Occam's Protocol - cadence vs. failure
I find that with some exercises, particularly leg press. I get to a point where I can't do more reps at the 5/5 cadence, but I could do more reps if I went faster. Is the 5/5 cadence just to get you close to failure as quick as possible, and then it's get the weight up any way you can once you can't go slow?
The Best Answer
The cadence is to keep your form good. Good form means you are going to get better results for your workout, and you will be less likely to injure yourself... and if you injure yourself, that time spent recovering is time that the rest of your muscles will stop growing at best, and atrophy at worst. Do that last rep as slowly as possible, like 5 seconds up, 12 seconds down. That's what I do, and by the end, I can't even do one more rep for a while.
It is important to keep the slow cadence since it removes all momentum that can be created in lifting, and since muscles do not have uniform strength over the full range of motion of a lift, it forces the muscle to remain under constant tension. This slow lifting protocol is put in place to exhaust the 4-5 different types of muscular fibers (slow twitch, and the other various high twitch forms while lifting), by lifting at this slow place. In fact many HIT (High intensity training) advocates have a 10/10 cadence even. Furthermore, lifting to complete exhaustion forces you to incorporate and exhaust the 4-5 different muscular types (with slow twitch being exhausted first and fast twitch being exhausted later in the set), this full exhaustion produces the largest muscular gains since most weight lifters only exhaust 1-2 types not all 5 (which incorporates the entire muscle).. Lastly it is very important to keep the muscle under constant tension (meaning don't lock out your joints during dips, bench, leg press or squat), slow twitch muscles have very quick recuperative response and this pause can ruin the exhaustion process.
I'm no expert but in my opinion, the 5/5 is part of your form - therefore, you need to fail with good form. If you can more reps fast it's because you're employing fast twitch fibres (I think!) and also using momentum to get the weight pushed out.
Remember, when failing with the 5/5 you have to really, I mean REALLY push / pull / lift whatever to move the weight until you really, really can't. Then don't just let it go, return the weight slowly using 5 seconds or so to return it.
You should be fine with that. If you're wiped out at the end of a work out (or can't lift your arms / stand properly on your legs etc) then you've done it right!
Also, keep monitoring your progress week on week in terms of reps / weight - so long as it's increasing each time then you're doing the right thing so just keep going.
Hope that helps
I think it is also about exerting / fatiguing your muscle as fast and efficiently as possibly to get to failure. The point is to trigger your body's response to build more muscle in the area that failed. You could do 8-12 reps for 3 sets, or you could do one set of 7 slowwwww reps which hurt like hell. Either way will probably trigger that muscle building response in your body. Tim says you might as well not waste time in the gym that is unnecessary.