Page 286 - Biomedical Engineering and Design Handbook Volume 1, Fundamentals

VIBRATION, MECHANICAL SHOCK, AND IMPACT 263

11.1.2 Human Response to Vibration, Mechanical Shock, and Impact
Mechanical damage can occur at large vibration magnitudes, which are usually associated with
exposure to shocks, and to objects impacting the body (e.g., bone fracture, brain injury, organ
hemorrhage, and tearing or crushing of soft tissues). At moderate magnitudes there can be physio-
logical effects leading to chronic injury, such as to the spine, and disorders affecting the hands. At
all magnitudes above the threshold for perception there can be behavioral responses ranging from
discomfort to interference with tasks involving visual or manual activities.

Injury from Vibration. Whole-Body Vibration. Small animals (e.g., mice and dogs) have been
killed by intense vibration lasting only a few minutes (see Griffin, 1990). The internal injuries
observed on postmortem examination (commonly heart and lung damage, and gastro intestinal
bleeding) are consistent with the organs beating against each other and the rib cage, and suggest a
resonance motion of the heart, and lungs, on their suspensions. In man, these organ suspension
resonances are at frequencies between 3 and 8 Hz.
Chronic exposure to whole-body vibration may result in an increased risk of low back pain, sciatic
pain, and prolapsed or herniated lumbar disks compared to control groups not exposed to vibration
(Seidel, 2005). These injuries occur predominantly in crane operators, tractor drivers, and drivers in
the transportation industry (Bovenzi and Hulshof, 1998). However, it is difficult to differentiate
between the roles of whole-body vibration and ergonomic risk factors, such as posture, in the
development of these disorders (Bovenzi et al., 2006).
Hand-Transmitted Vibration. Chronic injuries may be produced when the hand is exposed to
vibration. Symptoms of numbness or paresthesia in the fingers or hands are common. Reduced grip
strength and muscle weakness may also be experienced, and episodic finger blanching, often called
colloquially “white fingers,” “white hand,” or “dead hand,” may occur in occupational groups
(e.g., operators of pneumatic drills, grinders, chipping hammers, riveting guns, and chain saws).
The blood vessel, nerve, and muscle disorders associated with regular use of hand held power tools
are termed the hand-arm vibration syndrome (HAVS) (Pelmear et al., 1998). An exposure-response
relationship has been derived for the onset of finger blanching (Brammer, 1986). Attention has also
recently been drawn to the influence of vibration on task performance and on the manual control of
objects (Martin et al., 2001).
Repeated flexing of the wrist can injure the tendons, tendon sheaths, muscles, ligaments, joints
and nerves of the hand and forearm (Peterson et al., 2001). These repetitive strain injuries commonly
occur in occupations involving repeated hand-wrist deviations (e.g., keyboard and computer
operators), and frequently involve nerve compression at the wrist (e.g., carpal tunnel syndrome)
(Cherniack, 1999).
Injury from Shock and Impact. Physiological responses to shocks and objects impacting the body
include those discussed for whole-body vibration. For small contact areas, the injuries are often related
to the elastic and tensile limits of tissue (Haut, 2002; Brammer, in press). The responses are critically
dependent on the magnitude, direction, and time history of the acceleration and forces entering the
body, the posture, and on the nature of any body supports or restraints (e.g., seat belt or helmet).
Vertical Shocks. Exposure to single shocks applied to a seated person directed from the seat pan
toward the head (“headward”) has been studied in connection with the development of aircraft
ejection seats, from which the conditions for spinal injury and vertebral fractures have been docu-
mented (Anon., 1950; Eiband, 1959). Exposure to intense repeated vertical shocks is experienced in
some off-the-road vehicles and high-performance military aircraft, where spinal injury has also been
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reported. A headward shock with acceleration in excess of g = 9.81 m/s (the acceleration of gravity)
is likely to be accompanied by a downward (“tailward”) impact, when the mass of the torso returns
to being supported by the seat.
Horizontal Shocks. Exposure to rapid decelerations in the horizontal direction has been extensively
studied in connection with motor vehicle and aircraft crashes (“spineward” deceleration). Accident
statistics indicate that serious injuries to the occupants of motor vehicles involved in frontal collisions
are most commonly to the head, neck, and torso, including the abdomen (AGARD-AR-330, 1997).