Heart rate, steps, distance, and time are the typical sources of information that fitness trackers use to measure how much energy you’ve expended during a workout. It’s how they determine whether you’ve closed your rings or met your activity goals. But what about the other ways you’re putting your body to work. Namely, your muscles?

Now, there’s a new data source in town that taps into just that: Whoop has announced the launch of a feature called “Strength Trainer” that actually measures the effort your lifting sessions demand on your musculoskeletal system (muscles, joints, and bones). This is different from other fitness trackers, which focus on measuring the cardiovascular system and translating that energy output into calorie burn.

Now, using the accelerometer and gyroscope in the Whoop device, and training the Whoop strength trainer algorithm on what those measurements indicate about the “muscular load” of a workout, Whoop says it can now quantify a fuller picture of the impact that lifting weights has on your whole body. It takes your musculoskeletal work, as well as your cardiovascular work, into account while calculating your “strain score,” which is how much work your body has undergone, in addition to how many calories it’s burned.

Think back: Have you ever felt totally gassed after a lifting session, but your tracker is saying you only burned something like 75 calories? Now you have some data to back up what your body is telling you.

“Until now, the wearables industry hasn’t been able to quantify muscular load effectively,” Will Ahmed, WHOOP founder and CEO, tells Well+Good. “A lot of wearables capture cardiovascular load with varying degrees of accuracy. This means that they can understand the impact of running, cycling, and other aerobic activities on the body. But this measurement discounts the impact of weightlifting and strength training.”

Other fitness trackers, like the Samsung Galaxy Watch 4, allow you to track different lifting exercises, like bench presses or weighted squats, but it all still ladders up to caloric output. Tonal also measures the exercises you do, and in a “Muscle Readiness” view, actually gives you a picture of how fatigued each muscle group is based on your workouts (green muscles signify fresh muscles, tan are in recovery, and red zones are fatigued). You can use that data to guide your workout selections, but there’s no score, per se, or ability to track fatigue over time.

Whoop is known as the most data-driven fitness tracker, and is often a preferred tracker for athletes. So it makes sense that they’re expanding the definition of what tracking our fitness actually means beyond a cardio-driven view to a fuller picture that does not necessarily have to do with steps or weight loss. And trainers are enthusiastic about the possibilities.

“I definitely think it’s super useful,”says Kevin Mejia, CPT, a founding trainer of DOGPOUND. “Being able to see how muscular load affects stress levels and post-training effects can determine workouts and recovery. It can help maybe avoid injury, overuse, or overworking muscles. It’s super fascinating what technology can bring to improve training!”