The old "it'll make you stiff" argument is BS. Of course I'd recommend adding stretching anyway, because flexibility is good as well as strength, but it won't make you stiff.
A complete MMA conditioning routine should include circuit training, agility training, strength training, and flexibility. I'd recommend looking at Mark Hatmaker's "No Holds Barred: Conditioning" and Martin Rooney's "Training for Warriors" to get an idea of complete routines.
Powerlifting is one thing, MMA is another. You can't train A to be good at B. Training powerlifting will give you next to no special advantages in MMA, just as training MMA will give you no advantages for powerlifting.
I've read routines combining endurance sets of compound moves with 5x5 routines to build endurance and explosiveness.
However, I was told by my coaches that it makes a fighter stiff. Is it something I can supplement with some kind of stretching or should I discard it all together? Please give reasons
Thanks
