# Focal Vibration Therapy: Vibration Parameters of Effective Wearable Devices
###### Abstract  
Focal vibration therapy can provide neurophysiological benefits. Unfortunately, standardized protocols are non-existent. Previous research presents a wide range of protocols with a wide range of effectiveness. This paper is part of a broader effort to identify effective, standardized protocols for focal vibration therapy. In this study, the authors evaluated the vibration characteristics (frequency and peak-to-peak intensity) of four commercially available focal vibration devices: (1) Vibracool (wearable), (2) Novafon (hand-held), (3) Myovolt 3-actuator (wearable), and (4) Myovolt 2-actuator (wearable). An accelerometer was used for the measurements. Measurements were made under the following two conditions: (a) when the devices were free, i.e., unconstrained vibration, and (b) when the devices were strapped to the human body, i.e., constrained vibration. In the free vibration condition, frequency ranged from 120 to 225 Hz and peak-to-peak amplitude ranged from 2.0 to 7.9 g's. When the devices were strapped to the body (constrained), vibration amplitude decreased by up to 65.7%. These results identify effective ranges of focal vibration frequency and amplitude. They illustrate the importance of identifying vibration environment, free or constrained, when quoting vibration characteristics. Finally, the inconsistency output of multi-actuator devices is discussed. These results will guide protocol development for focal vibration and potentially better focal vibration devices. f 2021 20  
There is a lack of standardized application protocols for FV [15; 16; 17]. Different studies have used various combinations of vibration frequencies, amplitudes/intensities, and dosages as an intervention, but the reported values correspond to the vibration characteristics in the free vibration condition. FV has been used with vibration frequencies in the range of 80 [18] to 300 Hz [19] for upper limb post-stroke recovery. Similarly, vibration displacement has ranged from as low as 0.2 to 0.5 mm [20], up to 2 mm [19]. Different studies have demonstrated different degrees of success. This lack of consensus in the effectiveness of FV can be attributed to the change in characteristics of the vibrations delivered by the FV devices in the constrained condition. Therefore, an effective FV therapy protocol is still undefined. Fundamentally, an FV therapy protocol is defined by two parameters: vibration frequency and vibration intensity (amplitude or acceleration). [PERSON] and group [16] conducted a review on the acute as well as chronic changes in neuromuscular activity caused by FV and their findings indicate that the frequency of vibration plays a crucial role in adaptations in the central nervous system [16]. Tonic vibration reflex (TVR) is defined as a reflex contraction of the vibrated muscle due to the excitatory input on alpha motoneurons caused by the discharge of the Ia afferents during vibration [16]. While it has been reported that TVR due to muscle vibration helps in motor recovery, more studies on the effects of FV have reported that the response of the Ia afferents in the muscle spindles that contribute to the motor recovery. The muscle spindles are innervated with both primary (Ia) as well as secondary (II) afferents and the response of these afferents to vibration stimuli is dependent on the vibration characteristics of the stimulus, i.e., the frequency and amplitude of vibration. Smaller amplitudes (0.2-0.5 mm) and frequencies below 150 Hz are more effective in recruiting Ia afferents. Frequencies between 20 and 60 Hz result in the recruitment of II afferents in addition to Ia afferents, and frequencies >150 Hz result in a disharmonious discharge of the Ia afferents. Larger amplitudes (>0.5 mm) result in the recruitment of Ib and II afferents in addition to the Ia afferents and only the recruitment of the Ia afferents has been reported to be relatively more responsible for inducing motor recovery through neuromuscular changes/adaptations [16; 21].