If done correctly you should not get weak until your body gets very lean (5-7%6%) or depleted. I am no expert but i lost 88lbs for a contest back in October (291-203) over 21 weeks. I did not get weak, until I hit 240-250 range, and then I think it had more to do with eating 50 grams of carbs daily M-F while trying to work full time and doing 2 hrs cardio daily. And the weakness was more self limiting due to the fact I was feeling all the stress in the joints and I did not want to tear anything (albeit I had mutliple pec and quad strains , shoulder pain, and lateral epicondylitis.)