Update: You can work tie-in muscles best through combining their functions.
With the coracobrachialis, for example, combining flexion with horizontal adduction activates it more intensely regardless of how bent the elbow is (since it has no attachment here).
Nonetheless, these muscles can look odd when overdeveloped and likely get enough work indirectly through conventional exercises.
Therefore, I doubt the advantages originally detailed here make too much of a difference, more speculation than anything else.
Most bodybuilders employ a mix of compound and isolation exercises, attempting to hit all the important angles throughout their split routine.
While sensible enough, many avoid studying the nuances behind their selections. They may trust in diversity and variety, choosing various movements from bodybuilding lore for the major muscles. Deeper questions rarely appear…
Should we use a dumbbell fly instead of a flat bench press? Does the pull-over have advantages compared to what arm extensions, pull-ups, and rows already provide? Which exercises address smaller tie-in muscles like the coracobrachialis?
Some single-joint exercises involve straight arms. While a perfectly-extended elbow is unnecessary, you should keep your elbow fixed at no more than 30° beyond straight. Exercises in this subcategory include various raises, flyes or cross-overs, and pull-overs.
Straight-arm exercises hit smaller muscles otherwise barely addressed on compound exercises targeting the same large muscles. They require less weight yet hit them in nearly-equal fashion, though with less overall musculature involved. Other advantages come about on an exercise-by-exercise basis.
The shoulder functions of flexion, extension, adduction, abduction, transverse or horizontal adduction/flexion, and transverse or horizontal abduction/extension allow for straight-arm exercises.
We’ll analyze each straight-arm exercise that corresponds with these functions.
Front Raise (Flexion)
While the overhead press is far more common for addressing the anterior deltoid, the front raise has unique potential.
Studies have shown that the biceps are activated differently on elbow flexion versus forearm supination. Elbow flexion involves more lateral motor units while forearm supination recruits medially. On a related note, the proximal rectus femoris activates preferentially on hip flexion.
The biceps has a weak internal moment arm, or leverage, at the shoulder. Nonetheless, is it possible that shoulder flexion operates similarly in recruiting unique motor units?
A neutral grip with the elbow extended, possible on a dumbbell front raise, would place the biceps at a good length to contribute.
More definitively, the front raise involves the coracobrachialis to a much greater degree.
The coracobrachialis is a small tie-in muscle connecting the arms, lats, chest, and shoulders when seen on a front double biceps pose. It plays a role in adduction, flexion, and horizontal adduction or flexion. Without straight arms that provide it better leverage though, it contributes little.
The lower half of a front raise up until 90° may involve the front head of the deltoid more intensely, which the overhead press limits. The scapular muscles, especially the serratus anterior, contribute more and more beyond 90° of shoulder flexion for upward rotation.
More obviously, the triceps get more rest versus overhead pressing, even if not the main target for that exercise.
The lower weight required on front raises also gives the joints a break from heavy weights, though stimulating less connective tissue growth.
Lateral Raise (Abduction)
Upright rows are the less popular but compound analogue of the lateral raise for abduction. This exercise involves more muscle, overloading external rotation to some extent in addition to the forearms, arms, and upper back.
Lateral raises likely became standard though for several reasons.
Lateral raises require less internal rotation which can otherwise cause shoulder impingement.
Straight arms on lateral raises emphasize elbow over shoulder movement, so humeral over scapular motion. This means less trapezius involvement, which dominate compared to the serratus anterior on shoulder abduction.
Limiting the range of motion to 90°, more easily done on lateral raises, further limits trap engagement.
The lateral raise also hits the small forearm extensor muscles isometrically better than the upright row.
Bent-Over Lateral Raise (Horizontal Abduction)
The bent-over lateral raise, performed with dumbbells, cables that cross over, or a reverse fly machine, allows you to perform a sweeping motion. Like the lateral raise, this emphasizes the rear deltoid over the trapezius. Elbows more bent will focus on the middle trapezius.
Though again less common, you need a wide grip on barbell rows, done with the elbows up and out, if you want a compound equivalent of the bent-over lateral raise.
Avoiding rows will rest to the brachialis and brachioradialis, which can help on a split routine that usually overworks the single-joint elbow flexors.
Free Weight Pull-Over (Extension)
The pull-over, performed with a barbell or dumbbell on a flat bench, uniquely hits the long head of the triceps.
By fixing the elbow and using a high shoulder flexion angle, the latissimus dorsi and pectoralis major cannot dominate the movement. This stretched position focuses on the long head’s function at the shoulder.
The free weight pull-over also stretches the ribcage for a larger chest measurement while hitting the outer chest as you complete the movement.
Flyes & Cross-Overs (Horizontal Adduction & Adduction)
Dumbbell flyes, and cable cross-overs brought across your body, rank among the best exercises for hitting deep muscles of the thorax while building the chest.
The unique stretch possible overloads scapular muscles like the pectoralis minor and upper serratus anterior. It also hits the subscapularis of the anterior rotator cuff. This all increases your chest girth.
Like front raises versus overhead presses, flyes and cross-overs address the coracobrachialis far more intensely than bench pressing.
Bringing the handles down instead of just forward on the cable cross-over confers advantages of both a dip and a fly within a single exercise, hitting both the middle and outer portions of the chest.
Finally, Frank Zane pioneered an unusual exercise… the behind-the-neck cable cross-over. This serves as a possible substitute to the wide-grip pull-up or pull-down, which overloads adduction for the lateral and middle lats.
This again provides the single-joint elbow flexors some rest while allowing for an extreme range of motion which may emphasize distal lat development.
Straight-Arm Exercises for Bodybuilding
While straight-arm exercises do work fewer muscles, they include unusual muscles while allowing overworked ones to receive more rest.
We also have to be practical given our access to equipment. With many trainees at home, the extreme leverage on straight-arm exercises will maximize the resistance’s effect.
Most importantly though, completeness for bodybuilding will require straight-arm exercises, so include them in your routine to obtain their unique benefits.