When it comes to isolation work, not all muscles respond best to the same type of stimulation. Fiber dominance matters.
The hamstrings for instance are notorious for being more fast-twitch dominant than most other muscles. And it could also have underlying neural causes associated with the type of movements each muscle contributes to. Here are some observations about isolation work:
- Hamstrings are best trained with lower reps. Even when doing direct hypertrophy work, keep the reps below 10. This is mostly true of movements like leg curls and glute-ham raises. Exercises involving hip extension can be trained for higher reps since the glutes and lower back come into play.
- Quads can be trained for higher reps. As high as 20-30 reps can be effective.
- Pecs respond well to higher reps.
- Triceps respond better to lower reps.
- Delts generally respond better to high reps, though the front portion can respond well to lower reps. This is why delt-dominant bench pressers tend to have large front delts. But when training to get that rounded-shoulders look, higher reps of isolation work is best. This program will build yours.
- The mid upper back responds well to lower reps. Keep in mind that we're talking about bodybuilding work here, so lower means below 10, not below 6.
- The traps are very strong muscles, but because of the short range of motion with most isolation trap exercises, you need higher reps (15-20) to make them grow fast.
- Biceps are best suited for intermediate reps. High reps tend to make them look flatter once the pump subsides, but very low reps aren't effective at stimulating maximum growth.
The number of sets per exercise doesn't need vary between the muscles you're targeting – we're talking about more isolated work for a muscle. Do 3 or 4 sets per exercise most of the time, regardless of the number of reps per set you're using.