This graph show the breakdown of my HR zones. So although it was a high intensity ride 50% of my time was in zone 4. The Training effect score is also interesting in that its a 4, this means that "this activity was very demanding" The app also states that I exercised for an hour and burnt 921 calories.
Now lets compare this with the spin class I did on the 7th March. Also took an hour and I burnt 931 calories.
We can see that eleven days previously my average (147 vs. 141) and maximum (183 vs 182) heart rate were lower yet the training effect score was higher, so in theory I was working harder.