Evidently I was so taken with that hilarious photo of Loserboy in the post below that I omitted a few important points about training to failure. So, here they are:
1. It's a higher-risk technique, so don't do it unless you have a reason. All lifting stresses the body. That's why it works: it gives your body a reason to get stronger. No stress means no adaptation. But lifting to failure is particularly stressful and involves a greater risk of injury. So don't do it unless you feel you're at a point in your program where the potential benefits outweigh the risks. If you're continuing to make progress without going to failure, stick with what you're doing.
And be aware that in some instances the potential risks are always too great to justify training to failure. If, for instance, you suffer from hypertension, lifting to failure is not for you. Likewise if you are a teen do not lift to failure unless your doctor has confirmed that you are done growing.
2. It's not appropriate for novice lifters. There are a couple of reasons for this. First, if you're just beginning a resistance training program your body isn't ready to withstand the stress of heavy loads. As a general rule you're best off keeping your weights light enough that you can complete anywhere from 12 to 20 repetitions with good form. In this phase of training your goal is not so much to gain size and strength as to build muscle endurance and strengthen the connective tissues within the joints, and the higher rep range is what works best to meet those objectives.
Second, if you're new to weight training, you probably don't have all that good a sense of what the lifts are supposed to feel like, and as a consequence you may not be able to tell when things are starting to go wrong. If you don't know to jettison the weights or ask for assistance from your spotter before you lose control, you may end up injuring yourself severely.
3. The risk is not just to yourself, so use some common sense. If you think there's a chance you may have to drop your weights, be sure there's no one in your immediate vicinity on whom said weights might fall. (I know, I know, it's tempting to let them land on the cardio bunny in the Bebe sweats who thinks the area in front of the squat rack is a good place to do Pilates exercises ... but please don't, unless she is wearing a lot of perfume. )
(I am, of course, only kidding. Sort of.)
4. Don't feel like you need to train to failure every workout. By all means try to break new ground every workout, but use a variety of techniques to challenge yourself.
5. Be aware that if you train to failure, you'll need more recovery time between workouts. I can't say it enough: working out doesn't make you stronger, recovery makes you stronger. So if you lift to failure make sure you take all the time you need for your body to repair itself, or you'll lose the benefit of the workout. For that reason, if you're the sort of person who has trouble taking rest days, training to failure might not be productive for you and you should do something else. There's no sense in doing something high-risk just for the sake of doing it, know what I mean?
6. If you're having a "weak day" don't lift to failure even if it's on your training schedule. There are days to push hard, and there are days for playing it safe. Know what day you're in before you attempt to break new ground. If you're getting over a cold, or you slept funny and have a weird twinge in your back, or you're female and getting ready to menstruate, don't push the envelope! Save it for when you're feeling 100 percent.
Sometimes we know even before we set foot in the weight room that we're not having a good strength day, More often, however, it's a subtle thing that you may not pick up on until you start in with your preparatory joint mobility work and your warm-up sets. I like to start most client sessions with a short set of unloaded overhead squats, because this fantastic total-body exercise allows me to see at a glance whether the client has muscle imbalances that may prevent her from performing optimally during the workout. If we can't get the imbalances fixed with a little foam rolling and stretching, I'll make it a less intense session with more of a focus on corrective training even if it means deviating from plan.
It really is okay to do that. The bottom line is: what you do for your workout on any given day doesn't really matter; what counts is what you do over weeks, months and years. So be smart, and save the ultra-intense stuff for the days when you're mentally and physically up for it.
7. If kettlebells are your chosen training modality, don't lift to failure. The risks are just too high. Do as much as you can with good form, then set the kettlebell down or drop it if you must. Don't teach your body wrong habits by trying to force reps with poor form at the end of a workout when you are fatigued. Check your ego at the door and do only what you can, even if it turns out to be a lot less than you think.
In all honesty this is a real struggle for me, but I'm working on it.