Out of the box, the new Apple Watch doesn't look like it just underwent a massive redesign. It maintains the same rectangular shape and inoffensive design as its predecessors. It even accepts the same watch bands. But when you turn it on and tap the new, larger screen, it's hard to shake the feeling that the Apple Watch has grown up.

That's because it has. The Series 4 has a new, curved-corner display with 30 percent more screen than earlier models. There's a crown that provides haptic feedback as you scroll, a louder speaker, a faster processor, and a new wireless chip—all packed into a slimmer body. The new model also comes with an updated optical heart rate sensor and the ability to take ECGs. You get all this starting at $399 for a model with GPS, and $499 for a cellular model, in either a 40mm or 44mm case.

If you hit the ground and remain inactive for a minute, Siri will now call 911 for you.

It's the combination of hardware and software health features that makes this watch special, as senior writer Lauren Goode wrote in her initial hands-on with the Watch. It's the first Apple Watch to offer fall detection and heart screening, thanks to the new sensors. Apple updated the accelerometer and gyroscope and paired those with real-world research into how people slip, fall, and crash. If you hit the ground and remain inactive for a minute, Siri will now call 911 for you.

Apple advised me against trying to trick the watch into thinking I'd fallen, but I couldn't resist. I tried to trigger a false warning by tripping onto a yoga mat, jumping on the bed, and flailing around while attempting to powerlift. No dice.

The watch's heart screening features weren't available for testing, since they won't release until later this year. When they do, Apple says they'll improve health monitoring. The Series 4 ships with an optical heart rate sensor that keeps track of your heart rhythm throughout the day. It also has electrodes in the crown and back crystal that let you run an ECG to record the heart's electrical activity. If the watch detects an irregular rhythm that appears to be an atrial fibrillation, it will confirm the reading four more times and then send you an alert. You can then immediately take an ECG and give that reading to a doctor. While the ECG app is unique to the Series 4, the irregular rhythm warnings will be available later this year on the Series 1 and onward. Both screenings are cleared by the FDA for people 22 and older.

Both features have come under scrutiny. Senior writer Robbie Gonzalez argued that the new ECG feature could do more harm than good by leading to unnecessary testing, overdiagnosis, and treatment for subtypes of A-Fib that are best left unmedicated. The potential may be particularly real for smartwatch wearers, who tend to be young and active. That population is unlikely to experience A-Fib and unlikely to have occurrences of A-Fib that require treatment, says Eric Topol, a cardiologist and director of the Scripps Translational Science Institute. However, with features like fall detection and larger screens, Apple may also be targeting an older audience. And while future research is required on the subject, this type of device may prove useful to people with high-risk factors for A-Fib, including age, obesity, high-blood pressure, or certain genetic markers, according to Topol.

Watch This

Besides health tracking, the watch's new optical heart rate sensor should make a difference in capturing exercise data, which I tested while hiking, running, and lifting. In all instances, it performed comparably to the Series 3—good for most endurance training efforts. Optical heart rate sensors can get tripped up by cold weather and jostling, which I tested by hiking at elevation. The Series 4 captured the workout without issue.