The Science of Training: Deltoids

by Josh Bryant and Joe Giandonato

Surget Nubret had amazing shoulder development.

The metaphorical armor of a great physique is capped with a pair of cannonball delts.

Beyond aesthetics, strong, well-developed shoulders are an insurance policy for contact and combat sport athletes while offering both performance and injury-prevention benefits in sports ranging from Kushti to cricket to powerlifting! 

Initially, incremental progressive overload via basic exercises cuts the mustard, but eventually the spigot of newbie gains drips to a halt, propelling intermediate lifters into a continuum of rep ranges — rollercoasting from high to low — and more fancy exercises than Wilt Chamberlain has lovers. Unfortunately, the result is painful impingement, plunging pressing power, and becoming acquainted with the canvas in the backyard boxing brawls at the local mobile home turned afterhours sake house!

Information is at an all-time high and beyond elite-lifting circles, results do not reflect this. 

We are going to show you how to optimize size, strength, and the function of your shoulders.


The shoulder girdle serves as the foundation of the upper body and is comprised of four interdependent joints — glenohumeral, acromioclavicular, sternoclavicular, and scapulothoracic — which work in concert to articulate, or move the shoulder.

Poor alignment of these joints, often the result of a lack of stability and motor control of attaching muscles, structural abnormalities or deformities, or ligamentous laxity, will contribute to dysfunction and pain, therefore, some of the exercises, while boasting superiority in electromyographical studies in targeting specific aspects of the deltoid musculature may not work for you!

The overhead press and its kissing cousin, the seated military press, are not shoulder isolation exercises.  They are movements which activate and challenge a slew of muscles that range from agonist, or prime movers, to synergist, or helper, to antagonist, or stabilizer.

The mere act of lifting your arm over head, sans heavy pig iron, involves upward rotation of the scapula, or shoulder blade, as well as abduction, flexion, and internal rotation of the humerus, or shoulder bone, with the contribution of the deltoids differing throughout this routine movement.

Textbooks and traditions say the deltoids are comprised of three aspects — the anterior deltoid, lateral deltoid, and posterior deltoid.

This tradition has gone the way of the dodo bird.

A groundbreaking analysis involving the comparison of cadaveric dissection and positron emission tomography, a procedure encompassing the intravenous administration of a radioactive tracer to detect metabolism and function of tissue, commonly known as PET scanning, revealed (7) distinct intramuscular tendons encapsulating the humerus, all with varying lines of pull!

Bundles of fibers originate anteriorly from the distal clavicle, or lateral part of the collarbone, laterally from the acromion process of the scapula, and posteriorly from the spine of the scapula. Communally, these bundles, along with their respective embedded intramuscular tendons — three comprising the anterior portion, one comprising the lateral portion, and three rounding out the posterior portion — converge and blend onto the deltoid tuberosity of the lateral aspect of the proximal humerus, canvassing the glenohumeral joint.

The anterior bundle of fibers flex, adduct, and internally rotate the shoulder, similar to the pectoralis muscle that is separated only by deltopectoral sulcus, a groove in which the cephalic vein, the thickest and most visible of the upper arm, resides. The intramuscular tendon seated dead center within the convexity of the fibers comprising the deltoid, chiefly abducts the shoulder with a kick-start from the infraspinatus, a smaller muscle, nestled below the deltoid, which, along with three other muscles, make up the “rotator cuff”. The posterior bundle of fibers adducts, externally rotates, and along with the neighboring latissimus dorsi, extends the shoulder.

Fiber Type Distribution

A recent study involving digital imaging of muscle fiber morphometry among a diverse population with age, gender, exercise history, and health differences, revealed a balanced distribution of type I (47%) and type II (53%) muscle fibers. Though type II fibers were predominant among amateur and adult wrestlers, the posterior deltoid was found to be comprised of a greater proportion of type I fibers. Type IIx fibers were found to have greater capillary density, closely mimicking the properties of type I oxidative fiber characteristics, suggesting this adaptation was the result of repeated metabolic and biomechanical demands of training and competition. It can be inferred the fiber distribution of the posterior deltoid is balanced due its many assistive functions — adduction, external rotation, and extension, and stabilization of the shoulder, particularly while the arm is abducted and during higher velocity gait cycles. 

In Practice

The development of the lateral deltoid is the name of the game among aesthetically driven athletes — dramatizing a visual “v-taper” effect, in turn, providing an illusion of a smaller waist. 

In comparison to the posterior deltoids, most of the size potential is baked into the anterior and lateral portions which have greater involvement in dynamic movements, specifically shoulder flexion and abduction — think presses and raises — that can be loaded significantly. Author Michael Gundill ascertained this in a 2002 issue of Ironman, noting that that bodybuilders’ rear deltoids, on average, were just 10 to 15 percent bigger than sedentary people, conversed to lateral deltoids that were three times bigger and anterior deltoids that were six times bigger.

This should not be a complete shock since most bodybuilders vary their pressing range of motion and angles while peppering in an assortment of shoulder isolation work. 


An individualized approach is best when one wishes to transform their shoulders into boulders.

Those who regularly perform flat and incline presses should “migrate north”, temporarily replacing their flat pressing with overhead or seated shoulder pressing.

Don’t fret — your bench numbers won’t sink. In fact, they may actually improve since overhead pressing strength is directly correlated with bench press performance.


And for the small faction of lifters who overhead press with zeal, a changeup to a steady diet of dumbbell shoulder presses that offer a different path than barbell presses and often a greater range of motion and raise variations, will tap into swaths of lesser utilized muscle fibers.


Most of the time, lateral raises are performed with a straight arm, causing the elbow to buckle into hyperextension and shifting the emphasis away from the deltoids. Instead, the elbows should be slightly flexed, almost imperceptibly, roughly a handful of degrees off from full elbow extension, known in coaching circles as a “soft elbow” that brings the intramuscular tendons of the deltoids into their line of pull. Another seemingly minute, but profound, adjustment is focusing on raising the elbow away from the body as many times lifters will flail the dumbbells above shoulder height during raises with trapezius powered momentum.

For those with wonky shoulders or structural abnormalities, such as a beaked or upturned acromion process, that limit overhead movements, raises, if performed with intention and unwavering focus, can serve as a safer substitute. Taking it a step further, one can perform said raises within the scapular plane, characterized by a fixed shoulder abduction of 30 degrees from the body, not perpendicular like traditional raises.

And to assuage concerns of overhead movements being dangerous, or extra “shoulder work”, being counterproductive to strength gains, it should be noted that hypertrophy can be accomplished via metabolic stress and muscle damage through giant sets, super sets, and timed sets that do not require clavicle-crumbling loads.

Final Thoughts

In closing, whether goal is show or go, you now have the know!  Time to hit the pig iron!

Get Josh’s ISSA Bodybuilding Certification HERE.


Gundill, M. (2002). Pressing Issues: Building Better Shoulders with overhead presses. Ironman, Issue 8, p. 42.

Loukas, M., Myers, C.S., Wartmann, C.T., Tubbs, R.S., Judge, T., Curry, B., & Jordan, R. (2008). The clinical anatomy of the cephalic vein in the deltopectoral angle. Folia Morphologica, 67 (1), 72-77.

Mandroukas, A., Metaxas, T., Kesidis, N., Christoulas, K., Vamvakoudis, E., Stefanidis, P., Heller, J., Ekblom, B., & Mandroukas, K. (2010). Deltoid muscle fiber characteristics in adolescent and adult wrestlers. Journal of Sports Medicine and Physical Fitness, 50 (2), 113-120.

Moser, T., Lecours, J., Michaud, J., Bureau, N.J., Guillin, R., & Cardinal, É. (2013). The deltoid, a forgotten muscle of the shoulder. Skeletal Radiology, 42 (10), 1361-1375.

Omi, R., Sano, H., Ohnuma, M., Kishimoto, K.N., Watanuki, S., Tashiro, M., & Itoi, E. (2010). Function of the shoulder muscles during arm elevation: an assessment using positron emission tomography. Journal of Anatomy, 216 (5), 643-649

Reyes-Fernandez, P.C., Periou, B., Decrouy, X., Relaix, F., & Authier, F.J. (2019). Automated image-analysis method for the quantification of fiber morphometry and fiber type population in human skeletal muscle. Skeletal Muscle, 9, 15.