Physiological Effects of Acceleration

Dynamic amusement ride

Physiological Effects of Acceleration

Human tolerance to acceleration depends on the magnitude, direction, rate of onset, and duration of the acceleration stress, as well as differences in individual physiology and psychological reaction (i.e., the fear factor).

The response of the body to acceleration varies with length of exposure. Acceleration pulses of 0.2 seconds or less are considered to be "impacts" (e.g. slapping someone's back, crashing a car). The human body can accept a relatively high magnitude impact acceleration without harm as long as its duration is extremely short.

Accelerations lasting longer than 0.2 seconds are referred to as "sustained" or "prolonged" accelerations. It is sustained accelerations that create the exhilaration of a thrill ride experience. Body fluid and organ shifts become more important when studying the physiological effects of exposure to sustained accelerations. The overall effects of +Gz, and, to a lesser extent, +Gx, are very well documented in healthy populations and result in markedly increased heart workload and oxygen demand (ref: The Space Review).

Other aspects of thrill ride motion can impact a rider's comfort and may, in some cases, increase the risk of injury. These include jerk, vibration, and rotational acceleration.

Factors that May Affect Your Tolerance to Acceleration

An increase in heart rate has been one of the generally observed responses to +Gz in military testing. This response varied by individual, psychological stress, and the amount of muscular straining being performed by the experimental subject. The amount of absolute increase in heart rate is affected by the maximum G level reached and the rate of onset. Cardiovascular systems noted under G stress include loss of vision, loss of consciousness with accompanying seizures, convulsions, amnesia and confusion, cardiac dysrhythmias (tachycardia and bradycardia), heart blocks, and a stress cardiomyopathy.

Back, neck, and limb problems are the most frequently reported musculoskeletal problems. There are reported cases of intervertebral disk ruptures under high +Gz and many complaints of sore necks after centrifuge rides and flights in high-G aircraft. Permanent injury is rare enough to warrant a case report. (ref: USAF Flight Surgeon's Guide)

A number of factors are known to decrease tolerance to +Gz acceleration. The following information is based on Appendix D of the Australian standard for "Amusement Rides and Devices - Part 1: Design and Construction" (AS 3533.1-1997). Reprint permission granted by SAI Global Ltd. The full text of the standard may be purchased online at http://www.sai-global.com.

  • Heat stress - An increase in core temperature and dehydration from heat stress reduces the +Gz acceleration required to produce blackout.
  • Hyperventilation - Hyperventilation due to fear or anxiety can reduce the +Gz tolerance threshold by 0.6g.
  • Hypoxia - A reduction in blood oxygen level compounds the reduction in cerebral/retinal blood flow, reducing tolerance threshold to acceleration.
  • Alcohol - Alcohol reduces the acceleration effect threshold by 0.1g to 0.4g.
  • Hypoglycemia - A reduction in blood glucose level reduces the threshold for loss of consciousness by 0.5g.
  • Infections, such as colds, gastroenteritis - Infections and minor illnesses associated with dehydration or fever markedly reduce tolerance to acceleration stress.
  • Age - Persons over 60 years old should restrict exposure to acceleration.

Effects of Sustained Acceleration on Children

Parents should realize that children may be more vulnerable to the physical and psychological effects of acceleration than adults. Research on the effects of acceleration in humans has been restricted to healthy adults; there are no studies to guide ride designers in determining safe levels of acceleration for children. Fear can compound and escalate the effects of acceleration. Some high acceleration rides marketed to children may be too intense for younger children or even sensitive older children.

Note: Saferparks owes special thanks to the sources quoted in this article for helping the public understand some of the complex issues associated with thrill ride dynamics.

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