[zidan]

While failure is a crude gauge, it can be used as an effective tool in finding your baseline of intensity. For example, if you are performing three sets of fifteen reps with 200 pounds and during your first set you perform fifteen, your second set you perform fifteen but on your third set (to failure) you perform twenty six reps it is an indication that your first two sets were at sub maximal intensity and that you should be using a heavier weight or performing more reps. The limitation here should be obvious. How much lower was your intensity compared to what it could be? What are the units of measurement? How high is high? How low is low? When you use Power Factor and Power Index numbers in place of these vagaries your training is imbued with a precision behooving a proper science. Furthermore, if you are seriously overtrained you will reach failure at a point lower than the progressive intensity that you require in order to stimulate new muscle growth. For example, suppose that you started with a baseline intensity of one hundred pounds per minute and a few sessions later your intensity is 160 pounds per minute in the same exercise. At this point you become overtrained and next time in the gym you go to "failure" at 140 pounds per minute of muscular intensity. There is no possible way that can stimulate new muscle growth despite going to failure. Why? Because going to failure is not a requisite of muscle growth stimulation. Progressive intensity is!
To exaggerate the point, consider what your strength is when you are recovering from a serious bout of flu or perhaps a stay in the hospital. You can return to the gym and take every set to failure but the intensity will be so low that it cannot stimulate new muscle growth. That's the reason why some people can train to failure on every exercise for month after month and never show any sign of progress while they are convinced they are going all out and delivering 100% of momentary muscular effort. It's irrelevant, since what matters is a tangible progression of overload intensity. Since the Power Factor and Index measurements exist there is no excuse for a rational person not to employ these more precise measurements of muscular intensity into his or her workouts. No reason except blinkered dogmatism and a blind adherence to tradition and "the way its always been done".

Advocates of training to failure, particularly those who adhere to the Arthur Jones model of only one set to failure, believe that the last rep is the most productive rep of the set. As the rational goes, the first reps takes very little of your effort, the second, third and forth reps take corresponding more effort until you reach that last rep which requires all the effort you can muster and yet can not be completed. This most difficult rep is considered by some to be the most productive rep in the set as it is the one that triggers muscle growth stimulation. However, as we have already discussed, it is the progressive increase in intensity that triggers muscle growth and since that increase can be reached without ever going to failure, the last rep (as it's described as being impossible to complete) can be entirely unnecessary. For example, if the required increase in intensity would have been reached at the sixth rep of that set then the sixth rep is the one that triggers growth. The reps beyond the sixth were not even necessary to perform. If you have a baseline intensity of one hundred pounds per minute and you have set a goal intensity of 110 pounds per minute then the moment you reach 110 pounds per minute of intensity you can stop the exercise even if you've only completed five and a half reps. The number of reps is irrelevant, failure is irrelevant, it is the amount of intensity generated that is the only relevant factor.
And how does a scientific mind measure intensity? With a mathematical Power Factor and Power Index. How does the unscientific mind measure intensity? By feel, by perceived effort, by burn, by pump, by soreness, by failure, by rep count, by set count, by vague, non-specific, irrelevant intangibles that are not indispensable conditions of growth stimulation.
In science we measure the intensity of light, not by squint factor, or headache potential but by Lumens and Candle Power. We measure the intensity of sound not by ear pain or stomach vibration but by Decibels - precisely defined measurements that can be compared mathematically and used to discover other properties of the science. Anyone who claims there is a "science" of bodybuilding but rejects an objective measure of muscular intensity is a poseur and a dogmatist.