The 5×5 system is one of the oldest muscle building programs in the book and has stuck around forever. 5×5 is good and 5×5 is bad; it just depends on the situation. There are a few ways that the 5×5 muscle building system is implemented.
The first way that people use the 5×5 system to build muscle is that they warm up to a weight and then stick with that weight for five sets of five. So you need to choose a weight that you can handle for that many sets. In essence, the fifth set will be the only set that is quite difficult. This is a decent method to build muscle but largely a waste of time for anyone but a beginner. There is too much volume at an intensity that is too low building muscle. For beginners and early intermediates, I think the 5×5 system is a good one. When you are neurologically inefficient, you seem to respond better to a few more repeated efforts. I still don't know if five sets are really necessary though; 3×5 is probably better in most cases.
The next way people employ the 5×5 system in an attempt to build muscle is by starting with a heavy weight that they can barely get five reps with and then lowering the weight with each significant set. Again, I think this is too much. The only set that was worth doing was the first and maybe the second. This is not the optimal way to build muscle.
The third way I have seen this system employed to build muscle is to work up to a heavy weight that you can barely get five with and then keeping that weight for the next four sets, no matter how many reps you get. Progression is made each week by trying to get more reps in the subsequent sets. This is a favorite of one particular egomaniacal strength coach and like everything else he recommends, is total crap.
The last way that the 5×5 system is used to build muscle is by doing five increasingly heavier sets so that only your last set is tough. Basically the other sets are warm ups so you are really just warming up to a five rep max or very close to it. If this is the case, it's really 1×5 and not 5×5. It's a bit of a misnomer. But if I had to pick one, this would be the best method.
Working up to a five rep max and then a down set at 90% of your best is an even better option for building muscle. Or you could do a very heavy set of five but leave a little something in the tank and then go for broke on the second set. There is some evidence which shows that this second option may even be the better choice simply because the body may not function optimally during the first heavy set. This is due to the laws of homeostasis and various things of this nature. The first set causes shock and certain protective mechanisms may set it which prevents the first heavy set from actually being the best set. The job of the first heavy set may be to provide neural arousal and prepare the body to go all out on the second set. This is something that people have to play with on their own and see what approach helps them build muscle fastest.
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