A study carried out by The AP has shown that illegal hits to the head or neck area still take place almost once a game in the NFL.

The issue has been one of the league's main priorities in recent years, with new rules being introduced in an attempt to stop players tackling opponents when they are vulnerable and can not defend themselves.

However, according to the study, which looked at the first 11 weeks of this season, 491 of the 549 penalties reviewed were for major transgressions such as unnecessary roughness, unsportsmanlike conduct, facemasking and roughing the quarterback.

The AP found that 156 penalties from the opening 162 matches involved contact with the head and neck, an average of just under one per game. In comparison, the same period produced 35 penalties awarded for low hits.

Of those penalties, 93 were for hits to the head while 38 were for pulling face masks and 25 were for horse collars, with quarterbacks and receivers the positions to suffer most from the illegal plays.

The league has so far declined to comment on the study's results.