The posterior deltoid (rear delt) is a small but significant muscle group located on the back of the shoulder. Proper development is important for creating a balanced, three-dimensional look and promoting healthy shoulder mechanics and posture. Since the rear deltoid assists in many pulling motions, a common programming dilemma is whether to train this muscle alongside the large back muscles or group it with the anterior and medial heads of the shoulder. The decision depends on understanding the muscle’s function and considering individual training priorities.
Rear Deltoid Function and Synergy with Back Muscles
The primary biomechanical roles of the posterior deltoid are horizontal abduction and external rotation of the humerus (drawing the arm backward and rotating it outward). This action is distinct from, yet highly coordinated with, the large muscles of the back. For instance, the latissimus dorsi drives shoulder adduction and extension, while the rhomboids and middle trapezius perform scapular retraction.
During compound pulling movements, such as a seated cable row, the rear deltoids act as synergists, assisting the primary movers. Electromyography (EMG) studies confirm that the rear delts show substantial activation during exercises like face pulls and specific row variations. This inherent activation occurs because the muscle must stabilize the shoulder joint and contribute to the backward movement of the arm.
The structural proximity of the rear deltoid to the upper back muscles, particularly the middle and lower trapezius, reinforces this functional overlap. Even during isolation movements like reverse dumbbell flyes, surrounding back muscles stabilize the scapula. This established pattern of co-activation means that any comprehensive back workout already places a substantial, though often sub-maximal, stimulus on the posterior deltoid.
The Case for Including Rear Delts on Back Day
Training the rear deltoids on back day leverages training efficiency and muscle pre-activation. Since the muscle has already received a sub-maximal stimulus through heavy compound rows and pulldowns, dedicated isolation sets require less preparatory work. This approach allows a lifter to transition immediately into accessory work, saving valuable time.
Neurological activation from heavy pulling movements primes the motor units in the posterior deltoid, potentially enhancing the mind-muscle connection during subsequent isolation exercises like reverse pec deck flyes. EMG data shows that exercises like the 45-degree incline row can elicit activation levels comparable to specific isolation exercises, demonstrating movement synergy. Grouping them with back work also allows for a high-frequency approach to a smaller muscle group.
Many training splits involve training the back twice weekly, providing two opportunities to hit the rear delts without dedicating a separate third session. For individuals whose rear delts are proportionally developed or who prioritize time management over maximal growth, back day integration is practical. Finishing a back session with focused, high-rep isolation work is an effective strategy to accumulate necessary training volume without causing excessive fatigue that might interfere with a subsequent shoulder workout.
Why Rear Delts Might Be Better on Shoulder Day
The counter-argument for placing rear deltoids on a separate shoulder or accessory day centers on prioritization and intensity. When the rear delts are trained at the end of a demanding back workout (including heavy deadlifts or high-volume rows), the central nervous system and surrounding stabilizer muscles are fatigued. This pre-existing fatigue limits the weight or intensity applied to isolation exercises, potentially compromising the quality of the stimulus.
Training the muscle when the body is fresh allows for maximum effort and a higher neural drive, which aids the mind-muscle connection in smaller muscle groups. If the posterior deltoids are a lagging muscle group or a priority for aesthetic balance, training them at the beginning of a shoulder session ensures they receive the full benefit of peak energy levels. This placement guarantees that volume and intensity are not limited by the exhaustion caused by preceding large compound lifts.
Studies show that the reverse pec deck fly can produce significantly greater posterior deltoid activation than multi-joint movements like the seated row. Maximizing the effectiveness of this isolation movement requires the lifter to be fresh. Placing rear delt work on shoulder day reinforces the structural grouping of the deltoid heads, ensuring a balanced application of volume across the entire shoulder girdle. This strategy is useful for those who follow a low-volume training split, where maximizing focus during a dedicated “Push” or “Shoulder” day is necessary for growth.
Determining the Optimal Training Split for You
The choice between back day and shoulder day depends on your individual goals and current muscle development. If your primary objective is time efficiency and your rear delts are well-developed, integrating them as accessory finishers on a back day is the most logical choice. This strategy capitalizes on existing muscle activation and provides a high-frequency stimulus.
Conversely, if the posterior deltoids are a lagging muscle group requiring dedicated attention, they should be prioritized by training them first on a separate shoulder or accessory day. This ensures that maximum intensity and volume can be applied when energy levels are highest, avoiding limitations imposed by central fatigue from heavy back work. The optimal split aligns with your weakest links and your overall schedule.