I brought up 100 rep sets yesterday, and linked back to a previous post discussing them (which in turn links to a Jim Wendler article about them as well.)
But I've got a few more tidbits to share.
What exercises? - I tend to choose technically simple exercises with a low risk from tired technique. So curls, yes. Overhead barbell press - probably not. Bodyweight Bulgarian Split Squats, yes. Box jumps - probably not. Chest supported rows? Yes. Bent-over barbell rows? Maybe not. You will get tired, and I'd rather not have people rounding their back on deadlifts #80 - 100 or smashing into the box from a too-low jump. I also prefer people get tired, but not systemically tired, so smaller exercises (isolation, low-weight compound exercises) are favored over big, complex, and mentally taxing lifts.
In all cases, though, I prefer exercises (and weights!) that don't beat up your joints. You should finish these tired, but not clutching your elbow or rubbing your aching shoulder capsule. It should challenge your muscles, not beat up your connective tissue.
Expect soreness, even if you're done moderate to higher rep exercises before!
Variations - Here are some variations I use for 100-rep sets.
Straight Through - That is, 100 reps, no rest, no stopping. If you can't get 100 in one set, you stay at the chosen weight until you do. If you can, up the weight by the smallest possible amount and try for 100 the next time the same workout comes up.
Week 1: 100 x cable rows at 5 plates on the machine. Next week, go for 6 plates.
Week 2: Got 60, 30, 10. Next week, stay at 6.
Week 3: Got 90, 10. Next week, stay at 6.
Week 4: Got 100 in a row. Next week, 7.
I use these when I want strength-endurance first and foremost. Rehab exercises take this approach - I don't want failure, and I don't want to challenge you on mobility drills. Get everything you can out of that weight before you move up. Pick a lighter weight than you think you can do for 100, especially the first time. It will add up quickly.
Total Reps for Time - Get 100 reps in the minimum possible time. Over the course of a cycle, aim for shorter and shorter times.
This is ideal when you can non-variable resistance (a fixed weight barbell or dumbbell, only a specific set of bands, etc.). You make it harder without making it heavier. Also good for your mental game - you will push harder, knowing you have a time to beat. Be careful of sloppy partial reps just to beat time. You still want quality repetitions.
Total Reps in Minimal Sets - Get 100 reps, total. Pick a weight, lift it 100 times total. Your goal is to do the reps on less and less sets, no matter how much rest it takes. Once you get 100 in a row, move up more move on.
These are a little different than "straight through" in that you don't start with a very low weight. It's fine to start with a weight that you can't possibly get 100 times in a row.
You can either do these without putting weights down, or allow it - choose one. The first is harder.
I use these want I want a combination of strength-endurance, volume (generally for hypertrophy, aka muscle gain), and just the mental ability to push through hard work. 100 reps sounds intimidating - but doing them makes them no so.
Should I throw these in?
That depends - are you actually stuck at an exercise and need a change? Do you need strength endurance because your strength gains are started to dry up? Is one of your weak points contracting muscles strongly while fatigued? Do you need a challenge that you can swap in for one of your mirror muscle lifts?
Are we talking rehab exercises?
Then yes, go for it. Keep it light, get in high-quality reps.
If not, then I'd say no. Exhaust what's working, and then move these in.
If you have to try them anyway, try a technically simple exercise like band pull-aparts, curls, or triceps pushdowns. Or a simple bodyweight exercise that you perform well - air squats are a good choice.
Isn't this too light to get strong?
It won't increase your maximal, one-rep strength, but it will mean you are stronger at the other end of the curve than people who don't do these. You will get more strength-endurance, and this will help you eke out a few more reps with weights when you do 8 rep sets, or 10s, or 15s. That will add more volume (sets x reps x weight) and will ultimately help you get stronger.
People do say 100 rep sets are too light to be useful, but rarely say they're nothing special after trying them. At least consider giving them an honest try.
Isn't the muscle gain just "water weight">
I don't think there is any useless hypertrophy. Getting your muscles more endurance and size that's centered around endurance isn't a bad thing, at all.