American football

From Academic Kids

For details of the origins and development of American football, see the history of American football article. For information on other sports known as "football", see the football article.
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The ball used in American football has a pointed oval shape, and usually has a large set of stitches along one side.

American football, known in the United States simply as football, is a competitive team sport. The object of the game is to carry the ball to the opposing team's end zone and score points. The team with more points when time has expired wins.

Outside of the United States and Canada, the sport is referred to as 'American football' or sometimes 'Grid-iron' football to differentiate it from association football (soccer), a sport that it does not resemble. American football evolved as a separate sport from Rugby football in the early 20th century. Canadian football and arena football are basically forms of American football.



Since the 1990s, football has surpassed baseball as the most popular spectator sport in the U.S. The 32-team National Football League (NFL) is the most popular professional league. Its championship game, the Super Bowl, is watched by nearly half of US television households, and is also televised in over 150 other countries. Super Bowl Sunday has become an annual ritual in late January or early February. Additionally, top players in the league are selected to play an annual all-star game, the Pro Bowl, in Honolulu. The NFL also operates a sort of spring-training league, NFL Europe, with 6 teams based in European cities.

A  player runs with the ball as an  player tries to thwart his progress.
A Colorado State University player runs with the ball as an Air Force Academy player tries to thwart his progress.

College football is also popular, with many major colleges and universities playing NCAA (National Collegiate Athletic Association) Division I football, and consistently selling out huge stadiums. College games are widely televised and widely watched. Many institutions in lower NCAA divisions and the National Association of Intercollegiate Athletics (NAIA) also field varsity football teams, as do most high schools. High school football is popular in many parts of the U.S., notably the South, with games sometimes attracting tens of thousands of fans.

In addition, football is played by amateur, club and youth teams (such as teams in the Pop Warner leagues). There is an American Football World Cup. In addition, there are many "semi-pro" teams, where the players are paid to play, but at a small enough salary that they generally must also hold a full-time job.

Professional, college, and other leagues

Football is played at a number of levels in the United States and abroad. These include the following:

The descriptions in this article are based primarily on the current rules of the National Football League (NFL, 1920-present). Differences with college rules will be noted. Professional, college, high school, and amateur rules are similar.

Professional leagues that no longer exist include the World Football League (WFL, 1974-75), the United States Football League (USFL, 1983-1985), the XFL (XFL, 2001), the All-America Football Conference (1946-1949) (2 teams are now in the NFL), the World League of American Football (WLAF, 1991-1993 — now NFL Europe), and, the American Football League (AFL, 1960-1969). Only the AFL survives, as it merged with the NFL in 1970 and now exists (mostly) as the AFC.

As with most other American sports leagues, no promotion/relegation schemes exist between the NFL and these other leagues.

The game

Main article: American football rules

Game play in American football consists of a series of individual plays of short duration, outside of which the ball is not in play. These plays are often referred to as "downs." Unlimited substitutions can be made between plays.

Methods of scoring

Points can be scored in the following ways:

  • A field goal, worth 3 points, is scored by kicking the ball between the uprights of the goal posts. In the NFL Europe league, Field Goals over 50 yards (measured as the distance from the point where the ball is kicked to the end line) attract 4 points.
  • A touchdown, worth 6 points, is scored when the ball is advanced into the opponent's end zone. Immediately after a touchdown, the scoring team may run a single play called a try (more commonly called an extra point or conversion attempt) from just outside the end zone. They can either attempt to kick the ball through the uprights (worth one point, for a total of seven) or advance the ball into the end zone (worth 2, for a total of eight) on this play. College football and NFL Europe offer the non-scoring team the opportunity to score 2 points should they return a fumble, interception or blocked kick to the other team's end zone. In the NFL, a blocked extra point kick results in the play being whistled dead immediately.
  • A safety is a rare event occurring when a player causes the ball to become dead in his own end zone. When this occurs, the opposing team scores 2 points and gains possession from the ensuing free kick (see "Kickoff" below). Typically this occurs when a ballcarrier is tackled in his own end zone, a player's fumble goes out of bounds in his own end zone, a blocking or intentional grounding penalty is assessed against the offense in its own end zone, or a punt is blocked back out of the end zone.


Collegiate and professional football games are 1 hour long, divided into four quarters of 15 minutes each. Separating the second and third quarters is a halftime. If a game is tied at the end of four quarters, overtime, or sometimes called "OT" for short, is played. Professional overtime is played in 15-minute "sudden death" periods, meaning that the team that scores first, by any means, wins. In the regular season, a game that ends in overtime is a tie. However, in the post-season where a team must win, 2nd or even 3rd overtime is granted. The booth makes all challenges and a team is given only 2 time outs during overtime. In college football, a shootout style overtime system ensures that each team has equal opportunity to score. Many high schools play 12-minute quarters instead of 15.


A kickoff starts each half, and also restarts play following a field goal or touchdown. At the beginning of a half, the kicking team is determined by coin toss. After a team scores a field goal or touchdown, it kicks off the ball to its opponent. The ball is placed on a tee and kicked off at the kicking team's own 30-yard line in the NFL, or its own 35-yard line in college football. The ball is usually kicked as far as possible down the field, but sometimes a team will attempt to recover its own short kick, in a play known as an onside kick. The receiving team may catch and attempt to advance the ball at any time after the kick, but the kicking team may not touch the ball until it has traveled at least 10 yards.

Following a safety, a free kick similar to a kickoff is used to restart the game. The team that was scored upon kicks the ball from its own 20-yard line. In this case, a tee cannot be used, so the ball is usually punted. As a result, the ball usually travels a shorter distance than a regular kickoff, generally giving the receiving team better field position.

Challenging plays

In the NFL, a team's coach may "challenge" a play if he thinks an official's ruling is incorrect. He does so by tossing a red flag (like the yellow flags used by officials to denote penalties) onto the field. The referee then has 90 seconds to review multiple videotaped angles of the play, and then he either overturns or upholds the ruling. If the ruling is upheld, the team that challenged the play is charged with a time out. A team is given only two challenges per game. However, in the 2004 season the rules were changed so a team would be granted a third challenge if the first two were successful. A play can only be challenged before the next one has started.

In the last two minutes ("two-minute warning") of each half and in overtime periods, the "red flag" system is suspended, and only the instant replay official can decide whether to review a play. This is meant to curb abuse of the system in which a team could enjoy two free time outs at the end of the game by saving their challenges beforehand.

This system was introduced in 1999. [1] (http://www.nfl.com/news/990526replaytechnology.html) An earlier rule for the use of "instant replay" to reverse referees' rulings existed for six seasons starting in 1986. This earlier system did not utilize the red flags; an extra referee, called the "Instant Replay Official", would himself decide whether to review each play. The old system was criticized for slowing the pace of many games, and was killed by the NFL owners after the 1991 season.

The field

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  • NFL: 70? feet (21? m) from the sidelines; this means that the hash marks are separated from one another by the width of the goalposts.
  • College: 60 feet (18? m) from the sidelines.
  • High school: 53 1/3 feet (16? m) from the sidelines, dividing the field into thirds.


At the center of each end line is a set of goal posts, which have two upright posts extending above a crossbar. The distance between upright posts is 18? feet in general and 23 1/3 feet in high schools (5? m, 7 m), and the top of the crossbar is 10 feet (3 m) above the ground.

Play of the game

A game consists of many individual plays. The vast majority of these are scrimmage plays. Each play from scrimmage is one of a series of downs given to the team with possession. These two concepts, the concept of scrimmage, and the concept of downs, are fundamental to American football, and are what distinguish it, as well as Canadian football, from most other forms of football. However, rugby leagues do have a similar system where each side is allowed to be tackled six times while in possession before surrendering possession (see the entry for rugby league for an explanation of the play-the-ball and the limited tackles rule).

The team with possession of the ball is called the offensive team, and the other team the defensive team. A set of downs begins with a first down, which is given to a team either after it has just gained possession on the previous play, or it has gained the necessary yardage from a previous set of downs. On a first down, the offensive team is given four downs to gain 10 yards. This is commonly referred to as "first and ten", meaning that it is first down, and ten yards are needed to get another first down. The line a team must reach in order to gain a first down is technically called the line to gain or the necessary line, although it is commonly called the first down line. In the event a team gains a first down and the new line of scrimmage is within ten yards of their opponent's goal line, the goal line becomes line to gain. This is commonly known as "first and goal," signifying that no more first downs can be achieved, and it is necessary to score on the current set of downs. Failure to gain the necessary yardage on a set of downs results in a "turnover on downs."

Plays from scrimmage

Football players line up before the play during the 2002 annual .  , on the left, has possession.
Football players line up before the play during the 2002 annual Army-Navy college football game. Navy, on the left, has possession.
Each down is a play from scrimmage. Prior to each play from scrimmage, the two teams line up on opposite sides of a line of scrimmage, which is defined by the spot of the ball from the previous play. The spot is, in most cases, the yard line at which the ball became dead on the previous play, plus or minus any penalty yardage. A down, or play from scrimmage, begins with a snap and ends when the ball becomes dead for any reason. In a snap, the center either hands the ball between his legs to the quarterback, or tosses it backwards between his legs to the quarterback or sometimes another player, such as a punter or a holder for a field goal attempt. The ball may become dead, ending the down, because a player in possession is tackled, or because his forward progress is stopped, or because he goes out of bounds, or because a forward pass goes incomplete, or because a player makes a "fair catch" (see punts below). Each play from scrimmage can be either an attempt to advance the ball, a punt, or an attempt a field goal.

Advancing the ball

There are two methods of advancing the ball while still maintaining possession:

  • Running the ball - The quarterback, who normally receives the snap, either hands the ball or throws a lateral pass to a running back, who then becomes the ball carrier. Most other players on the offense have blocking assignments, and attempt to prevent the defense from tackling the ball carrier. The quarterback may also run the ball himself.
  • A forward pass - A forward pass may only be thrown on a play from scrimmage, and only from behind the line of scrimmage. It must be thrown to an eligible receiver (any player who is not an interior lineman). A completed pass is one caught by a member of the offense, although if the first player to touch the ball is not an eligible receiver, a penalty results. The player may run with the ball after catching it. To be considered "in bounds" a receiver must have clear possession of the ball and place both feet (NFL) or one foot (college) in bounds prior to stepping out of bounds. An incomplete pass is any forward pass that either hits the ground or goes out of bounds, at which point the ball becomes dead, and is spotted at the preceding line of scrimmage for the following play. An interception is a pass caught by the defense, which transfers possession to the defending team.

It is important for the offense to run a variety of running and passing plays in order to keep the defense uncertain of the next play.

Fourth down situations

If a team uses all four of its downs without gaining the yardage for a first down or a touchdown, and without kicking a field goal, possession shifts to the other team. This is called turning the ball over "on downs." Fourth down situations are therefore pivotal. The offense has the same three choices as on any other play from scrimmage: advance the ball, punt, or attempt a field goal — but the decision is often more difficult and important.

Offensive options on fourth down:

  • "Go for it" - despite the risk involved, a team may always elect to "go for it" on fourth down by making one last attempt to reach the first down marker or the goal line, mounting a regular running or passing play to get there (just as they did on the first three downs). This is most common when, due to a team's success on the first three downs, the distance required for a first down is short; or when it is trailing late in the game by more than three points (the value of a field goal). The risk is significant: failing to make the next first down or score gives possession of the ball to the opposing team, usually with better field position than would have resulted from a kick. It's often wise to kick on fourth down.
  • Punt - If the team thinks it is too far away to attempt a field goal, it may punt the ball to the other team in order to gain better field position, for their defense.
  • Attempt a field goal - Field goal attempts must be made with the ball on the ground (they cannot be punted), so a player called a holder holds the ball for a kicker. In times past, a kicker might have tried a "drop kick" — that is, dropping the ball and kicking it after it bounces off the ground — and if the kicker kicks it through the goalposts, it is a field goal. This is difficult to do, as the ball is in the shape of a prolate spheroid and its bounce is unpredictable. Nowadays, the only time you will see this is by a hurried kicker after a broken play. Failed field goal attempts, if they are short, can be returned by the opponent, but the ball usually goes past the end line and can't be returned. If the field goal attempt fails, the ball is spotted at the original line of scrimmage, and possession is given to the other team. (In the NFL, failed field goal attempts are spotted at the spot of the kick or the 20, whichever is farther from the goal line.) Field goals can also be attempted on other downs, but this is only seen in situations where a field goal will either win or tie the game and the distance to kick the field goal is well within range of the kicker.

A team will occasionally run a trick play on fourth down. They will line up in a punting or field goal formation, but will instead run the ball or pass it in an attempt to pick up a first down.

Specialized units and players

With its unlimited substitutions, American football is highly specialized, with most teams having three specialized units: an offensive unit, a defensive unit, and special teams. There are many specialized players within each unit. Some players may only be used in certain situations. (for details see: offensive unit, defensive unit, special teams, linemen, defensive schemes.)

A list of player types and definitions can also be found in the Glossary of American football.

Common penalties

There are many rules in American football which result in a penalty when broken. In most cases the offending team loses 5, 10 or 15 yards, meaning the ball is moved that distance towards their own end zone. There may also be a loss of down for some offensive penalties. Conversely, a defensive penalty may result in an automatic first down. Teams have the option to decline a penalty called against its opponent; this is sometimes beneficial. Usually, no penalty may move the ball more than half the distance toward the penalized team's goal line.

Some of the most common penalties are listed below. For a more detailed discussion, see American football rules.

Note: The neutral zone is the space defined by lines drawn through the ends of the ball parallel to the yard lines when the ball is spotted and ready for play. No player may legally have any part of his body in the neutral zone when the ball is snapped, with the exception of the center.

Penalties against the offense

  • Illegal procedure (5 yards) - Most commonly, any player moving after they have gotten in their set position before the snap in a way that simulates the start of the play. Other illegal procedure penalties include linemen not getting into a set position, not having at least 7 men on the line of scrimmage, and in college football, illegal substitution.
  • Delay of game (5 yards) - allowing the play clock to elapse before the snap
  • Holding (10 yards) - illegal use of the hands or arms while blocking; an automatic safety is assessed instead if spot of infraction is within the offensive team's own end zone.
  • Offensive pass interference (10 yards) - interfering with a defender attempting to catch a pass
  • Intentional grounding - throwing the ball into the ground to avoid being tackled
    • College penalty: Spot of foul and loss of down
    • In both NFL and college, intentional grounding from the offensive team's own end zone constitutes an automatic safety unless the defense chooses to decline the penalty, which might only ever happen if the infraction had occurred on a fourth-down play. If the quarterback has moved outside of the area between his offensive tackles (the "pocket"), there is no penalty for grounding the ball if the quarterback throws the ball past the line of scrimmage. There is also no penalty for "spiking" the ball to stop the game clock, by throwing it directly into the ground. However, such an action must be executed immediately after the snap of the ball, before the quarterback demonstrates intent to make a forward pass.
  • Illegal blocks
    • Illegal block in the back (10 yards) - an illegal block from behind and above the waist
    • Clipping (15 yards) - an illegal block from behind and below the waist
    • Illegal crackback block (15 yards) - an illegal block, from any direction, below the waist by any offensive player not on the offensive line (e.g. wide receivers, quarterbacks and running backs)
  • Ineligible receiver (10 yards) - The center, guard, and tackles are considered ineligible receivers(although they may become eligible before the play if the offensive team notifies the referee who will notify the other team. As such if the ineligible player goes downfield before the pass or touches the pass, this penalty is called. The exception is if the ball is tipped by a defender, then anyone is eligible to catch the ball. In the NFL ineligible receivers have the numbers 50-79 and 90-99.

Penalties against the defense

  • Offsides (5 yards) - Being across the line of scrimmage before the ball is snapped, or being in the neutral zone when the ball is snapped. When called, the official throws a flag, but the play continues.
  • Encroachment (5 yards) - A "defensive false start," called when a defensive player contacts an offensive player or crosses the offensive line before the snap. When called, the official throws a flag and blows his whistle, stopping the play.
  • Running into the kicker (5 yards) - during a kick from scrimmage
  • Pass interference - interfering with a receiver's attempt to catch the ball.
    • NFL: This can be a devastating penalty because the ball is moved forward to the location of the interference, as if it had been caught.
    • High school: 15 yards and an automatic first down
  • Defensive holding or Illegal use of hands (5 yards and an automatic first down; in college, the chains are not moved if the previous play was 1st and 10, making the next play 1st and 5, as in "Offsides" or "Encroachment" above) - illegal use of the hands or arms either while attempting to ward off a block, or to cover a receiver
  • Illegal contact (informally, illegal chuck, 5 yards and an automatic first down; in college, same exception noted in "Defensive holding" above applies) - any contact made between a defender and a receiver after the latter has traversed more than five yards beyond the line of scrimmage; this rule was adopted in 1978, and its enactment is regarded as contributing to the dramatic increase in both passing yardage and scoring the NFL has witnessed since that time.
  • Piling on (15 yards and automatic first down) - unnecessarily falling on or jumping on any player who has already been downed
  • Roughing the kicker (15 yards and automatic first down) - tackling the kicker after he has kicked the ball
  • Roughing the passer (15 yards and automatic first down) - tackling the quarterback after he has thrown a forward pass. The defender is expected to make a reasonable effort to avoid the passer; if, once the ball is thrown, contact is imminent or inevitable, no penalty is called.

Penalties against either team

  • Too many players on the field (5 yards)
  • Grabbing the face mask (5 or 15 yards) - If there is pulling, twisting or turning, 15 yards; otherwise 5 yards. In college, any face mask penalty on the defense results in an automatic first down; in the NFL, only the 15-yard ("flagrant") face mask penalty results in an automatic first down, but in the case of a 5-yard penalty the down remains the same.
  • Unsportsmanlike conduct (15 yards on the kickoff) - Any conduct by anyone involved in the game—usually a player, but occasionally a coach, and very rarely one or more spectators—deemed to be especially objectionable by the game officials, or by rule. The penalty is usually strictly enforced more in college football than in the NFL. However, after several outrageous incidents in the 2003 season (Joe Horn took a call from his cell phone hidden in the field goal stand, Terrell Owens signed a ball and threw it in the crowd, he also grabbed a cheerleaders pompoms, the list goes on) the NFL enforced a strict “no-nonsense” crackdown in 2004. Any introduction of a foreign object in celebration was a sure penalty, and anything else a referee deemed to be inappropriate, lewd, or excessive was also the usual 15-yard setback.
  • Unnecessary roughness (15 yards) - Tackling or striking another player after the ball is dead or when the player is out of bounds. Repeated infractions or especially severe fouls may result in the player's ejection from the game.

Development of the game

For more information see the article: History of American football.

Both American football and soccer have their origins in varieties of football played in the United Kingdom in the mid-19th century, and American football is directly descended from rugby football.

Rugby was first introduced to North America in Canada, brought by the British Army garrison in Montreal which played a series of games with McGill University. In 1874, McGill arranged to play a few games in the United States, at Harvard University, which liked the new game so much that it became a feature of the Ivy League. Both Canadian and American football evolved from this point. For an in-depth overview of the differences and similarities of Canadian football and American football see: Comparison of Canadian and American football

American football in its current form grew out of a series of three games between Harvard University and McGill University of Montreal in 1874. McGill played rugby football while Harvard played the Boston Game, which was closer to soccer. As often happened in those days of far from universal rules, the teams alternated rules so that both would have a fair chance. The Harvard players liked having the opportunity to run with the ball, and in 1875 persuaded Yale University to adopt rugby rules for their annual game. In 1876 Yale, Harvard, Princeton, and Columbia formed the Intercollegiate Football Association, which used the rugby code, except for a slight difference in scoring.

In 1880 Walter Camp introduced the scrimmage in place of the rugby scrum. In 1882 the system of downs was introduced to thwart Princeton's and Yale's strategy of controlling the ball without trying to score. In 1883 the number of players was reduced, at Camp's urging, to eleven, and Camp introduced the soon standard arrangement of a seven-man offensive line with a quarterback, two halfbacks, and a fullback.

On September 3, 1895 the first professional football game was played, in Latrobe, Pennsylvania, between the Latrobe YMCA and the Jeannette Athletic Club. (Latrobe won the contest 12-0.).

By the 1890s interlocking offensive formations such as the flying wedge and the practice of teammates physically dragging ball-carrying players forward had made the game extremely dangerous. Despite restrictions on the flying wedge and other precautions, in 1905 eighteen players were killed in games. President Theodore Roosevelt informed the universities that the game must be made safer. To force them to respond to his concerns, he threatened to pressure Congress to make playing football a federal crime.

In 1906, two rival organizing bodies, the Intercollegiate Rules Committee and the Intercollegiate Athletic Association, met in New York; eventually they agreed on several new rules intended to make the game safer, among them the addition of a neutral zone between the scrimmage lines and a requirement that at least six players from each team line up on them. The most far-reaching innovation they considered, though, was the legalization of the forward pass. This was very controversial at the time, much derided by purists. As an alternative means of opening out the play, Walter Camp would have preferred widening the field; but representatives from Harvard pointed to recently constructed Harvard Stadium, which could not be widened, and the forward pass was adopted; it has come to shape the whole history of American football, as opposed to its cousins around the world.

In 1910, after further deaths, interlocking formations were finally outlawed; and in 1912 the field was changed to its current size, the value of a touchdown increased to 6 points, and a fourth down added to each possession. The game had achieved its modern form.

Beyond recreation and entertainment

Football scandals

There is a long history in the second half of the 20th century of controversy over the tension in college football between values important to the institution's academic mission and the team's win-and-loss record. Many observers suggest that athletic talent plays a disproportionate role in college admission. Measures that are seen as effective in maintaining players' academic eligibility but not in furthering their graduation or competence in their fields of study attract similar criticism.

Recent attitudes toward and awareness of steroid use are also evolving. Fans and critics actively debate the role of steroids in professional and amateur football.

Football and drugs

Contemporary football players are larger than their predecessors of only 30 or 40 years ago. It is quite normal, for instance, for all the members of the offensive line of a major college or professional team to weigh more than 300 pounds (136 kg.), whereas in the 1960's linemen who weighed only 270 pounds were common. The increase in player size has led to an increase in the frequency and severity of injuries.

Since nutritional standards and weight-training technique were already quite advanced even in the 1960's, it has been conjectured that much of the increase in the size of the players is the result of the widespread availability of illegal anabolic steroids, which facilitate increased growth of muscle tissue. Such drugs are widely available even to high school players.

Because anabolic steroids have dangerous side effects, the National Football League tests its players for steroids, and penalizes those who are caught. However, it has recently emerged that new varieties of steroids are being developed in clandestine laboratories, which elude existing drug tests. Hence there is a kind of "arms race" between the scientists who develop new kinds of illegal steroids and those who develop tests to detect them.


Despite the helmets and heavy padding worn by all players on the field, injuries are common in football. An "Injury Report" section is ubiquitous in American newspapers' sports sections, detailing, for each injured player on each team, his injury and the amount of time he is expected to be out. Twice-weekly during the season (Wednesdays and Fridays), all NFL teams must report the status of their injured players, or be subject to a fine from the league. The standard severity descriptions are "out" (will not play in the coming game); "doubtful" (25% chance of playing); "questionable" (50% chance of playing); or "probable" (75% chance of playing). Note that teams occasionally manipulate their injury reports, minimizing or maximizing the extent of a player's injury, as an attempt to strategically deny their upcoming opponents a clear picture of the team's health. Similar systems are in place for most major American sports.

The NFL has a roster limit of 53 players per team during the season; 45 of which dress for a game plus an "emergency quarterback" who only plays if all the quarterbacks on the 45-man roster are out of the game. Players who are injured are frequently among the eight that do not dress. If it becomes certain that a player will not play for the rest of the season, the team may put him on the "Injured Reserve" list and replace the player on the roster.

An average of about eight players die each year in the United States as a result of injuries received in games at all levels. About 160 concussions occur every season, and the National Football League now collects benchmark measures of awareness for each player, which can be used during a game to judge whether he has been concussed.

Injuries sustained by football players often are permanent. Many former football players experience pain, sometimes severe, that lasts for the rest of their lives. Many players require surgery, even multiple surgeries, for injuries experienced years earlier.

Interestingly, newspaper reporters who have interviewed former football players who are crippled or in pain as a result of their former sport find that a player will never (or virtually never) express regret over his choice of career. The players often state that the thrill of playing football was worth the price of a lifetime of subsequent pain.

Deaths and long-term disability attributed to illegal use of anabolic steroids have become a new factor in this picture, starting in about the 1990s.

Instances of heat-related death, especially during professional practice sessions, have begun receiving press attention in the decade of the 2000s, and led to new standards intended to respond cautiously to possible danger signs that traditionally had been ignored. There is also the prospect that conventional first-aid technique has been in error, and an apparatus to circumvent this: apparently efforts to cool an overheated patient quickly, by wetting a large fraction of the body, are misguided, with the sudden chilling of the skin causing the body to reduce superficial circulation, and making that chilling near the surface ineffective at cooling the core of the body and thus the brain. A device suitable for professional teams has been developed, that provides for rapid cooling of small areas of skin where large blood vessels are near the surface, and is proposed as a means of cooling the blood quickly without evoking the reflex of isolating the body surface from the core.

Certain penalties have been implemented in an attempt to reduce the number of more serious injuries. An example of this is the illegal "crackback block", consisting of a block below the waist by a player entering the line of close play. These blocks are infamous for causing severe leg injuries.


  • National Football League. Digest of Rules (http://ww2.nfl.com/fans/rules/index.html). Retrieved January 15, 2005.
  • National Football League. History and the basics (http://ww2.nfl.com/basics/history_basics.html). Retrieved January 15, 2005.

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