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A: The lateral raise is an excellent movement for developing the middle head of the deltoid, and there's more than one way to do the exercise correctly. You can alter the emphasis placed on the middle head by changing up how high you raise your arms and the position of your body during the exercise. Here's what you need to know about your shoulders — and how to turn them into boulders.
Many muscles are recruited during the raise. The deltoid comprises three distinct sections, or heads: 1) anterior (front), 2) middle and 3) posterior (back). The supraspinatus is one of four small muscles — collectively known as the rotator cuff — that provide strength and stability to the shoulder joint while allowing a great degree of mobility. The supraspinatus lies underneath the deltoid and is the prime mover during the first 30 degrees of the lateral raise. The deltoid, particularly the middle head, provides the majority of force for the rest of the movement until the arm is about 45 degrees above parallel to the floor. The trapezius (traps) then takes over to bring the arm straight overhead.
Performing the lateral raise so that your arms are parallel to the floor in the top position significantly stresses the middle deltoid, providing good outer-shoulder development. Continuing the movement to about 45 degrees above parallel ensures maximal contraction and increases the range of motion, both of which contribute to maximal development. Going above this point, as in the overhead lateral raise, largely involves the traps.
Upper-body lean while doing a one-arm lateral raise alters the involvement of the supraspinatus and middle deltoid. Leaning toward the working arm, by holding onto a steady structure such as a power rack with your nonworking hand and leaning away from it, greatly reduces supraspinatus involvement and shifts emphasis to the middle deltoid. Leaning away from your working arm, by lying sideways on an incline bench, isolates the supraspinatus and reduces involvement of the middle delt.