# Focal Vibration Therapy: Vibration Parameters of Effective Wearable Devices
## 4 Discussion  
Little is known about the effective vibration characteristics of FV devices. In this study, we measured vibration characteristics of four commercially available FV devices. Anecdotal and literature evidence exists indicating that these devices can be effective, but the application of the FV therapy was limited. One factor influencing limited use is the lack of clinical practice guidelines for FV therapy. Much is unknown regarding how much vibration was actually delivered to the target muscles or body segments, as little research on the exact delivery of FV exists. Since the vibration characteristics of FV devices can be used as a starting point to devise FV therapy protocols, we conducted the experiments in this study to quantify the vibration delivery of four commercially available devices that have been used in clinical settings. We measured peak-to-peak acceleration and vibration frequency. In the case where the devices were free to vibrate, peak-to-peak acceleration ranged from about 2.0 to 7.9 g's. The vibration frequency ranged from about 120 to 225 Hz. When the devices were worn on the body, in general, the peak-to-peak vibration intensity dropped by 5.8% to 65.7%. This indicates that when quoting FV parameters, it is necessary to specify the vibration condition (free-vibration or constrained-vibration). The free-vibration condition is not an accurate representation of the vibration delivered to the body. The frequency drop in the Novafon vibration device can likely be attributed to the fact that the transducer is in direct contact with the body, and so the vibration mechanism and hence the driving (excitation) frequency can be physically hindered. This is different from typical vibration motor-based devices, where the vibration mechanism is enclosed, and the driving or excitation frequency cannot be physically hindered. From these data, it is apparent that, in general, vibration frequency does not change significantly when applied to the body. \begin{table}
\begin{tabular}{c c} \hline
**Device** & **Period (s)** \\ \hline Myovolt 3 actuator (Mode 1) & 4.21 \\ Myovolt 3 actuator (Mode 2) & 1.05 \\ Myovolt 2 actuator (Mode 1) & 4.19 \\ Myovolt 2 actuator (Mode 2) & 1.05 * \\ \hline \end{tabular}
\end{table} 
Table 2: Period of vibration pattern. \({}^{***}\) indicates an average from 2 tests instead of 6. Figure 8: Example of variation in vibration characteristics of the Myovolt multi-actuator vibration devices. These data are from a Myovolt two-actuator in Mode 3 (constant vibration) in the constrained setup. Plot shows the raw accelerometer data along the axis of vibration. Ideally, the peak-to-peak distance should remain constant. Vibration intensity in the constrained vibration case (Figure 6) is the most interesting because it reflects a realistic scenario where the FV device is applied to the body. Three clusters of changes occurred in intensity. First, there was a significant decrease ranging from 40.9% (Myovolt three-actuator, Mode 3) to 65.7% (Novafon Mode II, maximum). Next was the cluster of moderate decrease in intensity ranging from 5.8% (Myovolt two-actuator, Mode 2) to 22.3% (Vibracool). The third cluster with a lone data point showed a moderate increase in vibration intensity to 23.5% (Myovolt two-actuator, Mode 3). We can deduce that when worn on the body, the vibration intensity of a device can decrease anywhere from 5.8% to 65.7%. For the Myovolt two-actuator device, the increase in intensity can be attributed to the second vibration motor, which could have been vibrating in phase to amplify the vibration, i.e., constructive interference. Alternatively, this could be attributed to the structural dynamics of the Myovolt two-actuator device. For example, the resonant frequency in the constrained configuration may have been closer to the vibration motor frequency, resulting in higher vibration intensity.