Fast twitch muscle fibers are activated more by "explosive" movements.
here is a "scientific writing":
(sorry, but I'm doing a copy/paste!)
Fast twitch fibers are specialized for extremely short powerful bursts of activity. They have a very limited supply of mitochondria. Mitochondria can process glucose in a couple of ways. If it has oxygen the mitochondria will produce a very large amount of energy, but it takes a relatively long time. If there is no oxygen present the mitochondria will still process glucose, but the energy yield is very little. However, this second process is extremely fast. Fast twitch mitochondria sacrifice efficiency for speed. Fast twitch fibers also do not have the abundant amounts of myoglobin making oxygen available to the mitochondria. These type of muscle fibers fire extremely rapidly, but they don’t fire for very long. They get tired very quickly. Fast twitch muscles also have a large capacity for keeping calcium ions available. In the slow twitch muscles, the speed that they contract is limited in part by the availability of calcium ions. Because they can’t pig out on the ions all at once like the fast twitch muscles can, the slow twitch muscles can maintain contraction about five times longer than fast twitch.
------------------ Just because the majority believes it, does not make it true!
TSB might correct me on this one, but i believe that it depends on the amount of weight being used, which fibers will be activated. if you use a heavy weight for lower reps, done for example in a kind of slow 2/4 speed, you still be activating fast twitch fibers. on the other hand, if you do very light weights for very high reps (lets say for example 50) in a fast 1/1 speed, you would be activating mostly only slow twitch fibers (endurance fibers).
TSB, so if i go fast and light i will be using fast twitch fibers, just not as many, right? and when using heavy weight i will be activating most or much more of them, depending on the weight, correct?
explosively is what, 1 second up? or up to 2 sec up? and what about the lowering part, slow and controlled is better for muscle growth stimulation, right? i usually do 2/2. sometimes also slower on the eccentric part.
If you use a light weight and lift it explosively you will recruit high threshold fast twitch fibers (IIB). The thing is, you can only do this for a short period of time before fatigue sets in, so you have to do a lot of low rep, light weight, explosive sets (like, 8x2-3). This is known as "Compensatory acceleration training" (CAT) and it's been popularized quite a bit by Westside barbell. They idea is to train your nervous systewm to recruit more fast twitch fibers so when you go to lift a maximum load you can lift more (the acceleration training doesn't actually stimulate the fibers, just recruit them).
Explosively as in as fast as you possibly can. For CAT you lower quickly as well.
For general lifting, explosive up, and slow/controlled down, about a 3-1-1 tempo.
------------------ Complex problems have simple, easy to understand, wrong answers.
The human body is an amazing adaptive machine.Although there are 2 basic fiber types,fast/slow twitch,there are also a couple of types that have properties of both.For complete development we must stimulate all of our fibers and to do this we must vary the type of stimulus we impose on our muscles.Training must be consistent,progressive,and varied.We must utilize different rep schemes,rep tempos,TUT's(time under tensions),as well as varied exercises,training loads and volumes.In other words,don't train only one way,or you will limit your progress.