In the summer of 1895,  crowds flooded the Coney Island boardwalk to see the latest marvel  of roller coaster technology: the Flip Flap Railway. This was America’s first-ever looping coaster – but its thrilling flip came at a price. The ride caused numerous cases of  severe whiplash, neck injury and even ejections, all due to its signature loop. Today, coasters can pull off  far more exciting tricks, without resorting to the “thrill”  of a hospital visit. But what exactly are roller coasters  doing to your body, and how have they managed to get scarier and safer at the same time? At the center of every roller coaster  design is gravity. Unlike cars or transit trains, most coasters are propelled around their tracks almost entirely by gravitational energy. After the coaster crests the initial lift hill,  it begins an expertly engineered cycle – building potential energy on ascents and expending kinetic energy on descents. This rhythm repeats throughout the ride, acting out the coaster engineer’s choreographed dance of gravitational energy. But there’s a key variable in this cycle that wasn’t always so carefully considered: you. In the days of the Flip-Flap, ride designers were most concerned  with coasters getting stuck somewhere along the track. This led early builders to overcompensate, hurling trains down hills and pulling on the brakes  when they reached the station. But as gravity affects the cars,  it also affects the passengers. And under the intense conditions of a coaster, gravity’s effects are multiplied. There’s a common unit used by jet pilots, astronauts, and coaster designers called “g force”. One G force is the familiar tug of gravity you feel when standing on Earth – this is the force of Earth’s  gravitational pull on our bodies. But as riders accelerate and decelerate, they experience more or less  gravitational force. Modern ride designers know that the  body can handle up to roughly 5 Gs, but the Flip-Flap and its contemporaries  routinely reached up to 12 Gs. At those levels of gravitational pressure, blood is sent flying from your brain to your feet, leading to light-headedness or blackouts as the brain struggles to stay conscious. And oxygen deprivation in the retinal cells impairs their ability to process light, causing greyed out vision or temporary blindness. If the riders are upside down, blood can flood the skull, causing a bout of crimson vision called a “redout”. Conversely, negative G’s  create weightlessness. Within the body, short-term weightlessness is mostly harmless. It can contribute to a rider’s  motion sickness by suspending the fluid in  their inner ears which coordinates balance. But the bigger potential danger – and thrill – comes from what ride designers  call airtime. This is when riders typically experience seat separation, and, without the proper precautions, ejection. The numerous belts and harnesses  of modern coasters have largely solved this issue, but the passenger’s ever-changing position can make it difficult to determine what needs to be  strapped down. Fortunately, modern ride designers are well aware of what your body, and the coaster, can handle. Coaster engineers play these competing  forces against each other, to relieve periods of intense pressure with periods of no pressure at all. And since a quick transition from  positive to negative G-force can result in whiplash, headaches,  and back and neck pain, they avoid the extreme changes in speed and direction so common in thrill rides of old. Modern rides are also much sturdier, closely considering the amount of gravity they need to withstand. At 5 G’s, your body feels 5 times heavier; so if you weigh 100lbs, you’d exert the weight of 500 lbs on the coaster. Engineers have to account for  the multiplied weight of every passenger when designing  a coaster’s supports. Still, these rides aren’t for everyone. The floods of adrenaline,  light-headedness, and motion sickness aren’t going anywhere soon. But today’s redundant restraints,  3D modeling and simulation software have made roller coasters safer  and more thrilling than ever. Our precise knowledge about the  limits of the human body have helped us build coasters that are  faster, taller, and loopier – and all without going off the rails.