We may earn a commission from links on this page. If you've been eyeing the Garmin Forerunner 970 , you may be drawn to two of its buzz-worthy features: running economy and step speed loss. But there's a catch: You can't access either metric without purchasing a compatible Garmin accessory like the HRM-600 heart rate monitor , which will run you $170. Is this extra training insight worth the additional investment? To find out, I've been wearing one on my runs for the past several weeks, including a half-marathon race in early May. Here's what I've learned about myself, and what you need to know before buying an accessory to get the most out of your running data. What is "running economy," and how does Garmin calculate it? Running economy technically refers to how much oxygen you consume at a given pace. But when runners talk about it, we’re referring to a basic principle that says the more efficient you are, the less energy you'll burn to run at the same speed.…