First question: is the choice of working in the scapular plane made to reduce shoulder impingement (like in lateral raises)? If it is, can you explain me why? Does having a 30 degree angle between the elbows and the frontal plane creates space between the humerous and the AC joint for the rotator cuff tissues?
No, it's nonsense, seemingly made up by the author of the article. They cite two papers in support of this claim, stating that these papers demonstrate that an overhead press which begins with the upper arms in the scapular plane "to not only be a safer and more comfortable position for the shoulder joint to be in... But also more effective for overhead pressing". Neither of these papers actually makes any claims even remotely close to what the author is alleging. I'll summarise these papers below.
Reinold, M. M., Escamilla, R., & Wilk, K. E. (2009). Current Concepts in the Scientific and Clinical Rationale Behind Exercises for Glenohumeral and Scapulothoracic Musculature.
- The paper is a literature review, not a study, and consists of biomechanical explanations of the roles of the individual muscles of the shoulder, and recommended exercises for targeting those specific muscles in the context of a rehabilitation program. (Note: The paper is not a systematic review, so does not have any formal process of study inclusion. It's more of an evidence-supported opinion piece.)
- The only mention of any overhead pressing exercise is that the military press is listed among examples of exercises in which certain shoulder muscles are active. (Specifically, the deltoid, subscapularis, serratus anterior, and trapezius.) No recommendations are given for technique.
- The scapular plane is only mentioned as being relevant to the role of some muscles. E.g. The supraspinatus works as a shoulder abductor only during initial abduction (e.g. raising the arm from hanging downwards, up by just 30-60°) when the arm is in the scapular plane. It is never stated that the scapular plane is a specifically safer or more comfortable position for the shoulder.
Pink, M. M., & Tibone, J. E. (2000). THE PAINFUL SHOULDER IN THE SWIMMING ATHLETE.
- This paper is also a literature review, focussing on the mechanisms behind and techniques for rehabilitating overuse injuries in swimmers.
- The only mention of any overhead pressing exercise is that a "modified military press" is prescribed as an advanced exercise during the rehabilitation process. This exercise is defined as a seated dumbbell press, performed with the elbows kept out to the sides of the body, and is explicitly described as not in the scapular plane.
- The scapular plane is mentioned in a description of initial exercises for swimmers suffering chronic overuse injuries. These are extremely remedial exercises for swimmers with severe chronic shoulder pain - we're talking just unweighted arm raises, which the patient may not even be able to perform without pain.
- The paper does include the following statement: "Exercise should begin in the scapular plane. This plane allows for maximal congruency of the humeral head in the glenoid and the least stress on the capsule and ligaments." However, this is clearly in the context of the aforementioned unweighted rehabilitation exercises, being used only as an initial rehabilitation exercise patients with lax and unstable shoulders, and is definitely not intended as a exercise prescription in any other contexts.
I think that with even the most charitable reading of these papers or the overhead press article's claims, that no one could possibly claim that the author of that overhead press article is likely to have actually read these papers and have attempted to accurately represent them in his article.
Furthermore, it should be noted that the scapular plane is not at a fixed 30° angle from the frontal plane, but rather is defined relative to the scapulae, and so moves when the scapulae retract and protract. During an overhead press, the scapular plane actually goes from being much closer to the frontal plane at the bottom of the lift, to point more forward at the top. This makes attempting to use it to define an overhead press bar path especially futile.
Second question: why is [moving the elbows outwards towards the top of the movement] necessary? Why can't we continue the movement with our elbows in the scapular plane?
The scapular plane is a specific path of shoulder abduction, but all (complete) shoulder flexion and abduction has the same start and end point. The arm begins pointing downwards, and finish pointing upwards. The end position is the same regardless of whether you raised you arm to the front, to the side, or in the scapular plane. If you look at the scapular plane image in the question, you can see that the scapular plane, the frontal plane, and the sagittal plane all overlap in a line directly over the shoulder. So in this end position, you are actually in all three planes. It's not that you're moving out of the scapular plane.
Third question: How can we avoid shoulder impingement?
Impingement can (but does not necessarily) occur at the extremities of shoulder rotation. This is why exercises like the upright row and guillotine bench press are often maligned - because they put the shoulder into extreme internal rotation as the bar approaches the collarbones. Impingement is also a common diagnosis among baseball pitchers, who wind up by putting their shoulders into incredible external rotation.
A sensible approach to avoiding impingement would just be to stop and reassess any movements that cause you to experience shoulder pain. A more conservative and probably excessive approach would be to just avoid any exercise that pushes the limits of shoulder internal or external rotation.
That said, perhaps the most ridiculous thing about this overhead press article is that the technique he advocates actually does put the shoulders into extreme external rotation. At the bottom of the lift, he doesn't appear to even be able to lower the bar below his throat because his requirement of keeping the elbows in the scapular plane requires increasing amounts of shoulder external rotation as the bar is lowered. If you imagine giving someone a high-five, and they push your hand back in that position as far as it will go, that's the feeling of hitting the end of your range of external rotation. It could be argued that this scapular plane overhead press actually increases the risk of shoulder impingement.