The delts are more like the endurance muscles you see with calves or forearms and they can handle more reps. Some athletes who are really gifted can do heavy barbell or dumbbell presses and just grow easily. If you have problems gaining size try giant sets (where you combine 4-5 exercises and complete them consecutively with no rest between). It is really intense and you will feel a killer pump on your shoulders. Do a giant set with dumbbell lateral raises, dumbbell shoulder presses, upright rows and bent-over lateral raises for 10 reps each exercise, or 40 reps to total. If your shoulders are not responding from heavy training, this type of advanced supersetting will really work.
Competitive bodybuilders usually train for symmetry and proportion in an effort to build an aesthetically pleasing physique. There are many individuals who go to the gym to build up their beach muscles, disregarding symmetry and proportion in designing their training programs. Instead of training opposing muscle groups equally, they sometimes ignore the muscles they cannot see (back, hamstrings, calves, quads) and focus on the muscles they admire in the mirror everyday (chest, biceps, shoulders and abs). Although training specific muscle groups more than others can create an unbalanced-looking physique, disregarding a muscle group over time can potentially lead to other problems that may subject the body to injury.
One common mistake with regard to balanced training is performing an unbalanced training program between the chest and back muscles. The amount of time, energy, effort, volume and frequency between the agonist chest muscles and the antagonists back muscles may not be equal, or even close. A simple example would be an athlete who performs 4 sets os four exercises for the chest (for a total number of 16 sets) and does only 3 sets of three exercises for the back (for a total number of 9 working sets). Following this unbalanced training regime over time will result in back muscles much weaker than the chest muscles. This imbalance may lead to a slightly kyphotic posture (forward/rounded shoulders) that can potentially cause shoulder problems because of the faulty posture. In addition to the stronger chest muscles (compared to the back muscles) pullong the shoulders forward, inadequate stretching of the chest musculature can further contribute to this problem.
For some people, training legs consists of quads and that is it. The disregard their hamstrings while training only the quadriceps. Exercises such as the leg extensions, hack squats and front squats place a large amount of emphasis on the quads. Although these exercises are great you need to do an equal amount of hamstring work with leg curls and romanian deadlifts to stress both muscle groups. Distributing attention evenly to both muscle groups can give you healthier knees and lower back, as well as fewer hamstring strains because of an unbalanced hamstring-to-quad strength ration.
A less obvious training error woth regard to balanced training concerns the shoulders. Overhead shoulder press, incline bench press, flat bench press and other chest and shoulder pressing movements are critical for maximal muscle development. However, these exercises focus on the major muscle groups such as the deltoids, pectoralis major, pectoralis minor and other muscles known as prime movers. Often ignored in a training regime is the training or isolation of the smaller muscle groups in the shoulder known as the rotator cuff (the supraspinatus, infraspinatus, teres minor and subscapularis) that are critical in keeping the upper arm in the socket of the shoulder. Failure to train these smaller muscles groups, two of which are not even visible can create an imbalance between the large prime movers and small stabilisers. This lack of imbalance in strength may result in bursts of bursitis, tendinitis or even rotator cuff tears in the shoulder joint.
The theory makes sense, performing a bench press or military press on a Swiss ball should engage more of the stabilising muscles of your core. In reality, though this isn’t the case. A study released by the Human Performance Laboratory at California State University, suggests that abdominal muscle activation during a three-rep max chest press and shoulder press was similar whether performed on a Swiss ball or on a flat bench. Performing these lifts on a Swiss ball does have some benefit, however. It’s just not in developing core strength. Those lifts work the stabilising muscles of the shoulder, not the core. If you were to use the Swiss ball for that purpose it would have to be somewhere in the range of 15 to 25 repetitions with low weights.
Basic lifts on a stable surface, provide a better overall stimulus. A heavy squat, deadlift or military press is always going to be more effective at activating the core than any Swiss ball lift.