I’ve become an increasingly serious exerciser over the past five years, ramping up from the occasional elliptical workout to a regular running and swimming habit before finally adding a weekly weight lifting session. The progression has been slow, but I’ve seen and felt my body change over the course of that time: I’m leaner and more muscled than before, my resting heart rate is lower, and I can run faster, swim farther, and lift more than I could when I started. Exercising also helps me sleep better and improves my mood when I’m cranky or stressed out.

Until recently, I was working out 4-5 times a week, typically on a two-days-on one-day-off rotation. Like most people, I would push myself to the point of slight discomfort during my endurance workouts, figuring that if I pushed my limits a little more each time, I would slowly get stronger and faster. This worked pretty well for a while, but eventually I felt like my progress had slowed and then plateaued.

Then, a couple of months ago, I decided to take the plunge and train for a triathlon. Anytime I tackle a new project I try to read at least a couple of books on the subject, so I found two titles that seemed promising and ordered them. The first book “Your First Triathlon” by Joe Friel, contained some solid advice about race transitions, workouts, and nutrition.

The second book turned everything I thought I knew about endurance sports on its head.

The funny thing is, it wasn’t even a book about triathlon training. 80/20 Running by Matt Fitzgerald was written for runners, but the principles he outlines can be applied to all endurance sports.

The basic idea is this: if you’re trying to improve your performance in any endurance sport, you will do so most effectively if you complete 80% of your workouts at a low (or “aerobic”) intensity and only 20% of your workouts at moderate or high intensity. According to Fitzgerald, the vast majority of elite endurance athletes in the world utilize this principle, from runners to skiers to cyclists.

Compare that with my previous approach: I was spending almost all of my time at moderate intensity, with some brief stints of interval work at high intensity. According to Fitzgerald, the kind of exercising I was doing wears your body down and actually makes you more injury prone. He goes on to argue that it’s not particularly effective for improving your race time, either.

(Crossfit fanatics, don’t fear: Fitzgerald makes it very clear that this style of training is only appropriate for endurance sports, and that those interested in other sports such as weightlifting, wrestling, track and field events, or sprint distance running are better off with high intensity interval training (HiiT) methods.)

How do you know if you’re exercising at low intensity?  Fitzgerald goes on to outline several specific tests to determine your target heart rate for low intensity exercise, but basically, you should be exercising at a level that feels “natural”- like you’re not pushing yourself. You should feel like you could run, swim, or bike for a long time at your “low intensity” pace.

As it turns out, this is more difficult to do than it sounds. I started implementing Fitzgerald’s 80/20 principle before I had even finished the book, but I soon found myself waging a psychological battle with myself, worrying that I wasn’t working hard enough, allowing my pace (and then my heart rate) to creep upwards, and then bringing it back down again. I’m a competitive person by nature, and this internal battle was most difficult when I was running or swimming next to someone who I knew I could “beat” if I just allowed myself to swim or run at my usual pace.

After finishing the book and spending several weeks incorporating Fitzgerald’s 80/20 principle into my training, I’ve collected a few tips to share with you:

Tip #1: You can work out pretty much every day.

When I was exercising mostly at higher intensities, I *needed* that third day off. Now that I’ve shifted to training mostly at lower intensities, working out six days a week feels easy. (I’ve been taking one day a week off as a “rest day.”)  I’ve discovered that I have enough energy to train longer during each session, too, which means more time on the bike, in the pool, and on the treadmill.

Tip #2: To stay at “low intensity,” determine your ventilatory threshold and your lactate threshold

The lactate threshold is the heart rate at which lactate starts to build up in your blood, and you can determine yours by figuring out the fastest running pace and heart rate at which you’re able to talk comfortably. (Note that your lactate threshold heart rate will be different from someone else’s lactate threshold, and it will also be different for impact and non-impact sports.) For example, if you can talk comfortably while running a nine minute mile (but no faster) and your heart rate is 170 beats per minute at that pace, then 170 beats per minute is your lactate threshold. Your ventilatory threshold is less than 90% of your lactate threshold; if your lactate threshold is 170 beats per minute, your ventilatory threshold would be less than 153 beats per minute.

Fitzgerald defines “low intensity” exercise as exercise completed below the ventilatory threshold, so if your ventilatory threshold is less than 153 beats per minute, you should be maintaining a heart rate lower than 153 beats per minute during 80% of your training.

Check out the book for a more thorough explanation and more specific instructions for determining your personal heart rate zones.

Tip #3: Wear a heart rate monitor to fine-tune your workouts

After calculating your personal heart rate zones, continue to monitor your heart rate during workouts by wearing a heart rate monitor to make sure you’re not accidentally pushing yourself harder than you should be.

The Polar H7 seems to be the standard for wearable heart rate monitors, and it only costs about $50. You strap it around your chest and then it connects to an app on your mobile device via bluetooth, so you can monitor your heart rate throughout your workout. A fitbit would work just fine as well, though they’re a bit more expensive and a bit less precise. (If you go the fitbit route, remember that you need to wear it about an inch and a half higher on your wrist than a watch for the most accurate heart rate results.)

It’s possible to figure out the pace that keeps you below the ventilatory threshold and just stick to that, but that only really works if you’re on a treadmill or a stationary bike. If you’re running on a trail and you come to a hill, for example, you’re in trouble. (Plus, your heart rate can vary based on factors like fatigue and stress.)  Furthermore, a heart rate monitor will help you keep track of how hard you’re working when it’s time to push into the moderate and high intensity zones during that remaining 20% of workout time.

Tip #4: Vary the intensity of your workouts day by day and week by week

In order to prevent overtraining and burnout, Fitzgerald recommends completing a hard workout one day followed by an easier workout the next. You can make workouts harder by pushing into high intensity zones, but also by staying at low intensity and extending the length of your workout. Consider scheduling your most difficult workout the day before a rest day. Also, make sure to build in “rest weeks” every third or fourth week- weeks where you decrease the overall intensity of your training schedule, in order to give your body time to rest and recover.

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