Muscle engagement in HS PE2 iM

Which muscle groups should be engaged during the PE2 iM? My upper traps are always sore after the iM---is that normal? Should my upper traps be sore? I know---I need to post a form check!--but in the meantime, hoping for an academic discussion.

Generally when scapulare are retracted the upper traps work more than the lower and medium especially if you have the tendency to have the scapulae flared out. also triceps and rear delt are involved.

edited:  realised this was im not pe.    arent we meant to upwardly rotate the scapular while elevating rather than retract in this im?  as we would in a handstand. also how are triceps and rear deltoid involved in this aside from being elongated? 

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it is practically impossible to upward rotate the scapulae here, basically muscles that provides a certain amount of rotation are completely passive.