Middle Deltoid

This muscle is the most lateral on the body, (hopefully), and is the beginning of the classic V Taper that most people strive for. Training this muscle area correctly is imperative not only for results but to prevent shoulder injury. The shoulder complex is extremely mobile, yet intricate in function. Respecting its structure should be of utmost concern when applying resistance.
The primary job of the mid-portion of this muscle is to lift the arm out to the side, humeral abduction, therefore THE LATERAL RAISE is the most effective method in strengthening and/or enlarging.

How to do it:

Assume a stable stance, (knees and slightly bent, spine in neutral), and lean forward slightly. You must lean forward slightly in order to target the middle deltoid. This is the most common mistake. The muscle you are trying to emphasize must be on top of the axis, in this case, facing the ceiling. If you perform this exercise standing upright, the majority of the work will be done by the anterior, or front deltoid.

Keeping the arms straight, or slightly bent, slowly raise your arms out to the side just slightly to the front of the shoulders. Stop at shoulder height, although it is possible to go higher than this, the arm must externally rotate to avoid impingement, and this rotation will change the recruitment of muscles and emphasis away from the middle deltoid.

Other things to think about: SLOW & CONTROLLED! I cannot tell you how many people I see using momentum to hoist the weight up. Less is more when it comes to this exercise. Choose a weight that you can lift comfortably for 10-12 repetitions. Personally I know no one who has any business lifting over 25 lbs. for this exercise. This even includes those who bench press 300 lbs.