The push up plus has been demonstrated to have one of the HIGHEST EMG LEVELS of ANY SERRATUS ANTERIOR EXERCISE and should be a stable of your (P)REHAB routines. In addition to having high serratus anterior (SA) activation, it also has EXTREMELY LOW UPPER TRAPEZIUS (UT) activation. Building on Episode 221 , we can speculate that because the push up plus is performed in the prone position, it decreases the UT's role as a postural muscle working against gravity.
However, some patients may exhibit shoulder discomfort or pain when performing the push up plus. This may be due to the fact that kinematic studies have demonstrated that the scapula DOWNWARDLY and INWARDLY ROTATES during the "plus" or push phase of the push up plus. Why is that scapular motion significant? It's importance to consider because a downward and internally rotated scapula will decrease the SUBACROMIAL SPACE, that vital space in your shoulder where you supraspinatus tendon and subacrominal bursae lie - two commonly implicated anatomical structures in shoulder pain and discomfort.
So if you experience pain with the wall or prone push-up plus exercise or have a history of subacromial impingement syndrome, it may be BETTER to before the wall slide for targeting the serratus anterior.
Citations: Hardwick et al 2006 A Comparison of Serratus Anterior Muscle Activation During a Wall Slide Exercise and Other Traditional Exercises. Note, the article and the text refers to WALL push up plus vs wall slides, I forgot to specify that in the video Exercise Library