Use shoulder biomechanics to accurately target the rear delts and minimize compensatory patterns.

Whether you're training for aesthetics, athletics or rehab, the deltoid group will at some point become a primary focus of your training endeavours. Mechanically, the delt is a simple muscle to train due to its uniarticular nature (meaning that the delt only crosses one joint), but there are complicating factors – for example, the fact that it has three distinct fibre orientations or “heads”.

For training purposes, the deltoid is broken down into these three divisions:

• Anterior
• Medial
• Posterior

Of these three, the anterior and medial portions are the easiest to train – this is because, for better or worse, they are inextricably linked to the performance of compound pressing movements like bench press, dumbbell press, dips, etc. The evidence is plain: highly developed anterior and medial delts can be seen in any gym you walk into – in fact, they're something of a hallmark of the "gym-bro" – but highly developed rear delts are far more elusive, and seem to be exclusively found in professional bodybuilders and elite trainers.

The rest of this article is going to be dedicated to understanding why effective rear delt training is a target that most people miss. It will also provide some simple changes that can be made to immediately start improving your exercise selection and execution, and ultimately, your results.

You can't isolate the rear delts, but you can bias them.

By the book, the rear delt has a few basic muscle actions, and is classified as a shoulder hyper extender and a mild external rotator.

It’s rarely ever working alone.

As an extender of the shoulder, we have accompanying muscles like the triceps and lats, and as an external rotator, it has the help of the infraspinatus and the teres minor.

Now, the goal isn't to completely isolate the rear delt from its agonists and synergists. It's to learn how to select, set up, and execute movements in a way that grants the rear delt the lion's share of the load and lift.

And in the spirit of the goal, we need to understand some fundamental principles of the shoulder and how they pertain to effective rear delt training.

As mentioned before, the rear delt is a hyper extender of the shoulder, which means it's primarily responsible for when the upper arm starts to travel behind the neutral plane of the body, but not all hyper extension is created equal.

And this is where things get interesting…

The mechanisms of shoulder hyperextension change significantly based on the level of abduction the shoulder is in when it hyperextends. Simply put, how close or far our arm is to our side as we’re bringing our arm behind our body will drastically change the muscles we recruit when doing so.

How to find the right arm angle for maximum rear delt activation.

If our goal is to bias the rear delt, we want to be aiming for that hyperextension to occur somewhere between 45-60 degrees of abduction. Within this range, the humerus has the greatest range to move within the socket of the scapula without the scapula itself moving to contribute.

When we try to hyperextend with our arm at 0 degrees of abduction (when the arm is at the side of your body), the humerus runs out of space in the scapula and forces the scapula to move and compensate in order to facilitate the movement and complete the task. Seeing as the rear delt doesn't act as a primary mover of the scapula, this range is ineffective for emphasizing the rear delt.

The same is true when we try to hyperextend the shoulder with our arm at 90 degrees of abduction. The scapula is forced to move via the trapezius or rhomboid as the humerus runs out of room in the socket of the shoulder, and the shoulder blade will tilt in order to aim the arm further back. Again, moving the shoulder blade is not a primary action of the rear delt.

So, if we hyperextend at too little abduction, and we compensate, missing the rear delts. And if we hyperextend at too much abduction, and we compensate, missing the rear delts.

But in something of a “Goldilocks” range – there is a middle ground of abduction that's just right.

When we look to hyperextend the shoulder between 45-60 degrees of abduction, the position of the scapula, mixed with the rotation of the humerus, allows for the humerus to move into far greater degrees of hyperextension without the scapular movement compensating.

As a point of reference, this range of abduction is where most people feel most comfortable in their flat pressing movements and pec movements, and for the same reason.

Select movements that prioritize shoulder hyperextension between 45-60 degrees of abduction.

When we have a better understanding of the structural interaction of the humerus and the scapula, we can begin to make more informed decisions on how we choose, set up and execute movements.

In the case of the rear delts, we begin to prioritize exercises like cable rear delt flys and rowing variations, due to their ability to be customized and trained specifically within the range of abduction that we require.

And at the same time, we begin to deprioritize more conventional exercises like rear delt machine flys (also called a reverse pec deck), which tend to lock us into this 90 degrees of abduction position where our range will be limited, and compensations are almost guaranteed.

As a point of context, as we close, it's worth putting into context that compensations aren't necessarily a bad thing that have to be avoided at all costs; however, they do need to be understood at all costs. In the business of prescribing exercise, the devil is in the details, and understanding these deeper structure-function relationships is going to make you much more effective at getting the results you desire.