Lats and Traps role in depressing shoulder blades

Question

here it's written that shoulder blades depression is performed by the following muscles:

• Latissimus Dorsi
• Pectoralis Major
• Pectoralis Minor
• Subclavius
• Serratus Anterior
• Trapezius (lower part)

Let's focus on the Latissimus Dorsi and the Lower Trapezius. From what I understand, shoulder blades depression may occur when your lats shorten and your traps lengthen.

My question is: Do the two muscles' actions have to occur simultaneously to perform shoulder blades depression? Or is it sufficient to perform one of them?

My question arises since I've noticed that in different exercises it's advised to depress shoulder blades by acting only on one of these two muscles.

For instance:

1. Biceps curl. To avoid shrugging, here it's written to keep the traps lengthened to depress shoulder blades. Well, what are the lats doing in the meanwhile? Does "Shoulder blades depressed through traps" mean also "lats squeezed to depress shoulder blades"?

2. Bench press. To avoid shrugging, here it's said to squeeze the lats to depress shoulder blades. What is the traps doing in the meanwhile?

3. Deadlift. To avoid shrugging, I've heard to squeeze the lats to depress shoulder blades. What is the traps doing in the meanwhile?

4. Pull-ups. To avoid shrugging at the top, here it's written to start the pull-up with shoulder blades already depressed thanks to the traps lengthening. But, what are the lats doing in the meanwhile? Of course they cannot be squeezed from the beginning since they have to squeeze during the execution.

Answer 1

Firstly, muscles cannot push, only pull, so whilst relaxing the upper traps might make it easier to depress your scapula, that alone will not cause any movement.

The only muscle that can really depress the scapula is the lower part of the trapezius - whilst the lats do attach to the inferior angle of the scapula but these fibers are almost horizontal, so it can't play any significant role in scapular depression.