I've been lifting for quite a while but almost all of that time with a strength focus. It's been years since I attempted a single set of a compound lift with less than 90 seconds of rest between work sets. But I'm starting to try to incorporate more hypertrophy-focused / bodybuilding work, and just started a new routine which calls for short rest periods between every set, pyramiding up.

On Monday I did the first workout (Chest/Back), started on bench press, supposed to do a warm-up set of 30, then 12/10/8/6, with 45 seconds of rest between every set.

I did 95x30, 135x12, 155x10, 175x8 (barely), 195x4 (would have failed attempting rep 5).

I was surprised how hard this was. I can usually get at least 12, more like 15 reps with that top weight of 195. It felt like I hadn't really gone heavy at all, and hadn't accomplished much. After this, I rested around 5 minutes (someone started talking to me), then did 225x8 and a dropset of 185x10.

Today I did essentially the same workout again, except used the rep scheme in CT's article (22 proven rep schemes: http://www.T-Nation.com/training/22-proven-rep-schemes) -- 10/8/6/20.

I tried to hit the percentages listed there, using slightly longer rests (90 seconds to 2 minutes), but I still couldn't do it. I got 165x10, 185x8, 205x4 (supposed to get 6 here) and 135x13 (supposed to get 20).

My question is, given the ability to lift heavier at the same number of reps / more total volume with more rest, why is it more effective for hypertrophy to shorten the rests and induce cumulative fatigue? I.e., if on this day I was capable of lifting 165x12, 185x10, 205x5, 225x6 (or even heavier) if I rested 2-4 minutes between sets, why is it better to use less weight with shorter rests?

Second, if I'm going to use this pyramid technique, what is an optimal rest period for hypertrophy? 45 second? 90 seconds? Longer? I mean, there will still be some cumulative fatigue even if I rest 2 or 3 minutes, right?

Thanks for any replies.