ReadWriteBody is an ongoing series where ReadWrite covers networked fitness and the quantified self.

I had an epiphany the other day in the gym: I haven't been pushing myself as much as I could.

This wasn't some nebulous feeling of guilt or self-doubt. My personal trainer and I were looking at a chart that proved it to us—a chart of my heart rate.

Tell It To My Heart

Heart-rate monitoring isn't a new technology. Polar Electro, a Finnish company, pioneered the first wireless heart-rate monitor in 1977 for that nation's cross-country skiing team. But recently, the technology has spread beyond athletes to the masses, spurred by the popularity of wrist-based fitness trackers.

Indeed, it's become a baseline feature for smartwatches: Why take up wrist real estate if you're not taking advantage of that proximity to the body? (One critique of Samsung's new Galaxy Gear is that it lacks a heart-rate monitor.)

There are a number of different technological approaches to heart-rate monitoring. The EB Sync Burn tracker I've been using detects the heart's electrical impulses—but only intermittently, so it can't output a continuous graph of your heart rate. A chest strap, like one I've been testing from Pear Sports, can provide continuous monitoring, but you have to be pretty devoted to wear it. Newer monitors like smartwatches from Mio and Basis use optical detection to essentially watch your pulse—at the cost of a shorter battery life.

It's safe to assume that the technology behind heart-rate monitoring will get cheaper, less power-intensive and more available, which could mean that it's far easier to listen to your heart. And though there are some apps that can use optical techniques to measure your heart rate with a smartphone's camera, like Azumio's Instant Heart Rate, we'll likely need some kind of wearable device for continuous heart-rate monitoring.

Testing Myself At Intervals

For weeks, I've been using Pear's chest strap and companion smartphone app to measure my heart rate, with the goal of spiking it through high-intensity interval training, or HIIT. The theory behind HIIT is that elevating your heart rate through brief, intense exertion will bring you more physiological benefits than exercise that maintains your heart rate at a steady state.

What I discovered is that after years of trotting around San Francisco's Telegraph Hill with Ramona the Love Terrier, it's pretty hard to jack up my heart rate unless I'm tearing up the Filbert Steps or a similar incline. Here's a typical morning run:

My heart rate during a morning run with intervals, captured by Pear Sports. My heart rate during a morning run with intervals, captured by Pear Sports.

Even with Pear's coaching tools, I've found it hard to maintain a pace that keeps my heart rate up between intervals. (It doesn't help that I'm keeping pace with an 11-pound terrier—without my dog, I might be able to go faster, but the whole point of my morning runs is to give her some exercise, too.)

Contrast that to my experience at my gym, Fit Life Training Studio, earlier this week. It was an unusual situation: We were shooting video for an upcoming ReadWriteBody column, the results of which I hope to share with you soon. So I was in front of a camera—a high-pressure situation to begin with—and my trainer had designed a fast-paced workout so we could demonstrate lots of different exercises.

Instead of the usual three-minute rest periods I take between exercises, we used my heart-rate monitor as a guide, waiting only as long as it took to get my heart rate down from the 150s to about 110. The result was an exhausting but exhilarating workout that, according to the numbers, was far more vigorous than running straight up from San Francisco's waterfront up to Coit Tower at the top of Telegraph Hill. Here's the chart:

Here's my heart rate during a vigorous gym workout. Here's my heart rate during a vigorous gym workout.

The Prescription

So what's the takeaway? I'm not going to stop running intervals, since they're fun, and I think I can make progress by trying to keep my heart rate up more while running. But I think I can get more overall fitness benefit—both strength and cardiovascular improvement—by going all-out at the gym. While I always liked to think I was working hard with the weights, I never had rigorous proof. Until now.

How can heart-rate monitoring help you? That's a very individual question, but it's a good one to explore with your doctor, or your personal trainer if you have one. The interesting thing about the new generation of fitness trackers we're getting is that we have far more data available to inform those conversations.

Oh, and by the way, if your heart rate goes off the charts? Don't troubleshoot your gadgets. Use your smartphone and call 911.