Rear Delt Flyes: Muscles Worked, Benefits, and Proper Form

What Do Rear Delt Flyes Work?

Rear delt flyes primarily target the posterior deltoid muscles, contributing significantly to shoulder health, posture, and overall upper body balance by performing horizontal abduction of the humerus.

Anatomy of the Shoulder and the Rear Deltoid

The deltoid muscle is a large, triangular muscle that caps the shoulder joint, giving the shoulder its rounded contour. It is functionally divided into three distinct heads, each with unique anatomical attachments and primary actions:

• Anterior Deltoid: Located at the front of the shoulder, primarily responsible for shoulder flexion and internal rotation.
• Medial (Lateral) Deltoid: Situated on the side of the shoulder, its main action is shoulder abduction (lifting the arm out to the side).
• Posterior Deltoid (Rear Delt): Positioned at the back of the shoulder, originating from the spine of the scapula and inserting into the deltoid tuberosity of the humerus. This head is the primary focus of the rear delt fly exercise.

The posterior deltoid's key actions include horizontal abduction (pulling the arm backward in the horizontal plane, as if reaching out to the side and then pulling the arm back), external rotation, and extension of the shoulder joint.

Primary Muscles Worked

The rear delt fly is an isolation exercise designed to specifically target the posterior deltoid, but it also engages several synergistic muscles that assist in the movement.

• Posterior Deltoid: This is the prime mover and the main target of the exercise. Its activation is maximized when the movement emphasizes horizontal abduction with a slight external rotation, keeping the elbows slightly bent.
• Secondary Synergists:
  • Rhomboids (Major and Minor): Located between the spine and the scapula, these muscles assist in retracting (pulling back) and rotating the scapula, which is crucial for stabilizing the shoulder blade during the fly movement.
  • Trapezius (Middle and Lower Fibers): The middle trapezius aids in scapular retraction, while the lower trapezius assists in depressing and rotating the scapula, contributing to overall shoulder stability and proper movement mechanics.
  • Teres Minor: A small muscle of the rotator cuff, it assists in external rotation and adduction of the arm.
  • Infraspinatus: Another rotator cuff muscle, primarily responsible for external rotation of the humerus, it also contributes to the stability of the shoulder joint during the movement.

Biomechanics of the Rear Delt Fly

The rear delt fly typically involves either a bent-over position with dumbbells or using a reverse pec deck machine. The fundamental biomechanical action is the horizontal abduction of the humerus.

• Bent-Over Dumbbell Rear Delt Fly: In this variation, gravity provides the resistance. The exercise begins with the torso bent forward, arms hanging down, and the dumbbells positioned beneath the shoulders. As the arms are raised out to the sides, the posterior deltoids contract to pull the humerus horizontally backward, away from the midline of the body. The slight bend in the elbows helps to focus the tension on the rear delts rather than the triceps.
• Reverse Pec Deck Machine: This machine provides a more stable and controlled environment, often allowing for better isolation of the rear deltoids. The resistance is typically cam-based, providing consistent tension throughout the range of motion. Users press their elbows or forearms backward against pads, performing the same horizontal abduction movement.
• Cable Rear Delt Fly: Using cables allows for continuous tension throughout the entire range of motion, regardless of body position. This can be performed standing, seated, or bent over, with cables crossed or uncrossed, providing versatile options for targeting the rear delts.

Proper form is paramount to ensure the posterior deltoids are effectively targeted. Over-recruitment of the larger back muscles (like the latissimus dorsi or upper trapezius) often occurs if the weight is too heavy or if the movement becomes a shrug or row rather than a pure horizontal abduction.

Benefits of Training the Rear Deltoids

Developing strong posterior deltoids offers numerous benefits extending beyond just aesthetics.

• Shoulder Health and Stability: Many common strength training exercises (e.g., bench press, overhead press) disproportionately strengthen the anterior and medial deltoids. This can lead to muscular imbalances, pulling the shoulders forward and potentially causing impingement or instability. Strong rear deltoids help to counteract this by pulling the shoulders back and stabilizing the glenohumeral joint, promoting overall shoulder health.
• Improved Posture: Weak posterior deltoids contribute to rounded shoulders and a hunched posture. Strengthening these muscles helps to pull the shoulders back and down, improving thoracic posture and alignment.
• Enhanced Aesthetics: Well-developed rear deltoids contribute to a balanced and well-rounded shoulder appearance, creating a "3D" look that is often sought after in bodybuilding and physique training.
• Injury Prevention: By strengthening the muscles that externally rotate and stabilize the shoulder, rear delt training can reduce the risk of common shoulder injuries, especially those related to rotator cuff imbalances.
• Performance Enhancement: For athletes, strong rear deltoids improve performance in movements requiring powerful pulling, throwing, or overhead actions, such as in baseball, swimming, or rock climbing.

Common Mistakes to Avoid

To maximize the effectiveness of rear delt flyes and minimize injury risk, be mindful of these common errors:

• Using Excessive Weight: This is the most common mistake. Too much weight often leads to the recruitment of larger back muscles (like the rhomboids and trapezius) and a loss of isolation for the smaller rear deltoids. Focus on controlled movement rather than lifting heavy.
• Excessive Momentum: Swinging the weights or using momentum from the lower back or legs negates the isolation effect. The movement should be slow and controlled, especially during the eccentric (lowering) phase.
• Shrugging the Shoulders: Elevating the shoulders towards the ears (shrugging) indicates over-activation of the upper trapezius, taking tension away from the rear delts. Keep the shoulders depressed and retracted.
• Straightening the Arms: Locking out the elbows can shift tension to the triceps and reduce the focus on the rear deltoids. Maintain a slight, consistent bend in the elbows.
• Incomplete Range of Motion: Not bringing the arms back far enough limits the full contraction of the rear deltoids. Aim for a full, controlled sweep.

Incorporating Rear Delt Flyes into Your Routine

Rear delt flyes are best performed as an accessory or isolation exercise.

• Placement: They can be placed at the beginning of a workout to pre-exhaust the rear delts before compound pulling movements, or more commonly, at the end of a workout after heavier compound lifts (like rows or pull-ups) to specifically target the posterior deltoids without fatigue compromising form on larger lifts.
• Rep Ranges and Sets: Due to the relatively small size of the posterior deltoid and its role as an isolation muscle, higher repetition ranges (e.g., 10-20 repetitions per set) are often effective for building endurance and hypertrophy. Aim for 2-4 sets.
• Progression: Progress by gradually increasing the repetitions, sets, or the time under tension, rather than immediately jumping to heavier weights, to maintain proper form and maximize rear delt activation.

By understanding the anatomy, biomechanics, and common pitfalls associated with rear delt flyes, you can effectively integrate this crucial exercise into your training regimen to build balanced, strong, and healthy shoulders.