Fast Cars and Wild Rides:
Whiplash, Headbanging, and Sqeezeplays

Speeding, spinning vehicles and rapid changes of direction can apply significant stresses to the human body. The odds of injury from riding a thrill ride are very low for most people, but motion-related back/neck injuries and damage from impacting the containment system are the most frequently-reported ride-related injuries.

Human beings are the most complex and unpredictable component in ride design.  A ride designer can specify the precise width and strength of the bolts used to hold a vehicle on the track, or hold the lap bar onto the vehicle.  The engineer can count on those inanimate materials to react to stress in the same predictable ways.  Riders' bodies are not uniform in size, shape, or resistance to stress.  They do not always react in predictable ways.  Let's see how those human variables interact with dynamic force

Many industry-standard containment/restraint systems expose smaller riders and very large riders to higher risk of falls and ejection from moving rides, injuries from impacts within the containment system, and injuries from too-tight restraint fit.

• If a ride's lap bar doesn't fit closely against smaller riders -- and many don't -- young children may slide down under the lap bar or pull their feet up onto the seat, putting them into a position where they are at risk of ejection. A child who isn't closely contained by the restraint may become frightened at the feeling of instability, or confused as the ride comes to a temporary stop, and try to stand up or leave the ride.
• On spinning rides or rides with sharp turns, the lateral acceleration can cause the smaller child to be pressed into the larger riders. If this feels uncomfortable, or if the older brother yells at the smaller child, the smaller child may try to stand up or move away into an unsafe position.
• Ride designers often employ bracing points rather than barrier restraints to keep riders from sliding into unsafe positions during turns or drops on rides that have smaller accelerations. A common example is bracing with the feet to maintain an upright position during turns. Young children oftentimes cannot reach the floor with their feet on full-sized rides, and may be put at higher risk of sliding into an unsafe position.
• Young children like to imitate things they see older kids and adults doing, such as riding thrill rides with both hands in the air. Young children lack the experience to anticipate when they'll need to hold on for extra bracing, such as around a curve. If a child's hands are both up in the air when the ride takes a sharp turn, and if the ride has only an unfitted lap bar, then the friction of clothing against the plastic seat is all that's holding his tiny body inside the vehicle.
• Skinny riders may slide and/or bang around more within the confines of the restraint system.  They may not have as much neck strength as they need to hold their head upright under force.  They have less padding on their frame to cushion against impact.
• Lap-only restraint systems, even those with redundant paired restraints (e.g., lap belts with lap bars), may not safely restrain obese riders.  Seat belts tend to fasten over thighs instead of laps on larger riders and lap bars lock in a more vertical position, making it possible for the rider's body to be propelled out from underneath the lap bar on rides that induce negative accelerations.  Obese riders should avoid rides with steep drops if they do not have over-the-shoulder restraints.
• Patrons who are larger or taller than average may have trouble fitting comfortably in rigid restraint systems. Bruising can occur. In rare cases, an aggressive ride in too-tight restraints can break a rib or injure internal organs.

According to the U.S. Consumer Product Safety Commission (CPSC), neck sprain is the most common type of ride-related injury treated in hospital emergency rooms.  As thrill rides whip the human body around, the weight of the head exerts strong forces on the neck.  Risk factors for neck injury include:

• Previous injuries or pre-existing medical conditions affecting the neck.
• Acceleration profile of the ride, especially rapid changes in direction of movement.
• Seat and restraint design, including padding and neck support.  Coasters with low-backed bench seats may allow the rider's neck to snap backward.
• Strength of rider's neck.  Full-sized rides are designed for a median adult male weighing 170 pounds.  Younger, older, and more slender riders do not have as much muscle strength in their necks to hold their heads upright.  CPSC data from hospital emergency rooms indicates that women are twice as likely to suffer ride-related neck injuries as men.
• Flexibility of rider's neck.  Older riders and people who suffer from conditions affecting flexibility and bone strength, such as arthritis should avoid high-g rides that tend to whip the rider's head around.

On highly dynamic rides, it is very important that patrons keep keep their heads upright and facing forward.  One doctor studying ride-related neurological injuries noticed that many of those injuries happened when the rider turned his/her head (to check on a child seated next to them, for example) right before a change in direction or magnitude of acceleration.

Newer ride designs use several techniques to keep riders heads aligned properly.  Seats and restraints are designed to discourage side-to-side movement.  Heavily themed rides use carefully-positioned visual elements to keep the rider's attention focused forward. 

All patrons should pay attention to this warning.  Parents should make a special effort to teach children the importance of riding "eyes front".

According to Dr. Richard Brown, a neurophysiologist who evaluated human response to machine-induced accelerations for the amusement ride industry, it takes about 200 milliseconds for the body to react to accelerations.  At that point, the muscles will tense to counteract the force.  If the acceleration is reversed too quickly, the body's resistance actually magnifies the effect of acceleration.  A poorly designed ride can increase the likelihood of injury.

For example, imagine that a ride is exerting a 2g acceleration, pressing your head forward.  Your neck muscles will react to that pressure by creating an equal force pressing the head backward.  Now imagine that the forces of the ride reverse direction, so that they are exerting a 2g acceleration pressing your head backward.  Your neck muscles and the ride are suddenly working in the same direction.  Instead of equilibrium, you've got a 4-g force slamming your head back.  It will take 200 milliseconds for your body to turn off those muscles. 

Machines can be built with far quicker reaction time than humans.  Some percentage of the 300 million humans who visit amusement parks every year will have a slower muscle response than 200 msec.  Older people should beware of highly dynamic rides that might assume a muscle reaction time faster than their aging bodies can provide.

At a meeting of the ASTM F24 industry standards committee, Dr. Brown brought up a fascinating point about muscle strength and negative or reduced positive Gz accelerations.  Humans are accustomed to using their muscles under the action of normal earth's gravity.  If you tense your leg muscles to rise from a chair, for example, you automatically exert enough force to overcome 1g.  That same level of muscle resistance will move you much farther if you're in an amusement ride that's created an effective acceleration of less than 1g.  As Dr. Brown said, you can "be Superman" for brief moments in certain thrill rides.

Restraint design should account for this factor, by ensuring that sudden movements won't propel a rider out of the seat.  Patrons may want to take note of this phenomenon, however.  In particular, it's a good idea for parents to help children understand the importance of sitting still, especially if the child has a tendency to shift, wiggle, or otherwise move around when seated.

According to an article published in the January issue of the Annals of Emergency Medicine ("Amusement Park Injuries and Death"), reports of amusement ride-related brain injury have risen substantially since 1990.  The article raises concerns about the lack of regulation and the lack of data on the effects of amusement ride g-force.

"Case reports have demonstrated subdural hematomas, internal carotid and vertebral artery dissections, and subarachnoid hemorrhage in association with roller coaster-generated G forces, but no research has determined the human G-force threshold for these injuries.  The absolute strength (total Gs), the duration of G force, and the rate of intensification of G force are all important variables.  It is certainly possible that lateral G forces, rotational acceleration, abrupt directional changes, and predisposing anatomic factors play an important role in these types of injuries as well."

The article cited conclusions from several published medical articles:

• "Roller coaster rides induce marked rotary and other positional changes in a deformable brain that is moving within a rigid skull.  Tensile and shearing forces may be severe enough to rupture cortical veins leading to subdural hematoma."
• "In the absence of any other predisposing factors, the acceleration forces associated with roller coaster rides can cause tearing of bridging veins, resulting in subdural hemorrhage."
• "Carotid and vertebral artery dissections are often associated with indirect trauma or torsion of the neck.  The acceleration and abrupt changes in direction on a roller coaster may induce uncontrolled rotation of the head with stretching of the cervical vessels and aorta similar to that observed with acute deceleration in a motor vehicle crash."

Drs. Braksiek and Roberts concluded their article as follows:  "Although the current risk of injury, hospitalization, and death on amusement rides is extremely low, health care providers should be aware of a worrisome trend in the number and rate of amusement park injuries...Although data exist as to the threshold of G force needed to produce loss of consciousness in a controlled centrifuge, there is little or no data on the neurologic effects of intermediate duration G forces combined with rapid directional changes."

Notes: 

• In the first few months of 2003, several industry-affiliated studies were released that refuted the findings of Drs. Braksiek and Roberts.  For more information visit the Acceleration Injury Research page.
• In 2007, the New England Journal of Medicine published results of a Dutch study based on MRI scans of 2000 healthy adults with an average age of 63. The study, which looked at the causes and consequences of age-related brain changes, found that brain abnormalities are not that uncommon.
  • 7.2% of the participants had some dead brain tissue caused by a loss of blood flow. These so-called silent strokes don't usually result in a loss of speech or motion, but signal higher risk of vascular events.
  • 1.8% of the participants had bulging blood vessels, called aneurysms. This condition can, under certain conditions, put thrill riders at risk of serious brain injury.

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 g-force.