38
3 Mechanical Aspects of Biosystems
Table 3.1 Unusual ‘g’ effects
Sustained acceleration
Effect
Positive (toward head)
+3 to +5 g
Vascular pooling of blood in legs
Muscle cramps
Poor circulation
+5 to +9 g
Loss of vision
Loss of hearing
Blackout (‘G-LOC) (g-induced loss of consciousness’)
Negative (toward feet)
−3 g
Pressure in the eye socket
Headache
−3 to −5 g
Retina engorgement
Loss of vision (“redout”)
Above −5 g
Cerebral hemorrhages
Short-term zero g
Loss of a sense of balance
Possible vertigo and anxiety
Elation, feeling of strength
Long-term zero g
Loss of muscle tone
Loss of heart strength
Reduced red blood cell production
Degeneration of labyrinth function
Reduced bone strength
A concussion in a human will result from accelerations of over 90 g in less than a
second, particularly if rotation is involved. Collisions between American footballers
can generate 150 g’s. Helmets may reduce the maximum expected brain acceleration
to under 50 g’s. But repeated lower-level collisions can create permanent damage to
the human brain. Woodpecker brains, however, regularly undergo 500 g’s without
injury. Paracoccus denitrificans bacteria thrived living in a ultracentrifuge test tube
experiencing over 400,000 g’s. 21
For aviators, the effects of acceleration can be reduced to some degree by
wearing pressure suits which inflate when large g-forces are present, in order to
reduce the pooling of blood, and by lying in a personally contoured seat, with the
back perpendicular to the expected g-forces. This distributes the seat forces more
evenly, and also minimizes the pooling of blood by minimizing the longitudinal
separation between the high and low blood pressure regions in the body. Under
these conditions, up to 9 g’s can be tolerated for minutes, and 15 g’s can be suffered
for up to half a minute, after exercise and training (Fig. 3.2).
21 S. Deguchi, et al., Microbial growth at hyper-accelerations up to 403,627 g, Proc Natl Acad Sci
108:19, pp.7997–8002, NAS Press, Wash., DC (10 May 2011).
3 Mechanical Aspects of Biosystems
Table 3.1 Unusual ‘g’ effects
Sustained acceleration
Effect
Positive (toward head)
+3 to +5 g
Vascular pooling of blood in legs
Muscle cramps
Poor circulation
+5 to +9 g
Loss of vision
Loss of hearing
Blackout (‘G-LOC) (g-induced loss of consciousness’)
Negative (toward feet)
−3 g
Pressure in the eye socket
Headache
−3 to −5 g
Retina engorgement
Loss of vision (“redout”)
Above −5 g
Cerebral hemorrhages
Short-term zero g
Loss of a sense of balance
Possible vertigo and anxiety
Elation, feeling of strength
Long-term zero g
Loss of muscle tone
Loss of heart strength
Reduced red blood cell production
Degeneration of labyrinth function
Reduced bone strength
A concussion in a human will result from accelerations of over 90 g in less than a
second, particularly if rotation is involved. Collisions between American footballers
can generate 150 g’s. Helmets may reduce the maximum expected brain acceleration
to under 50 g’s. But repeated lower-level collisions can create permanent damage to
the human brain. Woodpecker brains, however, regularly undergo 500 g’s without
injury. Paracoccus denitrificans bacteria thrived living in a ultracentrifuge test tube
experiencing over 400,000 g’s. 21
For aviators, the effects of acceleration can be reduced to some degree by
wearing pressure suits which inflate when large g-forces are present, in order to
reduce the pooling of blood, and by lying in a personally contoured seat, with the
back perpendicular to the expected g-forces. This distributes the seat forces more
evenly, and also minimizes the pooling of blood by minimizing the longitudinal
separation between the high and low blood pressure regions in the body. Under
these conditions, up to 9 g’s can be tolerated for minutes, and 15 g’s can be suffered
for up to half a minute, after exercise and training (Fig. 3.2).
21 S. Deguchi, et al., Microbial growth at hyper-accelerations up to 403,627 g, Proc Natl Acad Sci
108:19, pp.7997–8002, NAS Press, Wash., DC (10 May 2011).