For athletes, high altitude produces two contradictory effects on performance. For explosive events (sprints up to 400 metres, long jump, triple jump) the reduction in atmospheric pressure means there is less resistance from the atmosphere and the athlete's performance will generally be better at high altitude. For endurance events (races of 800 metres or more), the predominant effect is the reduction in oxygen, which generally reduces the athlete's performance at high altitude. Sports organizations acknowledge the effects of altitude on performance: for example, the governing body for the sport of athletics, World Athletics, has ruled that performances achieved at an altitude greater than 1,000 metres will be approved for record purposes, but carry the notation of "A" to denote they were set at altitude. The 1968 Summer Olympics were held at altitude in Mexico City. Most short sprint and jump records were set there at altitude. Other records were also set at altitude in anticipation of those Olympics. Bob Beamon's record in the long jump held for almost 23 years and has only been beaten once without altitude or wind assistance. Many of the other records set at Mexico City were later surpassed by marks set at altitude.