I talk a lot about correct form around here and while this is important I did want to provide some additional insight concerning being too strict. I hear all the time that correct form is necessary for muscle growth but truth be told form is not neccessary to see gains.
Have you ever seen someone with terrible form who happens to be a monster? Some of this is partially genetic but mostly those huge gains come from simply lifting more weight over time. True, that individual may blow a disc or rotator cuff in the near future but bodybuilders run this risk with their training routines all the time. Bodybuilders as opposed to strength training or olympic training operate with different form in order to hit muscles in a way to achieve maximum gain. This does not always protect the muscles or joints in the way that regular strength training teaches.
Im not suggesting to compromise form in order to make gains but I am on the edge when it comes to reaching the next level. My concern for correct form falls into 90-95% of your training. Don't ever compromise form during the majority of your sets but don't be a form nazi when it comes to those last few reps.
The last few reps when you are feeling the "pump" is when you will make your gains and induce muscle hypertrophy. This will propel you to the next level in your training. The majority of trainees shy away from these last few reps because this is when you really feel the pain in your routine and form begins to deteriorate. It is this point in training that will seperate those that reach their goals and those that remain average. When you are reaching the end of your set, your muscles are burning and form is beginning to compromise; do not be afraid to squeeze out those last 2 reps. Many trainers will disagree on this point and instruct you to stop when your form is compromised. I will agree that for the majority of your sets this is the point where you stop.
There is a fine line and it has to be judged on a case by case basis. If you are squatting and another rep is severely compromising form I am not suggesting you proceed. Many of these compound lifts involving the lower back such as the squat and dead lift can be dangerous for the novice. However, if you are finishing a set of rows or chin ups or DB flys to failure (usually your first/last set) do not be terribly concerned if you are not getting 100% range of motion and locking out after each rep. The bend your arms form from not completely locking out will not slow your progress. On the contrary, these last few reps will increase strength. This in turn will allow for increased reps with reduced weight on subsequent sets because of the gains you make with those last few reps.
You will know if your form has fallen apart or if you are just holding it together. Do not be He-man and attempt reps when you have clearly lost all control but don't be She-ra either, make the call. I have taken heat for such views from others in the field but in order to progress you need to be able to show gains and this sometimes must be done by sacrificing some form. Notice I said some form. Do not make poor form a staple in your routine but understand that the final reps to failure will require some leeway in the perfect form. Be safe and take your workout All The Way.