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There's one thing that all great marathon runners have in common and no, it's not a Kenyan passport. (That helps, of course.)

But watch the first people to cross the finish line and you'll see that there's one common denominator of long-distance dominance: size. Almost without exception, elite marathon runners stand 5-foot-7, give or take two inches, and weigh 140 pounds, plus or minus a few pounds.

"There is a reason that most elite marathon runners are of moderate height and very thin," says Richard Hughson, a professor of kinesiology at the University of Waterloo and former world-class distance runner. "Running at high speed requires a lot of energy. Just imagine running with two two-kilogram weights in your hands or tied around your waist. If a marathon runner is four kilograms overweight, then he or she must carry that extra 'useless' weight for 42.2 kilometres. Therefore, all elite runners will have as little fat, bone and even muscle as possible."

Athletic performance can boil down to physics, with body type determining how well you will do out on the track, or the court or the pitch. When it comes to running, smaller individuals with slight frames will almost always do better than their bigger, bulkier peers, no matter how good their biomechanics or training. With less load to carry, smaller runners are simply lighter on their feet.

In addition, larger runners are more likely to be sidetracked by injury, Prof. Hughson says. "The pure physical loading on the skeleton, muscles and tendons is determined by the mass and velocity. So, heavy individuals will exert much greater forces on their bodies. Now, multiply those forces by the number of steps – not many in a sprint but of very high force, versus very many for the long-distance runner even if of lower force per step – and you end up with a recipe for trouble," he says.

But that doesn't mean that you can't run if you not the perfect ectomorph. Most injuries are usually a result of things that have nothing to do with body type, says Reed Ferber, director of the Running Injury Clinic, in Calgary. Mechanics, strength, flexibility and alignment play a much greater role in injury, and injury prevention, than body type, he says. "I firmly believe that anybody can run a marathon. It's taking into consideration those four factors," he says.

It is simply a matter of tailoring training to best fit your body type, says Paul Regensburg, co-founder, along with fellow former Olympic triathlon coach Lance Watson, of LifeSport Coaching, in Victoria. And while some body types may be more prone to injury than others, most injuries can be avoided by paying attention to factors that have nothing to do with body type, such as biomechanics and alignment.

Someone with a small, slight frame might be able to tackle a marathon after just four months of training, while someone who is stockier might need up to one year, for example.

Heavier runners should also begin by running shorter distances at a slower rate because more weight means more stress on the body, and taking on too much training volume too quickly can lead to injury.

As well, people who have more muscular physiques may want to cut down on hitting the weight room. If they really want to reduce their finishing times, they need to become more lean, Mr. Regensburg says.

But he adds, "For performance, it's a different game. The reality is that gravity-based sports do rely on basic physics. And basic physics are, if you're lighter, you'll generally go faster."

Just how much people should set out to adjust their physique for running should depend on why they are lacing up in the first place, Prof. Hughson says.

"Does a very large, muscular person really want to become an elite marathon runner, or is he or she perhaps going to be just as happy as an elite sprint triathlete where the ability to swim and ride a bicycle might benefit a bit from being more muscular?

"Or perhaps the objective is just to be fit and to enjoy the friendship of others while he or she completes a marathon or other endurance run?" he says. "There are genetic constraints and each of us must live within those, then develop what we want to be the best we can be."

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