If you don’t train hard enough, you won’t fulfill your potential as an athlete. But if you train too hard, you’ll hurt yourself. So how do you find the sweet spot in between?

That’s where Ithlete comes in. This smart phone app monitors your heart rate variability to give you clues about when you’ve gone too far. For the serious athlete, that’s an invaluable service.

To grow bigger muscles, blood vessels, tendons and everything else needed for exercise, you have to stress the ones you have. That usually hurts, at least a little. With experience, you can often gauge which kind of discomfort means progress and which kind means injury.

But new sensations will always keep you guessing. And moods muddle the messages your body sends. One day use a little twinge in your knee to justify laziness. Another day you crave victory so passionately you ignore the sensation of a tearing tendon.

Heart rate variability provides an objective measure. In contrast to heart rate (the speed at which your heart is beating), heart rate variability is the change in the amount of time between heart beats. Greater variability means that you have properly recovered from a stressful or exciting activity — whether it’s running a marathon or getting fired from a job — and are ready for a new one.

Research has suggested that this type of heart rate variability measurement can predict performance and detect overtraining.

Ithlete reduces the question to the simplicity of a traffic light: Green means go hard.  Yellow means train cautiously. Red means go back to bed.

My Experience with Ithlete

I tried Ithlete for about a month. The app ($8.99) runs on smart phones with iOS and Android operating systems. You can choose between a finger sensor ($69.99) and a chest strap monitor.

I chose the chest band because I’ve read that they are more accurate in general. The drawback is that you have to get it under your shirt, and for best results, moisten the contacts. Ithlete sells the Cardiosport Blue chest strap ($49.99), but the app is compatible with some other brands, so you might be able to use one you already own.

With the chest strap you’ll also need an ECG receiver ($54.99 or as a package with the Cardiosport chest strap, $79.99). Or you can get a Bluetooth chest strap monitor that doesn’t need a receiver ($79.99).

Next you plug in the receiver into the earphones jack on your phone. Then you open the app, which immediately begins taking a reading. The app coaches you to breathe in through your nose and out through your mouth in a prescribed rhythm as it finds and then measures your heart beat. If all goes well, this process takes about a minute. My receiver sometimes had trouble connecting.

The app then produces a number with a red, yellow or green background. (The first reading or two will be gray, because the software is establishing a baseline.) The number won’t mean anything to you unless you are well versed in the science of heart rate variability, but in general lower numbers indicate you are less ready for exercise.

Keeping an Ithlete Training Log

The next part is more important than I realized at first. The app gives you scales to rate fitness factors that might affect your heart rate variability since the last reading:

• Fatigue
• How well you’ve been sleeping
• Muscle soreness
• Stress
• Mood
• How well you’ve been eating

After saving these notations, you’ll see a graph. The fitness factors appear as bars. They are superimposed on two trend lines.

The first trend line is made up of each of the heart rate variability readings. By comparing the fitness factors to this trend line you can find correlations.

What seems to affect your heart rate variability the most? Did a day of subsisting on beer and French fries destroy your rating? Were you able to stay up all night working and still get a green light to hit the gym?

For me, the most consistent correlation seemed to be with my mood. If I felt sad, the readings dropped. When I perked up, the readings rose. It wasn’t an exact fit, and it would be nice if the app could calculate statistical significance rather than leaving me to guess how exactly the bars and lines have to match for me to pay attention.

Surprisingly, I did not see a close correlation to my previous workouts. Even on those days when I felt like I’d pushed myself to the limit the day before, I often got a green light for more punishment. That made me wonder if I’m not getting as close to my limits as I feel.

The second trend line averages your current reading with all your previous readings. This gives you a sense of whether your readings are trending in a particular direction. I saw a gradual drop over the month of February. This coincided with a period when the pace of my work was picking up, so it might have registered cumulative fatigue or emotional stress.

The Bottom Line

I often had trouble getting the sensor to pick up readings from the monitor. This problem increased at the end of a month, probably because the battery in the monitor ran out of juice. (I later learned from the Ithlete website that I should have removed the electronic module after each use. The battery is replaceable, though.) If you do get Ithlete, carefully read the company’s FAQs; mine didn’t come with many directions, so I missed some useful information at first.

Ithlete isn’t the only heart rate variability monitor on the market, but it’s one of the few that has been validated in a peer reviewed journal.  In this study, researchers at Auburn University at Mongomery, Alabama, showed that its readings were consistent with electrocardiographs of the type used in hospitals.

Overall Ithlete has a lot to offer serious athletes looking to optimize their training regimens, particularly those with an eye to an upcoming competition. You can figure out exactly how much training to do, which factors to change, and when to change them.

If you’ve used heart rate variability to analyze your own performance, please let me know about them by posting a comment below.

Photo: This Ithlete screen shot shows how my mood correlated to my heart rate variability.

If you liked this article, make sure you don’t miss the next one. Subscribe to our weekly newsletter. (We won’t share your information with anyone, and you can easily unsubscribe later.)