One of the most controversial exercises is a behind the neck press. There are many that say you should never do it, then there are those that refer to the fact that it was almost a staple for strength amongst old school lifters. This advice from opposite sides tends to leave most confused about what to do, and eventually people pick one side or another.
The behind the neck press works your front, lateral (side), and rear deltoids (shoulders); Trapezius muscles (upper back, below neck); triceps brachii (back upper arm); and serratus anterior (armpit over rib cage)
The truth is that it can be a great exercise to build size and press strength. However, the risk of injury is also very high when done by someone that has difficulty with the following physical setup. You will be seated with your legs at 90 degrees. That first portion seems simple enough, however there are many with drastic posterior pelvic tilt (forward posture from the pelvis observed in static posture which is greatly exacerbated with movement and affected in a seated position as well). If this is something that hinders you, adding a weight above is not in the stars for you. Stay away from it, there are other things you must work on to correct, and other ways to do various exercises.
Next you must create abdominal tension by bracing your core which will help hold the weight that is trying to return to the ground by giving into gravity. If bracing your core is a work in progress or is a foreign concept to you, then do not do behind the neck presses (or any overhead presses at that if you haven’t practiced bracing your core).
Next let us move to your upper back and shoulders. There is a great requirement for your shoulder to remain firmly in its socket. Any nagging shoulder injuries, hypermobility in the shoulder joint, or other weakness there, demands you address all that (and take the time you need before even thinking about doing behind the neck presses). It also requires you to retract and depress your scapula, and have proper anterior serratus* activation (yes the ability to use the muscles you are trying to work)
*not an easy muscle to access unless you have put in consistent work and have greater body awareness
Now, for those seasoned lifters and the exceptionally strong – you don’t have to worry about these various aspects because practice and habit take over soon upon putting your hands on the bar and the joints and muscles are placed and activated in the appropriate manner.
It is important to know that you must give it thought and practice. Work and hold everything that is required that will help you with the push and provide stability to bring it back down under control. Take your time and practice with a dowel. Then move to a light barbell, then an empty Olympic barbell, and keep working your way up slowly. This movement requires absolute knowledge of one’s own body, the capability to activate various muscles, patience, and no postural issues or weakness that puts your lumbar spine or cervical spine into an extended position so that spinal injuries can be avoided. Do not ever jump into this exercise if you are only beginning your journey towards strength, it can make you strong or stop you dead in your tracks.