Proper Knob wrote:
It's a hard one to gauge and can really only be learned by experience. For me, i stop when my technique feels like it is beginning to break down. How do i know when that happens? I do it by 'feel', that nice undescriptive word which doesn't help you in any way. For squats, i notice i start to lean forward a bit, when that happens i stop. Deadlifting, i can feel when my lower back starts to arch the wrong way. Benching is a bit trickier, in days gone by i would be happy squirming away under the bar with my technique flying out the window. Now as soon as i think the bar speed has decreased enough i rack it, for me once the bar speed has decreased enough i start to grind the rep and grinding usually means my technique has gone. At the moment i'm aiming towards better technique with good quality reps. The grind can come later.
Think "Technical Failure". If you know your next rep is going to be messy, don't do it. "If you train to failure, you train to fail". Maybe not so true with isolation type exercises but with the big compounds, let technique be your guide. Your joints AND your strength will thank you.
As for the benefits of leaving one in the tank, I always throw the question back - What are the benefits of deadlifting with the posture of a scared cat, or squatting with the hips shooting up, weight shifting forward, and knees caving in? Reps build form. Messy reps build messy form. Messy form on the big lifts will catch up on you. I would say this is one of the potential downfalls of 5-3-1, which Wendler has even warned against. You have a tendency to turn your AMRAP set into a personal war (do or die). Your last set of squats turn into breathing squats. That's not really the aim, though. Breathing squats are good for like, 3-4 weeks at a time maybe twice a year. Not 3 weeks of every month.
Training to technical failure is also basically the same as when I have talked about fighting "form" vs fighting "weight". For a while, even years, when your form breaks down, you will still have another rep or 2 in the tank. So, when it gets to max sets, you basically fight form/technique more than the weight. You're fighting to maintain form more than anything else. Eventually, though, by doing this what happens is your technique will very rarely change, even on max sets. A good example is to just look up good powerlifters. Particularly squat and bench press videos. The only difference between the opening lift and the last (and most difficult) attempt is bar speed. Form is almost always identical. Even when you see some videos of failed lifts, technique hardly changes even right up to the point that they fail (even better if you can see them in training, and not in competition, where they tend to unleash the "do or die" mentality). At this point, they're not "leaving some in the tank". They have just spent years "cementing" good technique and in my view, this is half the reason they have been able to keep getting stronger for years.
Remember strength training is a skill.
In practical terms, there are going to be times when you over shoot or under shoot i.e. you'll do another rep and it'll be messy or you'll fail, or you'll end the set and realise you could of done another 1-2 reps. However over time you'll hone the skill and get really in tune with what your body is telling you.