The behind the head military press is an exercise still commonly performed as a better way to isolate the shoulders and reduce pec involvement from a conventional military press. Unfortunately, the BHMT causes a multitude of problems for most lifters, especially those with limited shoulder and thoracic mobility. In other words, this is a shit exercise for about 95% percent of the population because kyphotic posture is so rampant due to sedentary desk jobs.
Okay smart guy well why is the exercise bad?
Well, when the barbell is lowered behind the head the shoulders move into an extreme external rotation, placing a boat-load of stress on the joint and rotator cuff muscles. In addition, the neck is flexed to move the head forward to create more room for the bar to pass behind the head, adding to the postural issues most face already with neck muscles.
Although the BHMT does stress the muscles to a great degree, the risk is not worth the reward. If you want beat up shoulders, a damaged rotator cuff, and to further emphasize poor posture this exercise may be for you. If not, never do them again.
Better Exercise: Perform a standard military press, which you can learn more about here**. This hits the shoulders, upper back, triceps, and core without the increased risk of severe shoulder injuries.