# Focal Muscle Vibration for Stroke Rehabilitation: A Review of Vibration Parameters and Protocols
## 4 Discussion
Future studies are warranted to explore their benefits with individuals with stroke. More than half of the studies used vibration frequencies from 85 to 120 Hz and vibration amplitudes of 0.01-2 mm. A reduction in spasticity was observed with various frequency ranges. Due to the variations in amplitude and treatment frequency and duration, and contradictory to what is stated in the recent review [28], we speculate that vibration frequency cannot be disregarded as a discriminative factor in FMV intervention. We believe that studies with rigorous design controlling for vibration amplitude and treatment protocol will be needed to investigate the impact of vibration frequency in FMV intervention. The vibration amplitude for stroke rehabilitation ranging from 0.01 mm to 2 mm was considered comparable to the vibration amplitude ranging from 0.005 mm to 10 mm in studies using FMV intervention for patients with spinal cord injury, multiple sclerosis, and other movement disorders [25; 29; 72]. About one third of articles on FMV did not report the amplitude delivered. The improvements observed in the outcome measurement scores were better in studies using FMV with amplitudes greater than or equal to 1 mm and frequencies in the range of 91-108 Hz or greater [45; 49; 52; 54; 55; 59; 60; 61], unless FMV was paired with other forms of intervention such as robotic assistive device [57] or progressive modular re-balancing (RMP) [58]. FMV alone with lower frequencies of less than 90 Hz and lower amplitudes of less than 1 mm seemed to have a lesser change in the outcome measures. Given that frequency range 75-120 Hz was particularly effective on the central nervous network underlying motor control [73], and amplitude of 1-2 mm was sufficient to drive Ia spindle afferents while remaining safe for the tonic vibration reflex [74] and avoiding muscle fiber injury [75], these frequency and amplitude combinations could be recommended for future studies. The duration of intervention did not seem to have much effect on the total improvement, although the change scores were slightly greater in studies with longer durations of intervention, which could be because of the long-lasting effects on cortical excitability. In addition to exploring the impact of vibration frequency, it is necessary to conduct basic science research on how muscle spindles, neurons, and human tissues respond to the different amplitudes delivered by the vibration motor to understand the individual and the combined impact of the vibration parameters as well as to optimize vibration parameters for individual patients. A single session of vibration while walking for 14 s was reported to improve the walking speed of patients with stroke [41]. Further, a single session lasting 5 min inhibited spasticity and improved muscle performance, as measured by EMG [45; 46; 49]. Although these studies were of high quality with larger sample sizes, the results were insufficient for generalization. These findings may implicate the acute effect of FMV in stroke rehabilitation, as also observed in professional athletes [69]. With more frequent and longer duration (5 min FMV + 30 min PT \(\times\) 3/week \(\times\) 8 week) and more FMV (30 min FMV \(\times\) 3/week \(\times\) 6 week), small to moderate effect sizes (0.11-0.52) were observed in studies with relatively high methodological quality [55; 61]. Other studies with less FMV (30 min FMV + 60 min PT \(\times\) 5/week \(\times\) 2 week) also reported significant reduction in spasticity in the experimental group compared with the control group [51; 52; 53; 59]. More FMV might lead to better outcomes, but there is a lack of evidence regarding the best vibration dosage and duration. Thus, future studies to investigate and standardize the protocol for FMV interventions are warranted.