As I share in my book, Kale and Coffee, I am  convinced there is a species-specific exercise that all humans are physically and genetically built to master. And the natural fitness activity for us humans doesn’t include fancy machines or swimming in the Arctic Ocean in late January. It’s gimmick-less, simple locomotion—walking, running, and jumping. But as simple as running is, if we understand our biology as runners and the physiology of running, we can apply that knowledge to make the most of just about any exercise we want to do. There are a convincing number of biological arguments, including muscle makeup and lung capacity, for why we humans are born to run. But if this is the case, we’ve gone pretty far off track. If running is so natural, why do so many people get hurt doing it?

America’s Great Exercise Confusion
On a suggestion from a friend, I signed up for the Hartford Marathon, never having run more than ten miles in one session. At that point, I was running three or four times a week and bicycling 25 to 50 miles every weekend, so I figured my body would be able to meet the challenge of running 26.2 miles. After all, I reasoned, we are built to be runners. The result was miserable. I finished the race but with a strained calf muscle that required eight months of rehab before I could get back in my running shoes.

This sort of thing happens way too often, because of our all-or nothing approach to fitness. We either do it or we don’t. And when we do it, we do it way too much and hurt ourselves, and when we don’t do it, we really don’t—in fact, we even lie about it. In my own first marathon, I made a classic American exercise mistake: I put my mental conditioning before my physical conditioning. I overstepped my body’s boundaries at the time, which is a form of overtraining. What causes the largest percentage of injuries, is overtraining, no matter what your level of fitness.

If less really is better, then why on earth do we work out like crazy in the first place? The answer, I concluded, is that we assume we can accomplish more than what we’re physically capable of—at least at the time—so we ignore the subtle and not-so-subtle warnings our body gives us. The number one
reason most people cite for exercising, or for resuming exercising if they’ve stopped, is to reach some short-term goal: weight loss, more muscle, flatter stomach, a toned butt. And there’s often a secondary goal behind that, such as looking good for an upcoming event like a wedding or a school reunion.
We’re unreasonably attached to our goals, so we hurt ourselves. It happened to me, and it definitely can happen to you too.

What’s confusing is that most of the science about what works fitness-wise is geared toward these faster short-term results. Sure, you can reach short-term goals in record time—lose that stubborn last ten pounds or strengthen your pecs—but it will be at the expense of your overall long-term physical well-being. So what is the threshold? How can you determine if your exercise is working for or against you? The answer is the number of times per minute your heart beats while you’re exercising, no matter what kind of exercise you do.

 The Better Fuel

The body has two major fuel systems: aerobic and anaerobic. The aerobic system uses fatty acids, glycerol, and oxygen for fuel; fatty acids and glycerol are formed when fat tissue is broken down. When your body goes into aerobic mode, it burns fat within a very specific range of heart beats per minute. This is known as your fat-burning zone. If you exercise at too high a heart rate, you will slip into using your anaerobic fuel system, which burns glucose (blood sugar) to fuel your muscles and creates lactic acid. The longer you exercise in this anaerobic state, your sugar-burning zone, the less likely you’ll be able to continue. Once more lactic acid builds up than your body can burn, you won’t be able to move well. So this zone has a more defined limit than the fat-burning zone.  Operating in this zone not only stresses your body but also creates sugar cravings, because you need to replenish the glucose stores you’ve depleted.

We seem to be addicted to getting into this anaerobic mode during exercise—pushing the body past its ideal aerobic limit. This is what happened to me at the CrossFit gym. It’s not all a foolish pursuit: there is some science saying we burn more fat and calories in the sugar-burning zone. But it’s important to note that exercising in the sugar-burning zone is not beneficial for long-term health, causing stress that is more damaging than any short-term benefit gained. Fat burning is where it’s at—regardless of the type of exercise you choose. 

What's Your Resting Heart Rate?
Cyclist Miguel Induran, who won the Tour de France five times from 1991 to 1995, is famous for something else as well: a resting heart rate of 28 beats per minute. That’s phenomenal, but a resting heart rate in the 35-beats-per-minute range is not unusual in professional athletes.

To find out your resting heart rate, all you need is a digital watch or a timepiece with a second hand, and your own two fingers.

Step 1. Make sure you’ve been sitting for at least 60 minutes.

Step 2. Turn your left wrist so your palm is facing up. Place the index and middle fingers of your right hand on the soft tissue at the right side of your wrist. Feel around until you find the pulse.

Step 3. Looking at your watch, count the number of beats in a 15-second period. Then multiply that number by four. That’s your resting heart rate in beats per minute.

Most people have a resting heart rate of 50 to 80 beats per minute. If yours is 80 or more, wait 15 minutes and try again. If it’s still 80 or above, consult your doctor for a checkup. Elevated blood pressure can be a sign of a more serious medical condition. The slower your heart beats, the more conditioned you are. Your heart doesn’t have to work so hard during exercise, and you’re more likely to burn fat, your body’s preferred fuel. Less stress on your body can help you realize the important goal of living longer.

Follow the Exercise Rules
Maybe running to you is like public speaking: you’d rather die than do it. Maybe you prefer lifting weights, CrossFit, spinning, tennis, or competitive curling. It doesn’t matter what you choose. Almost any form of exercise can be beneficial, as long as you follow certain rules.

Don’t overdo it.

If you want to avoid getting hurt, don’t jump into your first high-intensity training class like you’re a pro, or do a triathlon on a whim, or run 26.2 miles if you’ve never gone more than 10. If you’re just getting back to exercising, you could start with walking. Most people would be surprised at where their heart rate is after walking briskly up a steep hill. Pick a fitness activity you like.

The only exercise you’ll stick with for life is one you enjoy. There are fads in fitness as in everything else, so do what you love and keep doing it, no matter what science comes out that discounts it. If an activity involves your feet, legs, and arms, it’s suitable for human participation. Keep your heart rate in the fat-burning zone. 

Be Patient

Eventually your physical fitness will catch up to where your mind wants you to be. Even endurance athletes take a while to find the perfect training range. “Initially, training at a relatively low heart rate may be stressful for many athletes,” Phil Maffetone says. “But after a short time, you will feel better, and your pace will quicken at that same heart rate. You will not be stuck training at that relatively slow pace for too long.”

As much as a champion athlete might find it frustrating to train at a slower pace, I imagine you might actually be relieved to take it slow and not work too hard. In the long run, you’ll get better results because your body will experience less stress.