In my own humble opinion I think you are going to want to not do work sets more than once a week for a particular body part. This is not a rule, but a good guideline. Now some people love to go 2 times a week, or even three times a week, and it might work for them, but if you are lifting to failure, you shouldn't go more than once a week. The westside powerlifting philosophy goes twice a week for each body part, but they don't lift to failure on the suplementry muscles, and they only do max effort once a week (correct me if I'm wrong here, it's been a while since I've read their stuff). If you are going to failure, you need alot of time to recover. I've drawn a very crude picture to help illustrate this, hopefully it won't be too confusing.
When you lift, your muscles will be broken down, your strength will be deminished. During this time, you will feel significant muscle soreness known as delayed onset muscle soreness (DOMS for short). After you are done being sore, the muscles have still not recovered yet, but are still in the process of active recovery. Several days later, they will have recovered. Given more time (though not enough to allow atrophy) your muscles will make progress in their strength.
Many beginning lifters begin lifting and they do the same body part 2 or 3 times a week to failure, and seem to make progress. Their muscles are attempting to adapt to new stress and the first thing their muscles will do is increase glycogen stores (more energy available) and also increase lactic acid removal efficiency. This is often confused with progress, so they continue on using the same rutine. After 6 months they find they can no longer make any progress and don't know why. The reason is that they aren't allowin enough rest for their bodies to recover.
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