https://www.theverge.com/2017/9/21/16347244/apple-watch-os-4-heart-rate-series-original

Apple’s new Apple Watch software, WatchOS 4, includes several updates to the health and fitness tracking features of the Watch, the most interesting of which are new heart rate monitoring features. With the new software, the Watch will show resting heart rate, walking heart rate, and recovery rate, and flag abnormal spikes even when wearers aren’t working out.

But it turns out that not all Apple Watch users will experience those. As pointed out both in 9to5Mac’s WatchOS 4 writeup and by users on Twitter earlier today, the new heart rate monitoring features are limited to Series 1, Series 2, and brand-new Series 3 smartwatches. The original Apple Watch, the very first one that launched in 2015, won’t support the new heart rate features in WatchOS 4.