Alpine skiing, also known as downhill skiing, is a winter sport that involves sliding down snow-covered slopes on skis with fixed-heel bindings. It is one of the signature competitions of the Winter Olympics and has been held since 1936. Alpine skiing has five events: downhill, slalom, giant slalom, super-G, and combined. Downhill skiing is the simplest form of alpine skiing, where competitors ski down a slope as quickly as possible. Slalom is considered the fastest event among alpine skiing, where competitors ski down a slope that has a vertical descent of 180m to 220m for men or 140m to 180m for women. Giant slalom has courses that are set with more widely spaced turns than slalom. Super-G is a speed event that is contested in single runs down long, steep, fast courses featuring few and widely spaced turns. The combined event consists of a downhill and a slalom race, with the winner having the lowest combined time. Alpine skiing can be contrasted with skiing using free-heel bindings, ski mountaineering, and nordic skiing such as cross-country, ski jumping, and telemark. Ski trail ratings are used to indicate the difficulty of a ski run. Injuries can occur in alpine skiing, and learning how to fall correctly and safely can reduce the risk of injury. According to a 2004 Harvard Medical School study, alpine skiing burns between 360 and 532 calories per hour.