watchOS 4 introduces an expanded Heart Rate app that's able to track your current heart rate, your resting heart rate, average heart rate while walking, and your recovery rate after exercising, providing more information about your overall health. It's also able to send alerts if it detects a heart rate above 120 when you're not exercising.
These features are available on the new Apple Watch Series 3 models and the Apple Watch Series 2 and Series 1 models that were introduced in 2016, but some of the features are not available original Apple Watch models sold in 2015.
As Twitter users discovered after downloading watchOS 4 earlier this week, the original Apple Watch is only able to display current heart rate with a much simpler interface for the heart rate app, with no sign of resting heart rate or average walking heart rate. The app does offer the heart rate graph with a tap on the display, though.
It's not entirely clear why the original Apple Watch doesn't offer the full range of heart rate features, but it may be due to hardware limitations. The first Apple Watch offers an original S1 processor, which has since been significantly upgraded in Series 1, Series 2, and Series 3 updates, and its battery life is not as robust.
Apple discontinued the original Apple Watch in 2016 when the Series 2 was introduced, replacing it with the Series 1 Apple Watch. The Series 1 model is similar to the original Apple Watch, but features an upgraded S1P processor. The Series 2 Apple Watch uses an S2 processor, and the Series 3 Apple Watch uses an S3 processor.