The Pump Gets in the Way

The biceps attach on the scapulae in two places by way of the short and long-head tendons. That means that to get to the scapulae, the muscle tissue and tendons have to pass under plenty of deltoid tissue and enter the space under the acromion process.

With this in mind, filling up the biceps with blood via a good pump and a full biceps training workout could cause some trouble. You've decreased subacromial space in your shoulder just when you're about to proceed to a shoulder-dominant pressing workout.

Not Everyone's Built the Same

To add to this, tendon and joint structures aren't the same for everyone. Shorter tendon attachments means more thick muscle belly up higher towards the shoulders, which may create less space under the acromion process all on its own, let alone being inflamed from a biceps workout. It'll just make the shoulder workout to follow that much more uncomfortable.

It's also important to think about the three common types of shoulder joints, seen in the figure below.

Shoulder Sketch

The sketch on the left would show an ideal environment for pain-free overhead pressing movements due to the large space underneath the acromion process. Unfortunately, not every lifter is built this way.

In a worst case scenario, you'll have a situation like the shoulder on the right of the image, where a beaked tip prevents most overhead movements from being safely doable. This atmosphere, plus plenty of blood and muscle tissue, is a recipe for abrasions, tendinitis, impingement, and other bad stuff that deserves no business in our programs.

To play it safe, just train the biceps on a different day than shoulder day.

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