One of the most under trained body parts are the posterior deltoids in my opinion. Naturally most of the shoulder movements people do will target the anterior and medial heads, causing limited stimulation to the rear area. If this specific head of the deltoids remains under developed then the following issues you could fas include:
A) Visual imbalance from the side and back
B) Physical imbalance potentially leading to postural and shoulder issues
Here are ways I’d recommend you bring your posterior deltoids up if they’re lagging.
The posterior deltoids are particularly difficult to recruit especially if they’re under developed. With that said I’d recommend using 1-2 second isometric holds with every rep using a cable face pull. This exercise helps you isolate the rear heads of your deltoids as much as possible, allowing you to gradually improve engagement within the area.
2. Lighten The Load
One thing I notice when people do exercises like cable face pulls or bent over rear dumbbell raises is they go far too heavy. As a result this leads to excessive momentum created by other muscle groups, which drastically limits muscle stimulation within the target area. For such a small muscle group with a very specific requirement to achieve isolation I’d recommend going lighter.
3. Back & Shoulder Day
If your rear deltoids are particularly weak then you could potentially do some secondary work on back day as you’re already stimulating the posterior chain. This will help re-stimulate muscle protein synthesis within the area. However make sure if you do this that shoulder and back day are 4-5 days apart.
These steps will all help as will using Y3T because of the 3 dimensional approach to global muscle fibre stimulation.
PS Remember the start of a new week presents opportunity for improvements. Push hard this week!