Riding a high-speed roller coaster can sometimes cause lightheadedness, narrowing of vision, or even a brief period of unconsciousness. This phenomenon is a direct physical reaction to the forces acting on the body, causing a temporary reduction of blood flow to the brain. When the brain does not receive enough oxygenated blood, the condition is known as transient cerebral ischemia. Understanding the physics of the ride explains why the body responds in this way during a coaster experience.
Understanding Positive and Negative G-Forces
The sensations felt on a roller coaster are governed by G-forces, which measure acceleration as a multiple of Earth’s gravitational acceleration, or \(1g\). The body is accustomed to the constant force of \(1g\) when standing still, but rapid changes in speed or direction alter this familiar force. These forces are categorized as positive or negative, acting along the vertical axis of the body.
Positive G-forces, or \(+Gz\), create the sensation of being pressed heavily into the seat. This force is experienced when the coaster accelerates out of a steep drop or moves through the bottom of a loop or valley, pushing the body downward toward the feet. Conversely, negative G-forces, or \(-Gz\), create the sensation of weightlessness or “airtime.” This occurs when the coaster crests a hill, and the rider feels a pulling force upward, out of the seat. The temporary loss of consciousness is specifically a consequence of sustained, high positive G-forces.
The Mechanism of Blood Pooling
High positive G-forces are the direct physiological cause of a rider passing out because they overwhelm the body’s ability to circulate blood. As the coaster moves through a valley or a tight helix, the \(+Gz\) force increases the effective weight of the blood. This increased force acts against the heart, making it significantly harder for the organ to pump oxygenated blood upward to the brain.
The force causes a phenomenon called blood pooling, where blood is driven away from the head and accumulates primarily in the lower extremities, such as the legs and abdomen. This pooling reduces the volume of blood available for circulation to the upper body. The immediate consequence is a rapid drop in cerebral blood pressure, which starves the brain of necessary oxygen and nutrients. This lack of oxygen, or cerebral hypoxia, is the biological trigger for the temporary loss of consciousness, which manifests as fainting.
The Spectrum of Temporary Consciousness Loss
The temporary loss of consciousness is not an instantaneous event but a progression of distinct symptoms that correlate with the severity of reduced blood flow to the brain. The first sign is often the loss of peripheral vision, which results in a restricted, tunnel-like view. If the positive G-force is sustained, this tunnel vision progresses into a gray-out, where color vision is lost and the world appears in shades of gray.
Beyond the gray-out, complete visual loss occurs, known as a blackout, even though the rider may still be conscious. If the high positive G-forces are maintained for several seconds at a high level, above 4 to 5 Gs, the rider may reach G-force induced Loss of Consciousness, or G-LOC. This is the state of actually passing out, which is temporary and quickly reversed once the coaster exits the high-G element and blood flow to the brain is restored.
Individual Factors That Increase Vulnerability
Individual physical conditions significantly influence G-tolerance, meaning not every rider experiences the same symptoms on a high-G roller coaster. The body’s ability to compensate for the reduction in cerebral blood flow is important. For instance, being dehydrated or fatigued lowers a person’s G-tolerance threshold, making them more susceptible to gray-out or G-LOC.
Low blood pressure can also reduce tolerance, as the heart is already working with less pressure to push blood upward against the increased G-force. Riders can increase their tolerance by maintaining hydration and nutrition before a ride. Fighter pilots, who routinely experience high G-forces, often use specialized breathing and muscle-flexing techniques, such as straining the abdominal and leg muscles, to help push blood back toward the head.