Musculus Deltoideus or what we refer to as the 'delts' or 'deltods' is a large triangular muscle covering the shoulder joint and serving to abduct, flex, extend and rotate the arm.
The Masseter muscle in proportion to its size is the most powerful muscles in the body. It raises the jaw and clenches the teeth.
There is a co-activation of jaw and neck-shoulder muscles during jaw activities and the presence of neuromuscular connections between the jaw and neck regions.
Jaw and neck muscle actions are elicited and synchronized by neural commands in common for both the jaw and the neck motor systems and that these commands are preprogrammed, particularly at fast speed.
When you clench down on your mouthguard with your powerful masseter muscle or stiffen your jaw hard to protect yourself prior to contact, the bracing also increases the EMG activity in the musculus deltoideus shoulder region.
Studies earlier in this decade got many coaches fired up about training with mouthguards in the weight room, since studies were showing increased masseter force on a mouthguard improved shoulder strength.
The bottom line is Getting the Head and Neck muscles Strong will go along way in protecting an athlete's shoulders during contact.