Update:
This is simply not true. The problem with most compound exercises is that they fail to allow the muscle to actively change length, allowing for an intense contraction. This affects the biceps and the long head of the triceps. Even beyond these multi-joint muscles, you cannot achieve a full stretch without isolation movements. Various forms of curls, especially incline, along with arm extensions will maximize the size of the arms, not only compound exercises.
Get bigger arms by putting all of your effort into free weight compound exercises. Include both a push and pull. Go heavy and stay safe by using medium grips. This will best build your biceps, triceps, and forearm muscles. The barbell bench press and the underhand pull-up serve as two of the best options. Adding isolation through curls and extensions may harm your joints. These also allow less tension for growth.
Tension
Chins and dips, if properly performed, will stimulate muscular growth in your upper body and arms that will eventually lead to muscular size and strength that is very close to your potential.
– Arthur Jones
Your muscles work best at the mid-range of any normal motion. They form more connections here due to optimal overlap. At the endpoints of any motion, the sites that allow connections overcrowd or place too far apart. This means that although you may feel stronger or weaker at the endpoints, these positions always involve less tension. Tension is the main stimulus for size and strength. Therefore, heavy training must focus on the mid-point of any exercise.
At the lockout, such as during the bench press, you feel strong only because you have the best leverage. Since the moment arm, or the lever through which the resistance acts, is less when measuring the distance between your joint and the resistance, the muscles do not need to exert as much force. Think of a seesaw. When someone moves further away from the center, it makes the other person more likely to rise. Longer moment arms make the weight feel heavier. The longest moment arms usually occur in the middle. At the bottom of the bench press, your leverage is better than the midpoint, but it feels difficult here since your muscles stretch and grow weaker.
For this reason among others, compound exercises rank way ahead of isolation for building strength and size. They allow the most tension. They feel natural and work efficiently. Muscles tend to contract harder with other muscles. Isolation exercises place harmful shear forces on the joints. Isolation tends to change leverage too drastically. This fails to match the strength of your muscles with the right leverage.
Keep in mind that you may feel less of a burn in the arms during compound movements. This occurs due to less movement of the whole muscle, since the muscle stays closer to its ideal length. This will make more sense when discussing each muscle group later. Although the elbow moves, the muscle length stays nearly the same. This means less by-products of fatigue build up. Tension stimulates growth foremost, not movement. Tension does not require movement. Notice how hard your muscles can contract when you push against a wall as hard as possible.
These concepts set the foundation for the next sections. Any exercise that generates some tension can build size and strength. Isolation still works. It comes down to them working unsafely, poorly, or almost always both.
Biceps
Most try to build their biceps with curls of all types. These include incline, preacher, standing, close grip, wide grip, and concentration to name some options. You may be able to slightly affect different parts of the biceps. This may occur though only with very light loads irrelevant to weightlifting. Since the fibers run from attachment to attachment, you can only change the length of the whole biceps toward more or less tension.
Despite bodybuilders claiming to feel it differently depending on what exercise they use, most of these variations just overstretch or overshorten the biceps. This takes the muscle away from the optimal tension. Loading these areas just reduces the weight you can use. Fewer fibers activate in the more awkward positions. If you chose to curl, it would make the most sense to do a regular standing curl. This would allow the weight to feel heaviest exactly where your muscles work strongest. Curls are isolation exercises though, so carry those disadvantages.
The muscle fibers of each head of the biceps attach to the tendons of the elbow, scapula, and radioulnar joints. Changing the position of your arms may vary where you feel it, but not because you can emphasize parts. It comes because you disable the biceps. The elbow flexes due to other muscles beyond the biceps as well. The brachialis underneath the biceps and the brachioradialis of the forearm also bend the elbow. This explains why you feel preacher curls in the lower biceps. You feel it because the biceps shortens at the shoulder blade. This limits the tension of the biceps. This forces the other muscles to take over. The area bodybuilders describe as the lower biceps is actually the brachialis. Instead of developing these muscles better, you just make the biceps work less hard.
The strongest position of the biceps occurs at 90°. This happens when your arm lines up vertically with the side of your torso. This takes place at the midpoint of a typical standing curl. This serves as the midpoint on a proper row or pull-up too. On these exercises, the entire range of motion keeps the biceps in an good position. This occurs since the extension of the shoulder occurs as the elbow flexes. As the elbow straightens, the shoulder flexes. During a curl, you only bend at the elbow. This shortens and lengthens the biceps, which means less tension. Remember, tension is the main stimulus for greater size and strength. Think of a sawing motion. This works the muscles of the arm best.
You must use between an underhand and neutral grip. This must happen during any pulling motion to involve the biceps. This occurs since the biceps must unravel from the radius bone of the forearm. An overhand grip forces the biceps to tuck underneath this bone. Too much supination can irritate the wrist though. You can choose an underhand grip somewhere between full supination or a neutral wrist if possible. A straight bar for pull-ups can irritate the wrist of some trainees. In this case, consider different attachments or gymnastics rings that avoid fully turning the wrist.
Never flex the elbow with a pronated or overhand grip. This places the muscles in a weak position. It also makes the wrist less stable due to less support from the thumb. Bodybuilders often use an overhand grip to focus on their upper back muscles for rows, pull-ups, and pull-downs. They may want to focus on their forearms as well. They often would go wide with their grip, risking their shoulder joint. Instead, this just involves less muscle and risks injury.
Ignore most bodybuilding advice. Instead of building the muscles best, their advice will wreck your joints and overtrain the average person. Their points of view contain bad exercises done too much, too often, and too stupidly. They do not match our anatomy and biomechanics.
Some rightfully feel their upper back can take over when pulling. You instead need to use the proper grip and work hard enough. Choose a medium grip for either the row or pull-up. This will divide work fairly evenly between the arms and the upper back. The body will tend to emphasize the largest muscle groups during multi-joint motions. With heavy enough weight and effort though, the smaller muscles will match their intensity as well.
Do not follow the advice to imagine the arms as hooks. Bodybuilders do this to emphasize the back muscles. Instead, work heavily and progress. This will build the back and arms proportionately.
Triceps
Most use extensions, skull crushers, kickbacks, etc. to work the triceps. Others take normally good exercises, such as the dip while leaning forward or the bench press, and perform them with an upright posture or a close grip. These turn them into very unsafe choices. They make good compound movements have the properties of poor isolation exercises.
Since the long head of the triceps connects to the shoulder, it also plays a role during any pull. Other than this, you cannot choose to work any portion of the triceps during elbow extension. The long head of the triceps connects to the shoulder and elbow, and therefore benefits from the same ideal length concept as the biceps. All the triceps heads otherwise contribute equally to elbow extension during a horizontal push. The strongest position of the triceps also occurs at 90°. This would occur at the midpoint of a triceps extension. This happens when your arm lines up vertically with the side of your torso. A bench press will work the triceps at this ideal length. This keeps the long head of the triceps strong throughout the entire range of motion.
Bench press with a medium grip for joint safety. This also ensures the triceps and shoulders receive as much stress as the chest. Wrist position does not matter. An overhand grip works most securely during a push. The biceps play a small role in shoulder flexion, so the overhand grip gives no disadvantage. An underhand grip will harm the wrists during pushing. A neutral grip may work as well if you have the right bar, but an overhand grip using a barbell works fine.
Get Bigger Arms with Compound Exercises
Get bigger arms by pushing and pulling heavy weights. The push and pull needs to stay close to the horizontal plane. This improves safety for all the joints. It works the most amount of muscle and keeps the muscles of the arms at their best lengths. Make sure you use a medium grip. This will activate the arms evenly along with the larger muscles of the chest, shoulders, and back.
Make sure you eat enough and rest. Most trainees stimulate growth, but fail to realize it due to these factors. Many complain about their arms, and then do nothing outside of the gym to allow changes. They sleep poorly and therefore fail to put forth effort and concentration into the big compound lifts. They may work long and often, but fail to work hard. They fail to squat intensely and lose the indirect effect. They expect to mysteriously grow larger while eating poorly and not consuming enough calories. You cannot build something from nothing.
Avoid the temptation to ignore this advice. Many around you will do so. This comes from the belief that since many others do them, they should too. Focus on the compounds to build bigger arms.