I've tried to run. In high school I ran to get ready for football. In college I ran middling distances because … well, I'm not sure why. It seemed like the thing a health-conscious person was supposed to do. And then, in my late 30s, I tried running once more, this time with the goal of getting leaner.

It never worked. I could build up to five 12-minute miles, and then my body would fight back. My knees would ache, my allergies would flare up, and in general I would feel worse instead of better. A lot worse.

That last time was the most frustrating. I was fitness editor for Men's Health, with a lifelong interest in exercise and healthy living. Almost everyone I worked with enjoyed running, and many of them were pretty good at it. So why couldn't I do the simplest, cheapest, and most popular form of serious exercise?

It's a surprisingly difficult question to answer, but it's an important one to ask at a time when fitness advice is so polarized. On one side you have some writers and strength coaches saying no one should run, that it makes you sick and fat and destroys your muscle tissue. On the other you have … well, pretty much all of exercise science.

"I've never met a person who couldn't run, but that doesn't mean there aren't any," says Jon-Erik Kawamoto, MSc, CSCS, owner of JK Conditioning in St. John's, Newfoundland, and a former competitive middle-distance runner.

The "everyone can run" side has powerful corroboration from evolutionary biology. Let's start there.

Fred Flintstone: marathoner?

In 2004, Harvard biology professor Daniel Lieberman, Ph.D., published this paper in Nature, arguing that modern humans evolved for long-distance running. Not long-distance walking, which would make more sense for our ancient ancestors. Their survival depended on hunting, gathering, and exploring, and eventually led them to populate every habitable chunk of real estate on Earth.

These specific adaptations, he argued, could only have evolved to make us better at running:

  • Expansive gluteal muscles compared to apes, which help stabilize the trunk during running but aren't particularly important when walking on flat surfaces
  • The nuchal ligament in the neck, which keeps the head steady while running
  • Long Achilles tendons, which generate power and also help absorb the shock of landing
  • Short toes in general, and a big toe that's positioned alongside the others (as opposed to spread out like a thumb, as it is in apes), which would make running more efficient

Those are in addition to the changes that made us distinctly human: long, narrow, mostly hairless bodies that easily dissipate heat; shoulders that are relatively low and wide; and neck, shoulders, and hips that can all rotate independently of each other.

The endurance-running hypothesis, as it's now called, is one of those arguments that makes a lot of sense but also seems stretched too far. For example, the structure and mobility of our shoulders and hips made it possible to throw rocks and (eventually) spears, which gave us access to more and better food, and thus moved us to the top of the food chain. There's way more complexity in those movements than we need for long-distance running.

The obvious conclusion, I think, is expressed in this line from Lieberman's study: "Considering all the evidence together, it is reasonable to hypothesize that Homo evolved to travel long distances by both walking and running."

But that's not the way the paper was received. I remember a lot of headlines saying, in various ways, "Hey, we're supposed to run. We evolved to run. So why isn't everybody running?"

The Nature of the Beast
The short answer: a lot of us are.

We're so used to seeing runners that it's easy to forget they weren't always with us. The idea of running for fitness was completely bizarre to most Americans until a half-century ago. (When I ran as a teenager, in the mid 1970s, people used to stop and ask if I needed a ride. I'm pretty sure most of them were sincere.) Throughout Western civilization, going back to ancient Greece and Rome, you can find an appreciation of exercise. But exercise meant walking, hiking, gymnastics, wrestling. The longest race in the original Olympic games was about 3 miles.

It wasn't until Ken Cooper published Aerobics in 1968 that the idea of endurance running for fitness moved into the mainstream. Currently about 42 million Americans run at least six times a year, and about 10 million run at least twice a week. Those numbers have been increasing steadily for the past decade.

The health benefits of all that running are solid and well documented. But at the same time, injury rates are astonishingly high. In a poll at runnersworld.com, 66 percent of respondents said they'd suffered an injury in the previous year. Published research shows a range from 27 and 70 percent. The knees account for about half of those injuries. Most of the rest are below the knee.

What causes all that pain? Amby Burfoot wrote this in Runner's World:

"[R]unning injuries can be caused by being female, being male, being old, being young, pronating too much, pronating too little, training too much, and training too little."

In other words, take your pick. "Running is a repetitive activity, so overuse injuries are more common," Kawamoto says. The heavier you are, the greater the impact with each step. That makes running especially problematic for the subset of runners who are trying to lose weight.

No question, then, that successful running – that is, becoming good at it while staying relatively healthy – is in large part genetic.

At one extreme are members of the Kalenjin tribe of western Kenya. Even novice runners from that region can outperform elite professionals with just a few months' training. Their bodies are typically leaner and lighter than Western runners, and they obviously have some genetic predisposition toward aerobic fitness.

At the other extreme you'll find me, and people like me. Biomechanically, I'm a wreck. I love to walk, but the minute I go from walking to running, bad things happen. In any sport that involved running, I got hurt. It started with shin splints and ankle sprains in high school football and ended with chronic knee problems when I played pickup basketball in my 30s and 40s.

But a faulty structure isn't the only problem would-be runners might encounter.

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Why Can't Johnny Run?

When scientists study any type of exercise, some percentage of the participants will be nonresponders. That is, the program doesn't improve whatever it's supposed to. But you don't know this unless you read the studies. The news reports only tell you about the average gains (or losses, if that's the goal).

If you don't get those results, everyone assumes you did something wrong. You didn't work hard enough. Or you did too much too soon. Or your diet is messed up. Or, when all else fails, you just didn't want it enough.

It's all based on two assumptions:

1. Every form of exercise should work for everyone who tries it.

2. The sought-after benefits of the exercise are only available by doing that type of exercise.

The first assumption is mostly true, except for the 10 to 20 percent who are nonresponders. And #2 isn't really true at all. Nonresponders to aerobic exercise can sometimes improve their VO2 max – that is, their aerobic fitness – with strength training. Improving aerobic fitness, in turn, may or may not translate to an improvement in performance. It's not a linear relationship.

So if the goal of running is to improve your aerobic fitness, and thus your health and quality of life, it's possible to get those benefits without actually running.

The Lucky-Zygote Club
At the opposite end are the superresponders. Some people are simply programmed to get great results with no more initial effort than you or I put in. They improve fast, and become the best salesmen for a sport like running, since they make it look effortless, and sometimes even fun.

They also, more often than not, look like runners. That is, they're lean and long-limbed, with sinewy muscles and great posture. Most of us assume they look that way because they run. We could just as easily flip it around: They run because they look that way.

It's impossible to say which is more likely because each individual is his own case study. Nor does it matter. You can't predict your own results based on how something works for anyone else.

So let's talk about you.

If you're interested in running, and there's no obvious reason not to, of course you should give it a shot. The chances of being a superresponder or nonresponder are probably the same. You're most likely to end up somewhere in the middle, and you won't know until you try.

Once you start, give yourself a chance to succeed. Kawamoto says that the biggest risk factors for injury are total running volume – that, the number of times your feet hit the ground – and history of previous injuries. It's the same in any sport or activity. When you exceed your body's ability to recover from one workout to the next, you'll get hurt. Once you've been hurt, you're more likely to get hurt again.

The safest way to begin is with "a slow, steady progression of walk-runs," Kawamoto says. "Walk a bit, run a bit." See how your body responds. If it's all good, progress to steady runs. If that works, add volume. That can mean longer runs or more frequent runs, as long as the combination increases your total volume by no more than 10 percent per week.

Next comes intensity: running faster, working harder. Just remember that your body needs time to adapt to each change.

And if it refuses to adapt? Join the club.

Lou Schuler, C.S.C.S., is an award-winning journalist and the coauthor (with Alwyn Cosgrove) of The New Rules of Lifting Supercharged.