MX at the pro-level (Race duration: 30-minutes plus 2 laps) is an endurance sport, like soccer or medium distance running. This is apparent by the medium to very low amount of muscle mass the competitors can "preserve". Football is an explosive sport that does require a certain and very specific type of endurance, but the participants can, obviously, maintain a high degree of body weight and muscle mass that no MX athlete could ever keep, or even use because it requires too much aerobic capacity to support. Obviously the MX athlete is an endurance type of athlete. Being a "winning" pro MX athlete requires the same aerobic capacity as running for 30-36 minutes at approximately a 6-minute per mile pace and stopping to do a few push ups, pulls ups and squats at intervals along the way. In football only a few of the receivers, (skinnier guys) could ever do that. Endurance athletes are required to be skinny, even if they don't start out that way, and explosive athletes do not have to be, but can be. Its a different challenge, both are specialized. MX is more challenging for one reason; there are many times fewer people in the world that are genetically able to perform to the levels needed to be successful in MX, unlike football that has thousands of excellent athletes. So, MX is the most challenging, by far.