Notably, the sensory skin component stimulating the brain has potential for several purposes including improvements in brain-related disorders. Combining these two components by selecting the optimal settings in whole body vibration has clear potential for medical applications.
To realize this, the field needs more standardized and personalized protocols. Unraveling the underlying mechanisms by translational research can help to determine the optimal settings. Many systematic reviews on whole body vibration end with the conclusion that the findings are promising yet inconclusive. We are of the opinion that when part of these optimal settings are being realized, a much better estimate can be given about the true potential of whole body vibration as a medical application.
Vibrations are oscillations that occur around an equilibrium point. They propagate in a certain medium such as air, water, a branch of a tree, a leaf, or soil.
The waveforms of the vibrations can range from very regular sinusoidal to very irregular random. Such vibrations are all around us, produced, for example, by the wind, thunder in the air, or rain drops falling on the ground. They can also be caused by organisms of any size. Vibrations are often produced unintentionally while these creatures move around. Three basic forms of vibration transmission can be distinguished based on the medium in which the vibrations propagate: airborne, water-borne, and substrate-borne vibrations.
These vibrations are all intricate components of the natural environment. It is assumed that they played an important role in the evolution of life 1. Sensitivity to vibrations is found in even the simplest forms of life. The vibration sense is used across the animal kingdom for various reasons: to detect prey, to avoid predators, to assess and navigate within a habitat or environment, or to search for food, amongst others. Vibrations are also used intentionally, for example to communicate with other individuals or to ward off a predator 3 , 4.
Also, humans detect vibrations, and even more so when other senses fail 5. A present-day example is feeling a vibrating mobile phone in your hand or pocket. Also, sound is a form of vibration, which is collectively referred to as vibro-acoustics. A sound or loud noise can be both felt and heard by humans if both hearing sense and vibration sense are present.
Given our sensitivity to vibration, it is somewhat surprising that we do not employ our capacity of vibration detection to the fullest. In this paper, we highlight one approach to employ vibrational sense: the use of whole body vibration WBV and its potential for medical applications. We summarize the latest advances in the use of WBV and stress that WBV has great potential as a therapeutic treatment once some critical issues are solved.
Humans are endowed with a high density of mechanoreceptors in the skin notably in the fingertips and feet to detect vibrations Figure 1. Next to those, mechanoreceptors can be found less abundantly in ligament, joints, blood vessels, and organs 6. We evolutionarily inherited vibrational sensitivity, which is hard-wired in our body and brain. The mechanoreceptors project via the spinal cord and the thalamus to the somatosensory cortex. Various cortical brain regions are involved in vibrational information processing.
Vibrations of high enough energy are therefore consciously detected. Vibrations of low energy or beyond the detection level of hearing infrasound still reach the brain, and it is believed that it causes annoyance and distress in many people 7. Two main types of cutaneous mechanoreceptors receptive to vibrations are present in mammals, including humans see 8 for review.
Pacinian corpuscles, or pressure receptors, are deeply placed in the skin and less abundantly elsewhere in the body 6. They sense vibrations at a range of 20—1, Hz, with a peak sensitivity around Hz. Meissner corpuscles, or touch receptors, are more superficially placed in the skin. While touching a surface, the skin copies the surface via skin deformations and the corpuscles start to signal with their preferred frequency to the somatosensory cortex see also Figure 2.
Meissner corpuscles sense vibrations at a range of 5— Hz, with a peak sensitivity around 10—65 Hz. Meissner and Pacinian corpuscles are mandatory for the detection of vibrations. Stimulation of Meissner corpuscles results in the sensation of tapping-flutter-vibration, whereas stimulation of Pacinian corpuscles results in the sensation of vibration or tickling.
Pacinian corpuscles in the human hand also serve a function in active texture exploration 9. Under normal conditions, vibrational stimulation of the skin co-activates both types of skin mechanoreceptors either because the range of frequencies of the source is broad or because of harmonics for example, a 30 Hz vibration will also generate vibrations of 60, 90, Hz etc.
A schematic and simplified overview of the sensory and exercise components of WBV in humans panel A and small rodents panel B applied via a vibrating platform. The mechanical vibration has predominantly a sinusoidal waveform potentially beneficial , as more random and erratic waveforms are potentially harmful see also the European vibration directive General focus areas in WBV research are indicated in the boxes.
The dashed line from the gut microbiome indicates that WBV might directly affect the gut microbes instead of via mechanoreceptors. One recent advance in the field of WBV research is the increasing awareness that WBV, next to an exercise component, also has a sensory component affecting the brain via skin mechanoreceptors. A schematic and simplified overview of vibration detection influencing the brain. Mechanoreceptors in the skin Panel A: 1. Meissner corpuscles, 2.
Pacinian corpuscles detect the naturally caused vibrations depicted in panel B and relay the signal to the brain via the spinal cord. In the thalamus, the signal reaches the ventral posterolateral nucleus and the posterior thalamic nucleus. The incoming vibrational signal stimulates the prefrontal cortex and neurotransmitter systems, although the pathways of these brain connections are rather unclear. These vibrations have either a positive or a negative impact on human health depending on their features.
Detected vibrations can also influence the brain via increased heart rate and hence increased blood flow to the brain. Humans have a clear ability to detect vibrations of a wide range of frequencies and amplitudes.
It had evident evolutionary advantages for individual survival escaping predators or finding prey, and increased reproduction for our ancestors. The use of the skin mechanoreceptors for sensing the texture of fruit for its edibility is considered a crucial step in our survival. These original advantages seem not so relevant anymore in our current society. However, vibrations and vibration detection are still linked to health issues.
Numerous human studies on vibrations exist and are ongoing, examining under which conditions these vibrations become harmful. The aim of these studies is to determine which regulations are needed to prevent physical or mental damage. One aspect of harmful vibrations relates to the principle of resonance. Any part of the body has an intrinsic resonance frequency. The resonance frequencies will induce more physiological effects but if too strong should be considered harmful. For example, it has been estimated that the resonance frequency for the liver is 2—7 Hz in mice, while in humans the resonance frequency for the abdomen is 4—8 Hz, the thorax is 5—10 Hz, and the head is between 20 and 30 Hz WBV research was in part inspired by determining these health risks.
The mechanical vibration of these vibrating platforms is transferred to the body, which triggers physical and physiological responses see 12 for review. An example of how this can affect the outcome is given by Alizadeh-Meghrazi and co-workers in individuals with spinal cord injuries Most often, the waveform is sinusoidal and hence regular and predictable or predetermined.
With a shift in focus from harmful vibrations to beneficial vibrations, the number of WBV publications increased source: PubMed. Until around , the number of WBV publications was about 15 per year.
Then, the yearly number steadily increased and peaked in and about publications per year. The number of publications stabilized around in later years. The often-recurrent conclusion of the systematic reviews is that findings are inconsistent, of rather modest effect size if present, and require further research. The fastest vibration we can hear is 20, times per second, which would be a very high sound. Animals can hear different frequencies from humans. Cats can hear even higher frequencies than dogs, and porpoises can hear the fastest vibrations of all up to , times per second.
It takes 3 different vibrations to hear a sound, since sound is made when things vibrate or wiggle :. When sound waves move through the air, each air molecule vibrates back and forth, hitting the air molecule next to it, which then also vibrates back and forth. The individual air molecules do not "travel" with the wave. They just vibrate back and forth.
When the vibrations are fast high frequency , you hear a high note. When vibrations are slower, you hear a lower note. Describe how sound is produced. How many different vibrations are needed to hear a sound? All objects have the potential to vibrate. Can we hear all of them? If a tree falls in a forest and there is nobody around to hear it, does it still make a sound? Teacher Tip: This demonstration is a good way to introduce the topic of sound.
Details Activity Length 10 mins. Materials Per Class: balloons 1 for each student a chair 10—15 pairs of scissors whoopee cushion for optional activity in Part 3 Key Questions Part 1 When we are talking to each other, which parts of our bodies form the sounds and words we use? Part 2: Speaking involves air and muscles Blow up a balloon and make sounds by letting the air out.
Stretch the neck of the balloon to make a high sound and slacken it to make a low sound. This represents the stretching of the vocal cords to vary pitch. Hand out the balloons to the students. Challenge them to produce various pitches with their balloons. Repeat Part 1 several times, adjusting the pitch higher, lower and volume louder, softer.
Now try it without making your vocal cords move. Teacher Tip: This part also works well as a classroom demonstration. Part 3: A fun application of air, vibration and muscles: the fart whistle!
Choose a volunteer to sit on a chair facing the students. Ask them to stand up then sit down, and repeat several times.
Sneak a whoopee cushion onto their chair at some point, just before they sit down! The students blow through the neck of their balloons to mimic a farting sound. Extensions Try taking in a long breath and speak at the same time. How does it affect your speech?
For functional speech to occur, air must be going out, not coming in. Do an interpretive dance of how pitch is controlled.
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