If you want well-rounded, 3D shoulders, you cannot ignore side and rear deltoid training. Both the rear delt fly and lateral raise are staple isolation movements in gym routines. Many lifters mix them up or treat them as interchangeable exercises. While they both target the shoulder, they hit separate muscle heads, demand different movement patterns, and serve unique goals. Understanding their differences helps you build balanced shoulders, fix rounded posture, and avoid common training mistakes.
The lateral raise is the go-to exercise for your lateral delts, the side shoulder head that creates shoulder width. In a standard lateral raise, you hold light dumbbells or use a cable, keep your arms slightly bent, and lift your arms out directly to the sides until they reach shoulder height. The movement focuses on abduction of the shoulder joint. The lateral deltoid is the primary driver here. Secondary muscles include the supraspinatus, upper trapezius, and triceps, though you should minimize trap activation to get the best results. This exercise is what builds that coveted wide shoulder look, often called the “V-taper.” It works best with lighter weights and controlled tempo, as heavy loads easily pull tension away from the side delts and shift it to the upper traps.
On the other hand, the rear delt fly prioritizes your posterior delts, the often neglected rear shoulder head. For a rear delt fly, you lean forward at the hips, bend your elbows gently, and open your arms wide back and slightly up, as if you are squeezing a pencil between your shoulder blades. The main action here is shoulder horizontal extension and external rotation. Besides the rear delts, this movement heavily engages the upper back muscles: rhomboids, middle traps, and infraspinatus. Weak rear delts are extremely common, especially for people who spend long hours sitting at desks or bench pressing regularly. This weakness leads to rounded shoulders, poor posture, and increased shoulder injury risk. Rear delt flies are critical for balancing out chest and front delt dominance.
A simple way to tell them apart: lateral raises build shoulder width, rear delt flies build shoulder thickness and fix posture. Lateral raises move arms out to the sides; rear delt flies move arms backward. Both are isolation moves, so they do not let you use very heavy weight. Form quality always beats loading. Swinging momentum to lift heavier is the number-one error for both exercises. When you swing, the target delts stop doing the work, and tension transfers to bigger muscles.
So which one should you choose? If your priority is wider shoulders and filling out the side profile, add lateral raises to your shoulder day. If you struggle with rounded shoulders, have weak rear delts, or want to balance pressing movements, rear delt flies are non-negotiable. Most balanced training programs include both. You can pair them as a superset, or place them at the end of your shoulder workout after compound lifts like overhead press.
You do not need to pick one permanently. The best shoulder development comes from training all three delt heads equally. Use lateral raises to grow width, and rear delt flies to shore up the back of your shoulders. Master clean form on both, and you will notice stronger, healthier, more defined shoulders over time.
