Hip and knee replacements may be the next health care frontier for the Apple Watch, now that it can monitor users' heart rates.

What's new: Reuters reports that 4 hospitals and a handful of other providers are working with Zimmer Biomet, a medical device company, to test an app that would transmit patients’ heart rate, steps taken and standing hours to their doctors.

• The project is focused on patients who are either waiting for or recovering from a hip or knee surgery — two of the most common procedures in the country.

Why it matters: There are plenty of concerns about just how effective Apple’s heart monitoring will be, and whether the risk of false warning signs outweighs the potential to catch legitimate irregularities.

• But if the information collected here is accurate and reliable, it will help doctors know whether their patients are following the physical therapy regimens they’re supposed to — which can help avoid complications that would land patients back in the hospital after their surgeries.