drop sets
different people do different things that work for them...
if you are working to failure, do just that(work to failure), so that sometimes you may end up doing more sometimes less, it can all depend upon how much rest you are getting between sets, and how heavy your wieght is in relation to your strength(amongst other factors).
Sometimes(this works best on machines if you want very little rest), I do my last rep I can push out on the weight I have set drop the weight slightly and immediatly push out as many on that weight as possible, then drop and do the same again, and again, and again, etc., until I am left straining my a$$ on like a 1kg weight!!! thing is when you do that in the gym, you usually find that the girl everyone has the hots for in the gym is walking in and watchin you strain your a$$ over a weight she wouldn't even want to be warming up with.[not that i am talking from experience on that matter )
seriously though, if you are doing that be careful as you are potentially straining more muscle fibres by training that way, it just depends on your goals I guess...
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