I started wondering, are there any other muscle groups that are littered with slow twitch muscle fibers? Littered with 50/50 between slow and fast twitch? Littered with fast twitch? I know next to nothing about this topic, which is why I brought it to the brightest lifting minds on the web (which is like being the fastest quadriplegic, but I digress) to help me ensure that my reps are being used to their full potential.
Let's try to keep it as close to 3 groups as possible: Slow Twitch 50/50 Fast Twitch
I don't think knowing this would help you reach your goals better.
Muscle fibers don't change types, but they can behave a lot like another type if your training makes that change necessary. So (to a certain extent) you should train like you want your muscles to be.
All muscles respond pretty well to varied rep ranges. And the middle ground 5-12 reps often works well for size and strength. Use lower reps sometimes to get used to moving heavier weights, or higher reps to give your tendons a rest and build some strength-endurance and mental toughness.
Good rep ranges are often more exercise speficic than muscle specific. Deadlifts can be hard to use safe form on if you do higher reps, (olympic lifts as well). DB presses without a spotter are so hard to get into position for that you often end up doing higher reps. Kroc rows just work really well with high reps, but can be done for low reps as well etc.
Postural muscles respond well to isometrics and higher volume. These would be back and abs.
Everything else, blast it with maximum force production and heavy weight.
When you get more time under the bar, you might find that you happen to respond better to high or low volume...but until that time comes, just keep lifting heavy, eating heavy, sleeping heavy and adding more weight to everything.
I don't think there is really ever a time for slow reps. Squeezing and holding the contraction, sure. Isometrics, sure. Even slow burnout negatives, sure maybe occasionally/sometimes (even though I am not a fan of them at all). But slow rep speed--meaning lifting speed, no. I am firmly an advocate of thib's training philosophy in this subject. Only exception I can think of is brachialis vs. biceps braachi recruitment.
I really do think there is a lot of value in understanding slow vs. fast twitch composition, but I would think in terms of rep ranges rather than rep speed. Also there is a middle ground--like kakno mentioned there is a large chunk of the time where you really just want to train the muscles to perform your goal, even if that goes against "fiber makeup" because the fibers can take on behavioral qualities depending on your training style. I think the areas where slow/fast twitch changes are most important are pure hypertrophy and postural (think of endurance of stabilizer groups, or postural muscles to fix bad alignments).
If your goal is pure strength, or power, then it makes little sense to focus on higher rep ranges (meaning above 12) except just as a way to get volume in.
I generally lift explosively and it's worked well for me so far for most of my muscles. I give credit where credit is due, and that credit goes to C-Thibs. I haven't made much gain from this method in my "back-al region" though, so I'm beginning to experiment with squeezes at the peak contraction. I've only been doing this for a little under a month though so my gains haven't been mind-blowing or anything, but I've noticed a better "burn" back there. Just wondering if there are any other lifts I should focus more on the squeeze.
I realize that all muscles can benefit from squeezing the peak contraction, but I'm curious if any muscles benefit more than others.
Every single person is different, and muscle fibre composition is determined by genetics although the average is about 50/50. Also, every muscle in the body contains both slow twitch (type I) and fast twitch (types IIa and IIx), however after saying this, some muscles generally contain more than the other. Postural muscles like the back and soleus will be predominately type I fibres as their role is to keep the body upright all day and having them fatigue quickly wouldnt be beneficial to humans at all.
When speaking about other muscles, how you train plays a significant role in how they perform. No one can change their muscle fibre composition through training so by sprinting and lifting explosively you do not convert slow twitch to fast twitch and vice versa; however by sprinting and lifting explosively you will hypertrophy your type II fibres and change the relative area of muscle fibres in the muscle while making your type II fibres more efficient at generating force.
So, long story short muscle fibre composition genetically predetermines what you might best excel at, but that doesnt mean someone who has predominately slow twitch can't train to get faster and someone who is predominately fast twitch can't train to have more endurance it just means you might naturally excel at one over the other. Like I said at the beginning though most people are about 50/50 which means you can really train your body however you want it to perform.