibration exposures in the vertical (Z-direction) and anteroposterior (X-direction) directions using sinusoidal sweeps ranging from 2 to 18 Hz, with a constant amplitude of 0.4 g. For each vibration exposure, the accelerations and displacements of the seat and lumbar and thoracic spines were recorded. In addition, muscle activity in the lumbar and thoracic spines was recorded using electromyography (EMG) and surface electrodes in the lumbar and thoracic region. Transmissibility was determined, and peak transmissibility, displacement, and muscle activity were compared in each of the lumbar and thoracic regions. The peak transmissibility for vertical vibrations occurred at 4 Hz for both the lumbar (1.55 ± 0.34) and thoracic (1.49 ± 0.21) regions. For X-directed seat vibrations, the transmissibility ratio in both spinal regions was highest at 2 Hz but never exceeded a value of 1. The peak muscle response in both spinal regions occurred at frequencies corresponding to the peak transmissibility, regardless of the direction of imposed seat vibration: 4 Hz for the Z-direction and 2-3 Hz for the X-direction. In both vibration directions, spinal displacements occurred primarily in the direction of seat vibration, with little off-axis motion. The occurrence of peak muscle responses at frequencies of peak transmissibility suggests that such frequencies may induce greater muscle activity, leading to muscle fatigue, which could be a contributing mechanism of back pain.

Whole-body vibration (WBV) has been linked to chronic back pain but the mechanisms of its development are unknown, as is how biomechanical contributions modulate the pain response. An animal model of WBV was developed in which rats were exposed to 7 days of either 5Hz or 15Hz vibration. WBV induced immediate sensitivity (i.e. pain) that was sustained for 7 days after the cessation of the WBV exposure. There was no difference in the pain response between the two vibration frequencies. Increases in the inflammatory mediator, COX-2, were also induced following painful WBV in the lumbar spinal cord, suggesting a potential mechanism for the chronic pain.