Update:
Due to better leverage, the chest & back muscles will work harder than the deltoids when they are involved in pushing & pulling. This is because the body wants to optimize energy use in the long run. However, many trainees still find non-direct work develops them sufficiently (but not fully).
No, but you do need exercises that align resistance with the front & rear shoulder fibers. Fortunately, this includes popular compound movements like bench presses & rows that develop other vital muscles too.
Working these heads more directly just provide them with extra but unnecessary work.
What Is Direct Work?
Direct work means isolation. You try to work the muscle while excluding larger ones. This activates that muscle best… in theory.
If you have studied bodybuilding, then you likely feel that isolation is the best way to grow smaller muscles. I used to feel similarly too.
For instance, you do overhead presses to isolate the anterior delts or bent-over lateral raises for the posterior delts. Bench pressing and rows hit the chest and back muscles.
The concept is that on compound exercises, the largest muscles dominate so that growth toward synergists is reduced.
However, while the body does emphasize big muscles when it has to perform a less intense activity, to conserve energy it seems, this finesse disappears once you lift a heavy weight. All the muscles that can be prime movers, will be.
EMG studies can mislead here. When additional muscles share the load, EMG signal amplitude lessens for them all. If the exercise is taken close to failure though, all the muscles contributing at a relatively high level get stimulated.
When muscles don’t contribute, even when the fibers oppose the resistance, this can be explained.
The gastrocnemius & soleus have reciprocal activation, meaning when one muscle is more active, the other is less. This likely developed to transfer stored energy smoothly for locomotion.
The biceps only fail to work on pronated or neutral grip curls because it wraps around the radius bone of the forearm, disabling it.
Multi-joint muscles must be anchored at their non-dominant joint. This is why leg presses fail to work the rectus femoris well. Knee extension, without hip extension, is required.
I believe this reality shatters convention regarding how many exercises we need for bodybuilding.
Horizontal Push & Pull Hits Front & Rear Delts
When you abduct your arm to push on something, the front delt aligns with the chest fibers. This allows them to contribute maximally when pressing horizontally.
However, at anatomical position with your arm at your side, the anterior delt no longer aligns with most chest fibers. This explains why a front raise or overhead press focuses on the delts. The chest barely assists shoulder flexion.
Overhead pulling aligns resistance with the outer lats but not so directly for the rear delt, which angles obliquely. It doesn’t help with pulling down so much as pulling back throughout shoulder extension.
Transverse adduction, like on a pec deck machine, prevents much front delt involvement. You move toward the midline of your body, across the chest, with the elbows pointing down. The front delt doesn’t align with the chest in this unusual position.
Therefore, you cannot deviate far from horizontal pushing or pulling.
A dip or severely-angled decline press won’t allow the front delts to help, because once again, the fibers don’t align.
Rows done too upright or too forward resemble upright rows or vertical pulls, cutting out the posterior delts.
You train the chest when horizontal pushing. The upper lats, lower & middle trapezius plus the rhomboids & rotator cuff muscles (infraspinatus & teres minor) work on horizontal pulling.
So beyond the delts, these basic movements exhaust many other muscles.
Should I Ever Perform Direct Work?
Yes, but only under rare circumstances.
Performing front raises or overhead presses would circumvent pec development to fix an imbalance.
Transverse abduction or extension, like bent-over lateral raises or high rows, work the rear delts, middle trapezius & rhomboids, and rotator cuff muscles. They limit upper lat & lower trap work. Keep in mind that the extreme position is tough on the shoulders too.
Overhead pressing and front raises develop the serratus anterior, etching impressive detail into the torso, as you flex overhead since you protract while upwardly rotating the scapula.
However, you can still protract while in an upwardly-rotated position, ideally at least 90° relative to the body, while horizontal pressing. I like to do scapular protraction during skull crushers since this can feel too unstable on heavy bench presses.
Front raises and other straight-arm exercises like the dumbbell fly uniquely hit the coracobrachialis, but you must decide if that’s important enough to develop.
Do We Need Direct Front & Rear Delt Work?
You’ll notice that many bodybuilders say to avoid much front delt work. They could have gone a step further… you don’t need direct work at all!
Steve Reeves only performed front delt exercises, while hitting other muscles like his abs, right before a competition.
He also criticized John Grimek, saying that he overdeveloped thickness from all the pushing & pulling involved in Olympic weightlifting.
Steve’s approach instead created the aesthetic illusion of tremendous upper body width.
Thinking logically, a bench press plus an overhead press would have double the front delt volume for only half the outer chest, so this conclusion makes sense.
Here’s a terrific combination of exercises that work the whole deltoid while balancing other important muscles…
Perform a flat press over the middle chest or an incline press from a shallow 10-20° angle. The latter is easier on the shoulders. Either brings out the whole pec, and the front deltoid, for anterior upper body thickness.
The stiff-legged dead-lift, a comprehensive movement hitting the whole posterior chain, brings about rear upper body thickness. A row works as well but may not be needed alongside this near-magical exercise.
Ideally, overhead pressing would offset elbow flexion when vertical pulling. Alas, external rotation limits middle delt activation. The behind-the-neck overhead press fails us here.
A wide-grip upright row or a lateral raise, both with enough internal rotation and to 90°, widens your shoulders. Wide-grip pull-ups take care of the outer lats & teres major. They completely address upper body width together.
With these several movements, you can avoid direct front & rear delt exercises, improving efficiency while limiting stress to the shoulders.
With a tighter focus on what produces results, don’t be surprised when you more than surpass your bodybuilding expectations!