People tend to have differential opinions on this seeing that "failure" can refer to a variety of things depended upon definition.
Some people define "failure" as not being able to successfully complete the desired volume. For example: aiming for 5 X 5 and only getting 4 X 5 and 1 X 3.
Others define "failure" as if you complete 5 X 5, you weren't able to complete another rep. 25 total reps was all you can do.
The first time of "failure" is a big no no and is your nervous system giving you a red light.
Motor nerve units are the source of muscle contraction, a single muscle nerve cell gives muscle "permission" to contract. Sensory nerve cells sending impulses towards the CNS and motor nerve cells sending impulses away from the CNS.
If you would fail to contract on the last rep, ^ that's the reason and that's why failure is harder on the nervous system. When individual fibers contract, but the external resistance to the muscle’s major action is greater than the force generated, you fail to reach a rep due to failed contraction from the impulses from the nervous system, and this is why the first type of failure is a big no no. The motor units are too "exhausted" to permit further contraction concentrically.
However, the second type of failure grants that you are pushing your body to it's limits and therefore resulting in "overload" which is fatigue of the muscle greater than it's used to. Working out with 175lbs on an exercise for 10 weeks straight does not increase the overload, which therefore isn't productive. In order to use a progressive overload type approach, you would
have to use the second factor sooner or later.
Big ramble, but hope this answers your question.
<message edited by MVP on Wednesday, July 01, 2009 7:42 PM>