I have a treadmill, which I use for 20 minutes a time, about 3 or 4 times per week. I usually manage quite well, coming off tired and hot, but not particularly out of breath.
The other day, I realised that I was holding on to the handlebars for stability all the way through, and wondered how I would get on without. To my surprise, I found myself out of breath after only a minute or so, and found it quite a struggle to keep this up until the end of the session. Afterwards, my heart was beating far faster than it ever did when holding on, and it took a while before my breath was back to normal.
Can anyone explain to me why this was so? As I said, I was only using the handlebars for stability, I wasn't putting any weight on them.