There was a thread about a year or so ago that had a link to a neck exercise program to strengthen the neck. It was suggested to help those with subtle neck injuries that need to help their necks recover.

The concept that stronger necks can help prevent concussions is disputed. It is based on an observation that football players with stronger neck conditions suffer less concussions. This may be a misunderstanding of the issue. Univ of Pittsburgh has found that 80% of concussions have neck injuries and many of their symptoms are related to their neck injuries. This concept of stronger necks preventing concussions may simply be that the neck related symptoms of concussions are less thus it appears that stronger necks prevent or reduce concussions.

If one looks at the physics involved. A hit to the head is a hit to the head, regardless of the strength of the neck. The energy of the impact is absorbed either way. The only way to change how the same impact forces can cause less harm is to spread the forces over a longer period of time. This is usually done with helmet padding that distorts so an impact that may last 0.01 seconds lasts 0.03 seconds. This allows acceleration or deceleration forces to be reduced by a similar ratio.

Keep in mind that this concept is based on football or sports related concussions. Banging one's head into a concrete wall is not going to be less injurious with a stronger neck.