Effects of Vibration on the Body
A number of factors modify the effects of vibration on humans, including tissue resonance, duration of exposure, individual variations, and other simultaneous environmental stresses. For example, acceleration increases the body's rigidity, reducing its shock-absorbing properties and increasing the transmission of vibration energy to the internal organs (Antipov, Davydov, Verigo, & Svirezhev, 1975). The effects of vibration on the body are determined by the frequency ranges involved.
Effects at less than 2 Hz. Vibrations in the frequency range of 0.1 to 0.7 Hz most often produce motion sickness in humans. Vibrations of 1 to 2 Hz are generally associated with increases in pulmonary ventilation, heart rate, and sweat production above that level considered normal for any other stress present.
Effects from 2 to 12 Hz. Tolerance in this frequency range is usually limited by substernal or subcostal chest pain, with thresholds at approximately 1 to 2 Gz and 2 to 3 Gx. The etiology of the pain is the same for both axes of vibration:displacement of the abdominal and thoracic viscera induces stretching of the chest wall, with torsion at the costochondral junctions of the ribs. Dyspnea is the second most common symptom in this range, apparently with the same etiology as chest pain. Centrally induced hyperventilation can be produced by vibrations around two axes at acceleration amplitudes above 0.5 G in the range of 1 to 10 Hz.
Cardiovascular effects are maximized in Gz+gz (i.e. a Gz-acceleration environment with interposing +gz vibration) at 3 to 6 Hz and in Gx+gx at 6 to 10 Hz. The changes seen are increases in heart rate, arterial blood pressure, central venous pressure, and cardiac output; these are accompanied by a corresponding decrease in peripheral resistance. These changes all resemble nonspecific exercise responses.
Abdominal discomfort and testicular pain are common complaints due to stretching of viscera and force applied to the spermatic cord, respectively.
The headache commonly associated with this frequency range has several explanations. In a Gz+gz environment, the mechanical forces are not well attenuated by the skeletal system. In a Gz+gz environment, the head is forced out of phase with the headrest and repeatedly impacts against it. In Gx+gy environments, the problem is the same only more so; strain, spasm, and soreness of the neck are added to the symptoms.
Finally, bloody stools, transient albuminuria, and transient hematuria are occasionally seen in helicopter pilots flying heavy schedules. Such symptoms are attributed to vibration, and they usually disappear after a few days rest.
Effects above 12 Hz. In these frequencies, there is more concern about effects on performance (vision, speech, fatigue) than about injuries.
