Most people have understand slow tempo will yield greater strength gains than a very fast tempo (dont confuse strength with power here).
Well, I now know exactly why this is so and wanted to share it - and possibly gather a few comments that might add to this information.
First a little Muscle Composition:
A muscle such as the biceps is composed of muscle bundles. These muscle bundles are made of muscle fibers - a number which is believed to remain static and unaltered throughout life and predetermined at birh. Each muscle fiber is composed of myofibrils, which are extremely thin, but as long as the muscle fiber itself (about 15 cm in the biceps).
Each Myofibril consists of sarcomeres - rectangular compartments one in front of the other.
Each sarcomere has both Actin and Myosin. The actin looks like a bridge which has been destroyed at the middle, and the myosin (which ALTERNATES with the actin) looks like a canoe in that mid section. Let me draw this for you (or try to):
Strands of - are actin; strands of *are Myosin. When you contract a muscle, your brain sends a specific electrical signal to the muscl which activates these sarcomeres. The myosin in the middle will then form a number of cross-bridges with the actin strands at both ends and push in bothe directions so that the actin strands are pushed close together. It'll end up like this:
(of course I cant draw all the cross bridges due to technical limitations).
That is all the theory there is to it.
Now, Myosin can only build a few of these cross bridges with the Actin if the movement is done fast, but itll build many more bridges if you complete the movement over the course of a few seconds - in effect increasing the load you can handle for that particular exercise (unless you've been doing cheats). A greater load equals greater micro damage caused to the muscle fibers, which equals greater strength gain/hypertrophy potential.
And this is one of the main reasons Coach Poliquin's recommendations for slow tempo repetitions in his programs works so well (f.ex., a tempo of 4020 for biceps curls).
Anyhow, I thought it'd be interesting to throw in some technical stuff in.