Vibrations reduce the depression of neuronal excitability upon activation, increasing motor efficiency

Short-term whole body vibration (WBV) increases neuromuscular activation. Among the responsible mechanisms, the desensitization (reduction of sensitivity to excitation) of muscle spindles by local vibrations is postulated. In addition, it is believed that local vibrations, as well as WBV, contribute to a permanent reduction in the excitability of the spinal cord. This may be due to the mechanism of depression (reduction of neuronal excitability) after activation (post-activation depression, PAD). PAD is activated when at least two consecutive signals excite the same afferent Ia with an interstimulus interval lasting from 1 to 10 s (excitability decreases as a result of depletion of neurotransmitters). The German-Swiss group of scientists, the authors of the presented study, decided to check this mechanism, i.e. they asked whether 2 minutes of WBV could affect PAD and thus reduce the excitability of the spinal cord.

• The PAD was reduced after 2 minutes of administration of a 10 Hz vibration.
• PAD reduction after the vibration continued for several minutes.
• PAD is unlikely to explain the inhibition of spinal cord excitability observed after administration of vibration, but it may explain the increased motor performance after vibration therapy.

Prepared on the basis of:

Acute whole-body vibration reduces post-activation depression in the triceps surae muscle. Krause A, Gollhofer A, Lee K, Freyler K et al. Hum Mov Sci. 2020 Aug;72:102655.

Study population

28 volunteers participated in the study (16 men, age 25 ± 3 years, height 176 ± 10 cm, body weight 71 ± 13 kg).

Test procedure

Repeated-measurement tests were used. The effect of vibration was first evaluated, then the same tests were carried out on the same people, but without vibration – as a control (see below). The short-term effect of WBV on the inhibition of spinal cord excitability by PAD was assessed before and up to 10 minutes after training with repeated WBV exercises. To test the plausibility of the repeated measures model, exactly the same preliminary measurements were performed without vibration intervention twice (2 items) for the first 14 subjects and good values ​​(Cronbach’s α = 0.81) were obtained for the Hcond/ H ratios.

PAD was assessed in soleus muscle by paired stimulation before, 2 min, 4 min, and 10 min after 2 min WBV (10 Hz, 2 mm). The methodology included electromyography (soleus muscle, tibia anterior muscle) and goniometric recordings (range of motion: ankle, knee joint). H reflexes were induced by stimulation of peripheral nerves and assessed by conditioned reflexes.

Use of vibration in the study

Participants stood barefoot in an upright position on a Galileo Sport vibration platform (Novotec Medical, Pforzheim, Germany). Alternating forces were applied to the human body through mechanical oscillations around the sagittal axis. Vibration parameters included an amplitude of 2 mm and a frequency of 10 Hz, which gave an acceleration value of 0.8 g. The choice of 10 Hz was justified on the basis of previous studies in which it was claimed that natural frequencies lower than 20–30 Hz may have an impact on the human body. The heels were lifted, the knees were slightly bent, and the faces were turned forward. Two repetitions of 1-minute WBV workouts were used with 1-minute breaks to avoid fatigue.

Results

Hcond/ H was significantly increased, +55% (2 min), +32% (4 min) and +35% (10 min) after WBV (P < 0.05). Baseline muscle activity and joint position have been shown to be reliable (Cronbach’s α values > 0.990) throughout the test procedure.

Comment

The presented results showed that the vibration-induced inhibition of spinal cord excitability is accompanied by a reduction in PAD at the presynaptic terminals that connect the Ia afferents with the α motor neuron. Thus, PAD is unlikely to explain the inhibition of spinal cord excitability observed after administration of vibration elsewhere, while the reduction in PAD may explain the increased motor performance after vibration therapy, but further studies are needed to verify this assumption.

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