Polarm400 metrics and questions...

Myjourney2345

I made a similar post in the weight loss community board, but I figured it might be more appropriate to post here.

I recently stated tracking my heart rate during workouts. I am an avid cyclist and take spin classes at Flywheel. I was shocked to find out that I only burn 360 calories for a 45 minute session! Even though my "total power" score and the estimated calories burned according to Flywheel is around 1000. I get that it is impossible to burn that many calories, given my height (5'4.5), weight (143.2) and overall fairly good fitness level, but is there anything I can do to increase the calorie burn? Could the Polarm400 heart rate monitor not reporting accurate metrics? It just seems so awfully low and I am not sure what else I can do.

Also, if I am able to sustain a higher heart rate for a prolonged period of time does it mean that I am in good shape? Is my goal to increase the time spent in zone 4?