Apple introduced a new heart rate feature in watchOS 4 along with several other features. The most interesting among the entire feature list is the heart rate monitoring features. THe Watch is able to show resting heart rate, walking heart rate, and recovery rate. Then it will also flag abnormal spokes even while you’re not working out.
It seems that not all Apple Watch users will be able to take advantage of this feature. Pointed out by 9to5Mac‘s watchOS 4 roundup and by users on Twitter, the new heart rate monitoring features have been limited to Series 1, Series 2, and the new Series 3 smartwatches.
— jgirl (@jgirl125a) September 21, 2017
The original Apple Watch was first introduced to everyone in 2015 when it first launched. The original watch will not support some of the new heart rate features in watchOS 4. There are of course several reasons as to why the new features are not introduced in the original Apple Watch. The watches released after the original have faster chips and larger batteries than the original watch.