The National Football League will implement a series of new rules this season, some of them apparently designed to prevent the recurrence of controversial, and even tragic, plays from last year.

Among the changes for this year:

If an offensive player fumbles the ball anywhere on the field during a fourth-down play or on any down after the two-minute warning in either half, only the fumbling player is permitted to recover and/or advance the ball.

One of last season's most controversial plays was executed by the Oakland Raiders on fourth down in the closing minutes of a game with San Diego. The ball was "fumbled forward" into the end zone, where the Raiders recovered for a winning touchdown.

Also this season the kicking team will not be allowed to recover a fumble on a kickoff and convert it to a touchdown. The Minnesota Vikings were the victims of such a play last year, and films showed that the Viking kick returner had not fumbled in the first place.

This season if the kicking team recovers a fumble, it will assume possession where the ball was first touched by the receiver. If it was first touched in the end zone, play will resume at the one-yard line.

Following a 1978 preseason tragedy in which New England's Darryl Stingley was left paralyzed after being tackled by Oakland's Jack Tatum, the league has clarified its roughness violation.

It is now specifically illegal for a player to use his helmet to butt or ram an opponent or to use the crown of the helmet unnecessarily. In another move designed to curtail injuries, the league will allow officials to blow a play dead as soon as the quarterback is clearly in the grasp of a tackler, rather than waiting for him to be slammed to the ground.

On kicking plays this year, players on the receiving team can not block opponents below the waist during kickoffs, punt or field-goal attempts.

Other rule changes:

The zone in which crackback blocking is banned has been extended from three to five yards on either side of the line of scrimmage.

Defensive lineman can now wear numbers in the 90s.

Any player can be penalized for unsportsmanlike conduct when he commits a noncontact act such as throwing a punch, or a forearm or kicking at an opponent -- even if he misses.