If you're using the pushup-plus exercise to activate the serratus anterior, recent research concludes that traditional pushups work just as well. In a small study published in BMC Musculoskeletal Disorders [2015; 16, 23], researchers studied the effects of PUP variants on (among other things) the electromyographical activity of four shoulder muscles, including the SA, during concentric contraction. They found that the highest EMG activity of the SA occurred at 55 degrees of elbow extension during the concentric phase of the PUP and not at the plus phase.
One note: If, however, your clients are doing a modified pushup, add in the plus.This study found the highest SA activity during the plus phase of hands-and-knees pushups.
Another way to increase SA activity? Place the hands wider apart. A study in the Journal of Physical Therapy Science [2016; 28 (2), 446-49] reports that SA activity was greater during traditional pushups performed with palms spaced farther apart (150%) than when in neutral (100%) or narrow (50%) positioning.