## 1 Introduction  
Worldwide, low back pain (LBP) has become the main cause of disability [1]. However, its causes are unidentifiable in 85% of the cases [2]. Postural control impairment is considered one of the main causes of LBP [3]. Postural control is affected by sensory inputs from visual and vestibular senses and somatosensory receptors which provide superficial and proprioception. A previous study indicated that the feedback control on the trunk posture is mainly influenced by the visual and proprioception [4]. Also, another previous study suggested that the ability to select and reweigh sensory inputs to adapt to situations is a critical factor for postural control [5]. In addition, non-specific LBP (NSLBP) may alter postural control by refocusing proprioception from the trunk to the ankles [6]. Furthermore, reduced precision of trunk movement in NSLBP might be explained by proprioceptive deficits [7]. Therefore, NSLBP could be caused by a decline in proprioception leading to problems in postural control and LBP. Proprioception in the lower leg and trunk muscle plays an important role in maintaining postural stability [8]. Each proprioceptor has a response frequency of vibration. Meissner corpuscles are sensitive to lower vibration frequencies (peaks around 30 Hz), whereas the Vater-Pacini corpuscles are sensitive to higher frequencies (peaks around 240 Hz) [9]. Muscle spindles have a response frequency range of 40-100 Hz [10]. Mechanical vibration is commonly used to test externally induced balance control. This method has been widely used to investigate the role of muscle spindle in research focusing on postural control [11; 12; 13; 14]. However, the role of other proprioceptors in NSLBP has not been evaluated. Thus, comparing postural sway of NSLBP during various local frequency vibrations with that of non-LBP (NLBP) will allow determination of the cause of LBP and frequencies of proprioception deficit. Postural sway of NSLBP during local frequency vibrations of 30, 60 and 240 Hz has been compared with that of NLBP in our previous study. From the result, we found that not only does NSLBP involve the functional impairment of muscle spindles that other groups reported [11; 12; 13] but also Vater-Pacini corpuscles were functionally declined because of LBP [15; 16]. However, in previous studies, only one frequency of vibration stimulation was applied within the range of response frequencies for each proprioceptor [11; 12; 13; 15; 16]. In addition, other studies investigating muscle spindle response to multiple vibration frequencies reported different results. A previous study using 90 Hz vibration frequency suggested that the weighting of proprioceptive feedback from lumbar muscle spindles did not differ between NSLBP and NLBP [17]. On the other hand, another previous study using 60 Hz vibration frequency suggested that LBP patients rely less on proprioceptive feedback from lumbar muscle spindles [18]. From these results, it is implied that the postural response induced by mechanical vibration varies with the applied vibration frequency even in one proprioceptor. Also, it is assumed that there are individual differences in response frequencies of proprioceptors. To the best of our knowledge, a protocol for measuring postural control while applying local vibration stimulation in the entire response frequency band has not been developed. Thus, we considered that the inspection device and a protocol for evaluating postural response in the range of the entire response frequencies of the proprioceptors are necessary. Therefore, we developed a new experimental protocol to measure the center of pressure (CoP) of an individual on a gravicorder during local mechanical vibration with sweep frequencies, using our developed variable frequency vibratory stimulation device. This method can be analyzed by applying a vibration stimulus once in the entire frequency range covering the response frequencies of each proprioceptor.