Luge is considered one of the most difficult and dangerous sports in the Winter Olympics, primarily because it requires extreme physical precision while traveling at speeds up to 145 km/h (90 mph). Unlike a bobsled, which is an enclosed vehicle, a luge athlete lies on their back on a small, open sled with no brakes and very few mechanical controls. Steering is done entirely through subtle movements of the calves, shoulders, and head; even a slight over-correction or a tiny twitch at high speeds can cause the sled to "fishtail" and crash into the ice walls. The physical toll is also immense, as athletes experience forces up to 5G in the curves—five times the force of gravity—which makes holding their head up a significant athletic feat. For beginners, a "street luge" or a controlled recreational luge (like those found in New Zealand) is quite easy and fun, but "Olympic-style" ice luge requires years of specialized training, a high tolerance for risk, and a mastery of the complex physics of aerodynamics and friction on a sheet of vertical ice.