The app that will change the way you work out forever

Equinox instructor Gregg Cook tests the latest gadget to hit the fitness market for Details.com.

Recovery is like the broccoli of the fitness world—we know it's good for us, but many of us continue to avoid it. Yet it's the key to getting fitter. The BioForce HRV, a new app-heart-rate-monitor combo, is made for those who constantly wrestle with their exercise conscience (a workout in itself).

By measuring the minute variations between your heartbeats, which change daily (based on a slew of things, from sleep to diet to stress to physical exertion), the app predicts whether you should be taking it easy so your body can recover or hitting the gym hard.

How it works:
Each morning you hook up your heart-rate monitor to the device and lie still for a few minutes so it can get a read on your HRV, then the info is sent to the app for analysis. The readout tells you whether you're good to go (green), slightly fatigued (orange), or in full-on recovery mode (red).

What the pros say:
Gregg Cook, a top group-fitness instructor at Equinox in New York City, tested the gadget: "What surprised me about days when my readout was green was that sometimes I didn't feel like I was ready for a super-intense session. One day I really wasn't feeling it, but the device was green, so I did some heavy dead lifts and I had one of the best lifting sessions I'd had in a long time. If I hadn't used the BioForce, I probably wouldn't have worked out at all that day. It gives you the psychological confidence to go into the gym and try and take it to the next level."

$10 for the app, $279 for the heart-rate monitor; available at bioforcehrv.com.