Akiko Noda, Sayuri Tsukano, Seiko Miyata, Shinsuke Inoue, Kumiko Honda and Fumihiko Yasuma
Background: Short duration massage of the lower limbs reduces fatigue sensation in working adults. However, the effects of lower limb vibrations on autonomic activity, which potentially could improve sleep onset, have not been fully investigated. This study aimed to examine the effects of lower limb vibrations on heart rate variability (HRV) as a marker of autonomic activity.
Methods: The study involved two aspects. In the first experiment, the optimal frequency of vibration was determined in five healthy adults (one male and four females; mean age, 20.4 ± 0.5 years). The second experiment involved assessing the effects of vibration at the optimal frequency on nine healthy adults (four males and five females; mean age, 26.3 ± 16.8 years). Electrocardiograms (recorded with bipolar lead CM5) were obtained in the supine position, and HRV was used as an index for autonomic activity. The power spectra at 0.04-0.15 Hz (low frequency (LF) power) and 0.15-0.40 Hz (high frequency (HF) power) were quantified, and HF power and LF/HF ratio were calculated. In the first experiment, vibrations at four different frequencies (i.e., 25, 30, 35, and 40 Hz) were randomly applied for 10 minutes. In the second experiment, HF power and LF/HF ratio were measured at a vibration frequency of 35 Hz for 10 minutes and for 10 minutes of interruption. As a control, HF power and LF/HF ratio were recorded for 20 minutes without vibration.
Findings: The percent change in HF power tended to be higher at a vibration frequency of 35 Hz compared to vibrations at frequencies of 25, 30, and 40 Hz. Vibrations at this frequency significantly increased HF power and reduced the LF/HF ratio compared to when vibrations were not applied.
Conclusion: Our findings suggest that the lower limbs vibration in the supine position at the frequency of 35 Hz may increase parasympathetic activity and presumably promote the rapid onset of sleep.
