The phrase "neck muscles" refers to the group of muscles located in your neck area. These muscles allow you to move your head, support its weight, and control various movements of the neck and upper body. Full definition
This will help strengthen neck muscles and avoid flat spots on the head. (boppy.com)
In the first few months of life, babies can not control their heads because of still developing neck muscles. (cpsc.gov)
However, a 2014 study clearly showed that bigger and stronger neck muscles of football players were not particularly protective against head impact severity. (MomsTeam.com)