One of the Apple Watch Series 4’s biggest additions was support for electrocardiogram (ECG) measurements, a feature that officially became available to U.S. users this month. Now third-party developer RocketBody claims it will be the first to use that ECG data in an Apple Watch workout app, though there’s a key practical caveat.

RocketBody pitches itself as an AI-powered solution to help determine when a body is at the perfect time to work out, a state of metabolism the company calls “supercompensation.” Initially, RocketBody launched Kickstarter and Indiegogo campaigns for a $200 ECG-measuring wearable and AI personal trainer app, but with the release of the Apple Watch Series 4, the company believes it can use Apple’s ECG data for the same purpose.

The company tells VentureBeat that it will use four Apple Watch ECG readings per day to let its AI system determine the current state of a user’s metabolism: one reading each before sleep, after sleep, before training, and after training. Each reading provides two points on the metabolism curve, which RocketBody says can be analyzed to indirectly determine a heart’s aerobic and anaerobic processes.

But since Apple doesn’t offer an open API to get ECG data — and each ECG is only collected when a user presses a fingertip on the Series 4’s Digital Crown — users will need to stop and take each day’s four readings manually, then forward each one to the company’s mobile app. Rocketbody says its app will take the same ECG reports that would normally go to your doctor via the “Send your ECG to your doctor” forwarding route, and instead give users the option to “Send the ECG to your AI trainer.”

The company says the app will carry a weekly price tag, as well: $4.99. That’s a pretty steep cost to track one’s metabolism, particularly given the inconvenience of multiple manual readings every day.

Granted, that’s not all the app does. In addition to promising AI-powered data analysis, RocketBody claims that its software will offer real-time workout coaching, personalized diet and workout plans, and measurements of the user’s sleep and stress. If you’re interested in checking it out for yourself, RocketBody’s page on Product Hunt will go live on December 18.