Physical demands

One of the least understood aspects of motocross racing by non-participants is the extreme level of physical fitness required of competitors. Those unfamiliar with the sport often assume that the rider is doing nothing more strenuous than steering a motorized vehicle around a field, no more demanding than driving the family car around the block. Motocross is very physically demanding on the arms, shoulder, and quads. Observing in detail a rider's actions while at speed on the track reveals why. He or she must maintain ultra-precise control of a machine traversing terrain that most people would have difficulty walking across while maintaining as high of a rate of speed as possible. The rider is astride a machine weighing at least two hundred pounds and, at the most elite professional level, has an engine that produces at least fifty horsepower. A rider's arms and legs are in constant motion during a race, fighting for control of the motorcycle and absorbing the energy produced by high-speed landings from heights that often exceed twenty feet or from two-foot high stutter bumps (called whoops) that jackhammer the motorcycle and the rider. The G forces produced test the absolute limits of a rider's strength and endurance. Finally, a typical professional moto (heat race) lasts at least thirty minutes. That represents half of an hour in which the faster the rider goes, the more violently and frequently he or she is put to the test. And there are no pauses, breaks or pit stops. At least not if a rider expects to win.
The National Sport Health Institute in Englewood, California tested several professional motocross racers in the early 1980s as part of a comparative study of the cardio-vascular fitness of athletes from various disciplines. Athletes from track, American football and soccer were tested, among others. The cardiac stress and strength test results compiled there revealed that the motocross subjects had as high a fitness level as any other discipline tested. (Original article appeared in DirtBike magazine in 1980. Interview with Brad Lackey, World Motocross Champion and one of the test's participants appeared in Racer X Illustrated in 2004 and is recounted

source wikipedia