Last Rep Is Squeezing Out that Last Rep Really the Best Thing?

You know the rule of RM (repetition max) in weight lifting: The last rep of any exercise should be all you can do.

Don't forget this important corollary: The last rep should be all you can do with good form. Squeezing out a few extra reps after your form has fallen apart sets you up for injury and doesn't work the targeted muscles anyway.
You know you've lost your form on a bench press, for example, if you arch your back on the lift, says eFit fitness expert Joseph Quatrochi, Ph.D.
On a biceps curl, if your upper body rocks or you move your elbows away from your torso, it's time to put the weight down.