The bastard cycle machine in the physio gym. It's a sit one (so I don't need to use my arms I guess!) and I do 10 - 20 mins depending on what else I've done or got planned physio wise.
Ok, I know I'm not going very 'fast' - I do 4-6 km it says. And I do get quite warm from it.
The bastard machine has a load of readings including calories used. Today I did 20 minutes and it didn't register that I'd used a single calorie

I was dripping (inappropriate clothes and the gym shorts were still wet from washing so I was in denim knee length shorts) and it is way warmer in there than I'd choose!! LOL
But seriously?? I know you don't use many calories on a gentle cycle, but NONE in 20 minutes?? The most it's ever shown is 2
