We have been tracking an issue with data from the wrist-based heart rate sensor and have an upcoming fix. I am not sure when that will be able to be released.
If this data is coming from a chest strap, however, then it would be something different. I recommend reaching out to Polar for troubleshooting tips, but feel free to view ours as well: My Heart Rate Monitor Strap is Reporting Erratic Data
Make sure the strap is clean, and try warming up before your next few activities!
