Many people are familiar with Michael Phelps, who is widely considered to be one of the greatest athletes in history. Phelps has won more Olympic medals. Not only than any swimmer, but also more than any Olympian in any sport.

Fewer people know the name of Hicham El Guerrouj, a Moroccan runner who holds two Olympic gold medals and is one of the greatest middle-distance runners of all times. These two athletes are wildly different in many ways. El Guerrouj is five feet, 9 inches tall. Phelps is 6 feet, 4 inches tall. Despite this 7-inch difference in height, the two men wear the same length inseam on their pants. How is this possible? Phelps has relatively short legs for his height and a very long torso, the perfect build for swimming. El Guerrouj has incredibly long legs and a short upper body, an ideal frame for distance running.

Now, imagine if these world class athletes were to switch sports. Could Michael Phelps become an Olympic caliber distance runner with enough training? It's unlikely. At peak fitness, Phelps weighed 194 pounds, which is 52% higher than El Guerrouj, who competed at an ultra-light 128 pounds. Taller runners are heavier runners, and every extra pound is a curse when it comes to long distance running. Similarly, El Guerrouj would never qualify for the Olympics as a swimmer. Swimmers tend to be tall and have long backs and arms, which are ideal for pulling through the water. The secret to maximizing your odds of success is to choose the right field of competition.