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.
The biceps and triceps should be trained using rep ranges between 5-20 reps to best maximize strength, muscle growth, and overall development of the muscles.
Use Higher Reps
The rule of thumb for gaining size is to use a rep range of 8-12. With legs, however, I always found – and research backs me up – that higher reps produce the biggest gains. I experienced great results with squats, lunges, leg presses and leg extensions when doing sets of 15-20 reps.
Muscle Growth
As explained by Charles Poliquin, your quadriceps (or quads) are mostly made up of type 2a muscle fibers, which are more aerobic than type 2b fibers so they respond better to higher reps. The time you spend doing a set of high rep squats can quickly pack on lower body muscle.
If not overused, high reps are especially effective when training forearms, as well as calves, because these areas are built to respond to endurance work.
If you want to build muscle in the shoulders and triceps, you should do high rep overhead presses. If you do choose to do high reps in the overhead press, you should push the sets quite hard and train close towards failure.
More repetitions with lighter weights can build muscle as well as heavier weights -- assuming they are done to the point of exercise-induced fatigue. And fatigue is the important point. That means even with light weight, the last two to three reps should be hard.
Keep the reps low and the sets high. The hamstrings are predominantly fast twitch, meaning there is little point in training them with high repetitions. You need to train with a heavy resistance otherwise you will not create sufficient tension to evoke a hypertrophy or strength response.
Lastly, due to the slow twitch nature of the calves, performing higher reps is likely necessary to force growth in these muscles.
There is a delicate balance of power between the quadriceps (quads) and the hamstrings. It's natural for the quads on the front of the leg to be stronger than the hamstrings. The strength of the hamstrings should be between 50 to 80 percent of the quad strength, with 70 being the optimum goal.
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.
It turns out that 8-12 reps is actually good advice. It sits in the Goldilocks Zone of reps and loading, it's time efficient in that it can maximize the number of tough reps per set, it's energy efficient in that it doesn't require too many reps to stimulate hypertrophy, or involve loads that are unnecessarily heavy.
Generally, exercises with higher reps are used to improve muscular endurance, while higher weights with fewer reps are used to increase muscle size and strength.
The Case For High Reps
If you train with high reps, your goal is to build a bigger muscle. Some folks call this "structural hypertrophy" since the higher rep sets allow you to focus primarily on the muscles themselves. They also lend themselves to fewer total sets per exercise.
It all comes down to reps
People lift weights with the goal of making their muscles stronger (and, for some, to get those bulky biceps or lean-looking arms). For those looking to develop large muscles, they will likely opt for a heavier weight, while people who want to get lean will stick to something smaller.
“You must always remember to train your triceps heavy if you want to see maximum growth. Heavy exercises like the close grip bench press, lying ez bar extension and even weighted dips are all candidates to help pack on some size on your triceps while fitting the bill of the heavy movement required to get the job done.”
To create perfect symmetry it is suggested that your biceps and calves should be the same size. It therefore makes sense to train these two muscle groups together. The superset workout below should help you on your way to getting both your biceps and calves looking bigger and fuller.
Your calves need a little of everything to grow: Heavy weight, high reps, short rest, and most importantly, solid programming to tie it all together. This is going to hurt, but don't worry—it'll be over soon! Perform triple dropsets after the last 2 sets, using weights that cause you to fail at 10 reps.
The larger the calf complex, the more the lower leg weighs. Having more weight below the knee and towards the ankle can actually hinder running speed, according to Charlie Francis. It increases the lower leg lever, and slows leg turnover.
Tendons have very little vascularisation which might explain why mega high reps will help develop the tendons and potentially also speed up tendon recovery by increasing capitalization.
To properly execute a leg curl, always start with a lighter weight. You don't want to force your body to overcompensate by lifting the hips and flexing your lower back. This fails to isolate the calves and hamstrings and can cause injury to your back.
1 – Heavy, Low Reps
Your hamstrings are primarily fast-twitch dominant. Anecdotally, this may contribute to the massive hamstring development in sprint athletes. In the gym, this means you should incorporate heavy, explosive, and/or low-rep training.
If you're looking to build muscle quickly, whether you've been training for years or are just starting out, then doing slower reps is the way to go. Workouts with slower reps cause your muscles to experience more time under tension, much more than with faster reps.
In general, exercises that use heavier weights at low repetitions result in increased muscle bulk and power. Conversely, exercises performed with lighter weights and higher reps lead to muscle toning and muscular endurance.
Reps for muscle growth
In order to get bigger and stronger, you must ensure your muscles work harder than they are used to. Generally, between 6-12 reps for 3-6 sets will help to build overall muscle size.