Weight loss with a bad knee and back

[jhonydeep3305]

I am a 46 yr old 6ft2 and weigh about 350lbs/160kg/25stones.

I need some help to step up my training in the fat burning zone - level 2. I use a Polar FT80

I had surgery on my knee last year to repair damaged cartilage. I can use the cross trainer for 1hr or 2hrs on level 3 but if I try and increase the resistance my knee becomes very weak and painful and gives way afterwards. Any suggestions on what I can do to increase training in the fat burning zone without affecting my knee?