When you say increase do you mean build size of, and/or strengthen?
OR
When you say increase do you mean, develop NEW fast-twitch fibers?
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From my understanding the amount of fast-twitch fibers we have.. or the % of them compared to slow-twitch is all genetically based (don't quote me on that, I don't have a study to reinforce that statement). BUT, that doesn't mean you can't train the ones you already have and develop them to their fullest potential.
From what I've read in terms of fast-twitch development.. you'll want to train with great intensity. Of which, there are many ways to do such thing!
Intensity in Bodybuilding is a very diverse statement.. We can say volume for example, and typically just a few things come to mind. High reps, High sets, Low (relative) weight. Longer workouts follow suite. However, with intensity.. we have many things come into play.
- High % of Maximal Weight per Rep Range
- Light Rest Intervals And/Or SuperSets (1 rest interval per 2 sets)
- Mini Reps within standard reps ( full negatives, half negatives, half reps, quarter reps within full reps, anything to get the maximal load distributed evenly and total destruction of muscle complete )
- Less Sets (relative to volume)
- Animal-Like Mindset ( Mindset is commonly an overlooked aspect of intensity )
- Sprint vs. Marathon. Anaerobic vs. Aerobic. It's the ability to take force and apply it to a single task or object with 100% maximum effort. 400 x 1 (most intense) vs. 225 x 10 (much less intense)
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Train with intensity for development of fast-twitch fibers. It is the more dangerous option generally speaking (not most are seasoned lifters), and if AAS is involved you will be at greater risk as you up the weight continually to hit PR's and such. But, training with maximal load, no matter what the task.. will put a novice lifter at great risk. However, if you are experienced and used to training with great deals of weight regularly.. I'd say get on it! High intensity training is cool.
If you are looking to exchange a volume based program for an HIT program.. I would recommend giving yourself extra rest time in between at first. As you develop up to your maximal load for 4, 6, or 8 reps you will be using significantly higher weight ALL the time. Your CNS will take a beating from the stress at first and could leave you immunosuppressed (at risk for sickness), blown out feeling, joint/tendon damage or stress, or something more serious if you jump too quickly with the loading of weight by week. Can't stress it enough.. maximal effort is a developed trait.. it takes a while to be capable of pushing through and truly hitting close to 100% effort for any rep range, let alone the real low-rep sets with the heavier weights.
Good luck on your endeavor!