Very interesting machine, but the tension?

I find very interesting machine because it makes work a lot. Moreover it is cardio ... Levels 1 and 2 being a bit "weak", I use it mainly for Level 3 now with 3 or 4 sets of 15 or 20 moves. I try to stick to it every day even though it's not easy and I think it could bear fruit if we still something else next (jogging, cycling ...). I not against a little afraid for my back because I did not find anything relevant about it. Thank you to contact me if you have any info on this level. When there is very little time, it's really the ideal, I also recommend to others. Negative point: I have not received tensioners with and knowledge received with the purchase of the same product ...