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|First played||November 6, 1869, Princeton vs. Rutgers|
|Team members||11 per side|
Both teams can substitute players freely between downs
|Categorization||Team sport, ball game|
Pads (shoulder and knee)
|Venue||Football field (or gridiron)|
|Olympic||No; demonstrated at the 1932 Summer Olympics|
|First played||November 6, 1869, Princeton vs. Rutgers|
|Team members||11 per side|
Both teams can substitute players freely between downs
|Categorization||Team sport, ball game|
Pads (shoulder and knee)
|Venue||Football field (or gridiron)|
|Olympic||No; demonstrated at the 1932 Summer Olympics|
American football (known as football in the United States and gridiron in some other countries) is a sport played by two teams of eleven players on a rectangular field 120 yards long by 53.33 yards wide with goalposts at each end. The offense attempts to advance an oval ball (the football) down the field by running with or passing it. They must advance it at least ten yards in four downs to receive a new set of four downs and continue the drive; if not, they turn over the football to the opposing team. Most points are scored by advancing the ball into the opposing team's end zone for a touchdown or kicking the ball through the opponent's goalposts for a field goal. The team with the most points at the end of a game wins.
American football evolved in the United States, originating from the sport of rugby football. The first game of American football was played on November 6, 1869, between two college teams, Rutgers and Princeton, under rules resembling rugby and soccer. A set of rule changes drawn up from 1880 onward by Walter Camp, the "Father of American Football", established the snap, eleven-player teams and the concept of downs, and later rule changes legalized the forward pass, created the neutral zone and specified the size and shape of the football.
American football as a whole is the most popular sport in the United States; professional football and college football are the most popular forms of the game, with the other major levels being high school and youth football. The NFL's championship game, the Super Bowl, is the most popular single-day sporting event in the United States and among the most-watched club sporting events in the world.
American football evolved from the sport of rugby football. The first American football game was played on November 6, 1869 between Rutgers and Princeton. The game was played between two teams of 25 players each, used a round ball, and resembled a combination of rugby and soccer in its rules; the ball could not be picked up or carried, but it could be kicked or batted with the feet, hands, head or sides, with the ultimate goal of advancing it into the opponent's goal. Rutgers won the game 6-4.
Collegiate play continued for several years in which matches were played using the rules of the host school. Representatives of Yale, Columbia, Princeton and Rutgers met on October 19, 1873 to create a standard set of rules for all schools to adhere to. Teams were set at 20 players each, and fields of 400 by 250 feet were specified. Harvard abstained from the conference, as they favored a rugby-style game that allowed running with the ball.
An 1875 Harvard-Yale game played under rugby-style rules was observed by two impressed Princeton athletes. These players introduced the sport to Princeton, a feat the Professional Football Researchers Association compared to "selling refrigerators to Eskimos." Princeton, Harvard, Yale and Columbia then agreed to intercollegiate play using a form of rugby union rules with a modified scoring system. These schools formed the Intercollegiate Football Association, although Yale did not join until 1879. Yale player Walter Camp, now regarded as the "Father of American Football," passed rule changes in 1880 that reduced the team size from 15 to 11 players and instituted the snap to replace the chaotic and inconsistent scrum.
The introduction of the snap resulted in unexpected strategy changes. Previously, the strategy had been to punt if a scrum resulted in bad field position. A group of Princeton players realized that, as the snap was uncontested, they now could hold the ball indefinitely to prevent their opponent from scoring. In 1881, both contestants in a Yale-Princeton game used this strategy to maintain their undefeated records. Each team held the ball, gaining no ground, for an entire half. This "block game" proved extremely unpopular with spectators and fans of both teams.
A rule change was necessary to prevent this strategy, and a reversion to the scrum was considered until Camp passed a rule in 1882 that stated that a team would have three downs, or tackles, to advance the ball five yards. Failure to do so would forfeit control of the ball to the other team. This change made American football a separate sport from rugby, and the resulting five-yard lines added to the field made it resemble a gridiron in appearance. Other major rules changes included a reduction of the field size to 110 yards yards long by 53.33 yards wide, and the adoption of a scoring system that awarded four points for a touchdown, two for a safety and a goal following a touchdown, and five for a goal from field. In 1888 tackling below the waist was legalized.
Football remained a violent sport despite these innovations. Dangerous mass-formations like the flying wedge resulted in serious injuries and occasional deaths among players. A 1905 peak of 19 fatalities nationwide resulted in a threat by President Theodore Roosevelt to abolish the game unless major changes were made. Sixty-two schools met in New York City to discuss rule changes on December 28, 1905, and these proceedings resulted in the formation of the Intercollegiate Athletic Association of the United States, later named the National Collegiate Athletic Association (NCAA).
The legal forward pass was introduced in 1906 after its suggestion by John Heisman, although its impact was limited due to the restrictions placed on its use. Further 1906 rules changes included the reduction of the time of play from 70 to 60 minutes and the increase of the distance requirement for a first down to 10 yards over three downs. To reduce infighting and dirty play between teams, the neutral zone was created along the width of the football. Field goals were lowered to three points in 1909 and touchdowns raised to six points in 1912. The field was also reduced to 100 yards long, but two 10-yard-long end zones were created, and teams were given four downs instead of three to advance the ball 10 yards. The roughing-the-passer penalty was implemented in 1914, and eligible players were first allowed to catch the ball anywhere on the field in 1918.
The first instance of professional play in American football was on November 12, 1892, when William "Pudge" Heffelfinger was paid $500 to play a game for the Allegheny Athletic Association in a match against the Pittsburgh Athletic Club. This is the first recorded instance of a player being paid to participate in a game of American football, although many athletic clubs in the 1880s offered to help players attain employment, gave out trophies or watches that players would pawn for money, or paid double in expense money. Despite these extra benefits, the game had a strict sense of amateurism at the time, and direct payment to players was frowned upon, if not outright illegal.
Over time, professional play became increasingly common, and with it came rising salaries and unpredictable player movement, as well as the illegal payment of college players who were still in school. The National Football League (NFL), a group of professional teams that was originally established in 1920 as the American Professional Football Association, aimed to solve these problems. This new league's stated goals included an end to bidding wars over players, prevention of the use of college players, and abolition of the practice of paying players to leave another team. The NFL by 1922 had established itself as the premier professional football league.
The dominant form of football at the time was played at the collegiate level, but the upstart NFL received a boost to its legitimacy in 1925 when an NFL team, the Pottsville Maroons, defeated a team of Notre Dame all-stars in an exhibition game. A greater emphasis on the passing game helped professional football to further distinguish itself from the college game during the late 1930s. Football in general became increasingly popular following the 1958 NFL Championship game, a match between the Baltimore Colts and the New York Giants that is still referred to as the "Greatest Game Ever Played". The game, a 23–17 overtime victory by the Colts, was seen by millions of television viewers and had a major impact on the popularity of the sport. This helped football to become the most popular sport in the United States by the mid-1960s.
A rival, the American Football League (AFL), arose in 1960 and challenged the NFL's dominance. The AFL began in relative obscurity but survived for several years due to a television contract with the ABC network. Competition for players heated up in 1965, when the AFL New York Jets signed rookie Joe Namath to a then-record US $437,000 contract. A five-year, $40 million NBC television contract followed, which helped to sustain the young league. The bidding war for players ended in 1966, when the two leagues agreed on a merger that would take full effect in 1970. This agreement provided for a common draft that would take place each year, and it instituted an annual championship game to be played between the champions of each league. That game began play in 1966 and came to be known as the Super Bowl.
College football maintained a tradition of postseason bowl games. Each bowl game would be associated with a particular conference, and earning a spot in a bowl game was the reward for winning a conference. This arrangement was profitable, but it tended to prevent the two top-ranked teams from meeting in a true national championship game, as they would normally be committed to the bowl games of their respective conferences. Several systems have been used since 1992 to determine a national champion of college football. The first was the Bowl Coalition, in place from 1992 to 1994. This was replaced in 1995 by the Bowl Alliance, which gave way in 1997 to the Bowl Championship Series (BCS). The BCS arrangement has been controversial, and will be replaced in 2014 by a four-team playoff system.
Football is the most popular sport in the United States. In a 2013 poll conducted by Harris Interactive, professional and college football were the first and third most popular sports, respectively, and 45% of participants ranked some form of the game as their favorite sport; professional football has ranked as the most popular sport in the poll since 1965, when it surpassed baseball for the first time. The Super Bowl is the most popular single-day sporting event in the United States and is among the biggest club sporting events in the world.
In the United States, American football is referred to as "football". The term "football" was officially established in the rulebook for the 1876 college football season, when the sport first shifted from soccer-style rules to rugby-style rules; although it could easily have been called "rugby" at this point, Harvard, one of the primary proponents of the rugby-style game, compromised and did not request the name of the sport be changed to "rugby". In countries where other codes of football are popular, such as the United Kingdom and Australia, the terms "gridiron" or "American football" are favored.
A football game is played between two teams of 11 players each. It is legal to have fewer players on the field, but playing with more on the field is punishable by a penalty. Teams may substitute any number of their players between downs; this "platoon" system replaced the original system, which featured limited substitution rules, and has resulted in teams utilizing specialized offensive, defensive and special teams squads.
Individual players in a football game must be designated with a uniform number between 1 and 99. NFL teams are required to number their players by a league-approved numbering system, and any exceptions must be approved by the Commissioner. NCAA and NFHS teams are "strongly advised" to number their offensive players according to a league-suggested numbering scheme.
The offensive team must line up in a legal formation before they can snap the ball. An offensive formation is considered illegal if there are more than four players in the backfield or fewer than five players numbered 50-79 on the offensive line. Players can temporarily line up in a position whose eligibility is different from what their number permits as long as they immediately report the change to the referee, who then informs the defensive team of the change. Neither team's players, with the exception of the snapper, are allowed to line up in or cross the neutral zone until the ball is snapped. Interior offensive linemen are not allowed to move until the snap of the ball.
The main backfield positions are the quarterback (QB), halfback/tailback (HB/TB) and fullback (FB). The quarterback is the leader of the offense. Either he or a coach calls the plays. Quarterbacks typically inform the rest of the offense of the play in the huddle before the team lines up. The quarterback lines up behind the center to take the snap and then hands the ball off, throws it or runs with it.
The primary role of the halfback, who is also known as the tailback, is to carry the ball on running plays. Halfbacks may also serve as receivers. Fullbacks tend to be larger than halfbacks and function primarily as blockers, but they are sometimes used as runners in short-yardage situations and often are not used in passing situations.
The offensive line consists of several players whose primary function is to block members of the defensive line from tackling the ball carrier on running plays or sacking the quarterback on passing plays. The leader of the offensive line is the center (C), who is responsible for snapping the ball to the quarterback, blocking, and for making sure that the other linemen do their jobs during the play. On either side of the center are the guards (G), while tackles (T) line up outside of the guards.
The principal receivers are the wide receivers (WR) and the tight ends (TE). Wide receivers line up on or near the line of scrimmage, split outside of the line. The main goal of the wide receiver is to catch passes thrown by the quarterback, but they may also function as decoys or as blockers during running plays. Tight ends line up outside of the tackles and function both as receivers and as blockers.
The defensive line consists of defensive ends (DE) and defensive tackles (DT). Defensive ends line up on the ends of the line, while defensive tackles line up inside, between the defensive ends. The primary responsibilities of defensive ends and defensive tackles is to stop running plays on the inside and outside, respectively, to pressure the quarterback on passing plays, and to occupy the line so that the linebackers can break through.
Linebackers line up behind the defensive line but in front of the defensive backfield. They are divided into two types: middle linebackers (MLB) and outside linebackers (OLB). Linebackers are the defensive leaders and call the defensive plays. Their diverse roles include defending the run, pressuring the quarterback, and guarding backs, wide receivers and tight ends in the passing game.
The defensive backfield, often called the "secondary," consists of cornerbacks (CB) and safeties (S). Safeties are themselves divided into free safeties (FS) and strong safeties (SS). Cornerbacks line up outside the defensive formation, typically opposite of a receiver so as to be able to cover him, while safeties line up between the cornerbacks but farther back in the secondary. Safeties are the last line of defense, and are responsible for stopping deep passing plays as well as running plays.
The special teams unit are responsible for all kicking plays. The special teams unit of the team in control of the ball will try and execute field goal attempts, punts and kickoffs, while the opposing's unit will aim to block or return them.
Three positions are specific to the field goal and PAT (point after touchdown) unit: the placekicker (K or PK), holder (H) and long snapper (LS). The long snapper's job is to snap the football to the holder, who will catch and position it for the placekicker. There is not usually a holder on kickoffs, because the ball is kicked off of a tee; however, a holder may be used in certain situations, such as if wind is preventing the ball from remaining upright on the tee. The player on the receiving team who catches the ball is known as the kickoff returner (KR).
The positions specific to punt plays are the punter (P), long snapper, and gunner. The long snapper snaps the football directly to the punter, who then drops and kicks it before it hits the ground. Gunners line up split outside of the line and race down the field, aiming to tackle the punt returner (PR) - the player that catches the punt.
In American football, the winner is the team that has scored the most points at the end of the game. There are multiple ways to score in a football game.
The touchdown, worth six points, is the most valuable scoring play in American football. A touchdown is scored when a live ball is advanced into, caught in, or recovered in the end zone of the opposing team. The scoring team then attempts a try or conversion, more commonly known as the point(s)-after-touchdown (PAT), which is a single scoring opportunity. A PAT is attempted from the two- or three-yard line, depending on the level of play. If scored by a placekick or dropkick through the goal posts, it is worth one point, and is typically called the extra point. If it is scored by what would normally be a touchdown, it is called the two-point conversion and is worth two points. No points are awarded on a failed extra point or two-point conversion attempt. In general, the extra point is almost always successful in professional play and is slightly less successful at amateur levels, while the two-point conversion is a much riskier play with a higher probability of failure.
A field goal, worth three points, is scored when the ball is placekicked or dropkicked through the uprights and over the crossbars of the defense's goalposts. After a PAT attempt or successful field goal the scoring team must kick the ball off to the other team. A safety is worth two points and is scored by the defense. It occurs when the ball carrier is tackled in his own end zone. After a safety has been scored, the team that conceded the safety must return the ball to the scoring team via a free kick, which in professional football often resembles a punt.
Football games are played on a rectangular field that measures 120 yards (110 meters) long and 53.33 yards (48.76 m) wide. Lines marked along the ends and sides of the field are known respectively as the end lines and side lines, and goal lines are marked 10 yards (9 m) outward from each end line. Weighted pylons are placed on the inside corner of the intersections of the goal lines and end lines.
White markings on the field identify the distance from the end zone. Inbound lines, or hash marks, are short parallel lines that mark off one-yard increments. Yard lines, which run the width of the field, are marked every five yards. A line one yard wide is placed at each end of the field. This line is marked at the center of the two-yard line in professional play and at the three-yard line in college play. Numerals that display the yard lines in multiples of ten are placed along both sides of the field.
Goalposts are at the center of the plane of each of the two end lines. The crossbar of these posts is ten feet (3 meters) above the ground, with vertical uprights at the end of the crossbar 18 feet 6 inches (6 m) apart for professional and collegiate play and 23 feet 4 inches (7 m) apart for high school play. The uprights extend vertically 10 yards on professional fields, a minimum of 10 yards on college fields, and a minimum of ten feet on high school fields. Goal posts are padded at the base, and orange ribbons are normally placed at the tip of each upright.
Football games last for a total of 60 minutes in professional and college play and are divided into two halves of 30 minutes and four quarters of 15 minutes. High school football games are 48 minutes in length with two halves of 24 minutes and four quarters of 12 minutes. The two halves are separated by a halftime period, and the first and third quarters are also followed by a short break. Prior to the start of the game, the referee and team captains for each team meet at midfield for a coin toss. The visiting team is allowed to call 'heads' or 'tails'; the winner of the toss is allowed to decide from between choosing whether to receive or kick off the ball or choosing which goal they want to defend, but they can also defer their choice until the second half. The losing team, unless the winning team decides to defer, is allowed to choose the option the winning team did not select, and receives the option to receive, kick, or select a goal to defend to begin the second half. Most teams choose to receive or defer, because choosing to kick the ball to start the game would allow the other team to choose which goal to defend. Teams switch goals following the first and third quarters. If a down is in progress when a quarter ends, play continues until the down is completed.
Games last longer than their defined length due to play stoppages - the average NFL game lasts slightly over three hours. Time in a football game is measured by the game clock. An operator is responsible for starting, stopping and operating the game clock based on the direction of the appropriate official. A separate clock, the play clock, is used to determine if a delay of game infraction has been committed. If the play clock expires before the ball has been snapped or free-kicked, a delay of game foul is called on the offense. The play clock is set to 40 seconds in professional and college football and to 25 seconds in high school play or following certain administrative stoppages in the former levels of play.
There are two main ways that the offense can advance the ball: running and passing. In a typical play, the quarterback calls the play, and the center passes the ball backwards and under his legs to the quarterback in a process known as the snap. The quarterback then either hands the ball off to a back, throws the ball or runs with it himself. The play ends when the player with the ball is tackled or goes out of bounds, or a pass hits the ground without a player having caught it. A forward pass can only be legally attempted if the passer is behind the line of scrimmage.
The offense is given a series of four plays, known as downs. If the offense advances ten or more yards in the four downs, they are awarded a new set of four downs. If they fail to advance ten yards, possession of the football is turned over to the defense. In most situations, if the offense reaches their fourth down they will punt the ball to the other team, which forces them to begin their drive from further down the field; if they are in field goal range, they might also attempt to score a field goal. A group of officials, the chain crew, keeps track of both the downs and the distance measurements. On television, a yellow line is electronically superimposed on the field to show the first down line to the viewing audience.
There are two categories of kicks in football: scrimmage kicks, which can be executed by the offensive team on any down from behind or on the line of scrimmage, and free kicks. The free kicks are the kickoff, which starts the first and third quarters and overtime and follows a try attempt or a successful field goal, and the safety kick, which follows a safety.
On a kickoff, the ball is placed at the 35-yard line of the kicking team in professional and college play and at the 40-yard line in high school play. The ball may be drop-kicked or place-kicked. If a place kick is chosen, the ball can be placed on the ground or on a tee, and a holder may be used in either case. On a safety kick, the kicking team kicks the ball from their own 20-yard line. They can punt, drop-kick or place-kick the ball, but a tee may not be used in professional play. Any member of the receiving team may catch or advance the ball, and the ball may be recovered by the kicking team once it has gone at least ten yards or has been touched by any member of the receiving team.
The three types of scrimmage kicks are place kicks, drop kicks, and punts. Only place kicks and drop kicks can score points. The place kick is the standard method used to score points, because the pointy shape of the football makes it difficult to reliably drop kick. Once the ball has been kicked from a scrimmage kick, it can be advanced by the kicking team only if it is caught or recovered behind the line of scrimmage. If it is touched or recovered by the kicking team beyond this line, it becomes dead at the spot where it was touched. The kicking team is prohibited from interfering with the receiver's opportunity to catch the ball, and the receiving team has the option of signaling for a fair catch. This prohibits the defense from blocking into or tackling the receiver, but the play ends as soon as the ball is caught and the ball may not be advanced.
Officials are responsible for enforcing game rules and monitoring the clock. All officials carry a whistle and wear black-and-white striped shirts and black hats except for the referee, whose hat is white. Each carries a weighted yellow flag that is thrown to the ground to signal that a foul has been called. An official who spots multiple fouls will throw his hat as a secondary signal. The seven officials on the field are each tasked with a different set of responsibilities:
Another set of officials, the chain crew, are responsible for moving the chains. The chains, consisting of two large sticks with a 10 yard-long chain between them, are used to measure for a first down. The chain crew stays on the sidelines during the game, but if requested by the officials they will briefly bring the chains on to the field to measure. A typical chain crew will have at least three people - two members of the chain crew will hold either of the two sticks, while a third will hold the down marker. The down marker, a large stick with a dial on it, is flipped after each play to indicate the current down, and is typically moved to the approximate spot of the ball. The chain crew system has been used for over 100 years and is considered to be an accurate measure of distance, rarely subject to criticism from either side.
The National Football League (NFL) and the National Collegiate Athletic Association (NCAA) are the most popular football leagues in the United States. The National Football League was founded in 1920 and has since become the largest and most popular sport in the United States. The NFL has the highest average attendance of any sporting league in the world, with an average attendance of almost 70,000 persons during the 2011 NFL Season. The NFL championship game, the Super Bowl, is among the biggest events in club sports worldwide. It is played between the champions of the National Football Conference (NFC) and the American Football Conference (AFC), and its winner is awarded the Vince Lombardi Trophy.
Collegiate football ranks third in overall popularity in the United States, behind baseball and pro football. The NCAA, the largest collegiate organization, is divided into three Divisions: Division I, Division II and Division III. Division I football is further divided into two subdivisions: the Football Bowl Subdivision (FBS) and the Football Championship Subdivision (FCS). The champions of Division I-FCS, Division II and Division III are determined through playoff systems, and the Division I-FBS champion is determined through the Bowl Championship Series (BCS). Division I-FBS will switch to a four-team playoff system in 2014.
High school football is the most popular sport in the United States played by boys; over 1.1 million boys participated in the sport from 2007 to 2008 according to a survey by the National Federation of State High School Associations (NFHS). The NFHS is the largest organization for high school football, with member associations in all 50 states as well as the District of Columbia. USA Football is the governing body for youth and amateur football, and Pop Warner Little Scholars is the largest organization for youth football.
Several football leagues have been formed as rival leagues to the NFL. The XFL was created in 2001 by Vince McMahon and lasted for only one season. Despite television contracts with NBC and UPN and high expectations, the XFL suffered from poor television ratings and a low quality of play. The United Football League (UFL) has suspended its 2012 season, its fourth, due to financial issues. The United States Football League (USFL) operated for three seasons from 1983 to 1985 but collapsed due to poor business decisions and monetary problems. A subsequent USD $1.5 billion antitrust lawsuit against the NFL was successful in court, but the league was awarded only three dollars in damages. The World Football League (WFL) played for two seasons, in 1974 and 1975, but faced monetary issues so severe that the league could not pay its players. In its second and final season the WFL attempted to establish a stable credit rating, but the league disbanded before its second season could be completed.
The most successful minor league was the American Football League (AFL), which existed from 1960 to 1969. The AFL operated as a minor league from 1960 to 1964 and then signed a five-year, USD $36 million television deal with NBC and competed directly against the NFL. AFL teams signed NFL players to contracts, and their popularity grew to challenge that of the NFL. The two leagues merged in the 1970 season, and all AFL teams joined the NFL. An earlier minor league, the All-America Football Conference, was in play from 1946 to 1949. Two AAFC teams, the Cleveland Browns and the San Francisco 49ers, became members of the NFL.
American football leagues exist throughout the world, but the game has yet to achieve the international success and popularity of baseball and basketball. NFL Europa, the developmental league of the NFL, operated from 1991 to 1992 and then from 1995 to 2007. At the time of its closure, NFL Europa had five teams based in Germany and one in the Netherlands.
The European Football League (EFL), run by the European Federation of American Football (EFAF), is an annual invitational tournament between the champions or co-champions of competitions run by EFAF members. The league's championship game is the Eurobowl. Other EFAF tournaments include the EFAF Cup, played between the top teams from national leagues in a similar manner to the UEFA Cup, the Atlantic Cup, played between teams from the Atlantic region of Europe, and the Challenge Cup, played between teams from newer federations that are not eligible to play in the EFL or EFAF Cup. American football federations are also present in Asia, Oceania, and Pan America, and a total of 64 national football federations exist as of July 2012. The International Federation of American Football (IFAF), an international body composed of American football federations, runs tournaments such as the IFAF World Championship, which is held every four years since 1999, the IFAF Women's World Championship, the IFAF U-19 World Championship and the Flag Football World Championship. The IFAF also organizes the annual International Bowl game. At the international level, Canada, Mexico, and Japan are considered to be second-tier, while Austria, Germany, and France would rank among a third tier. All of these countries rank far below the United States, which is dominant at the international level.
Football is not an Olympic sport, but it was a demonstration sport at the 1932 Summer Olympics. The IFAF was seeking recognition from the International Olympic Committee (IOC), and finally received provisional recognition in December 2013 with a vote expected to take place by 2017 and could be a medal sport by the 2024 Olympics. Several major obstacles hinder the IFAF goal of achieving status as an Olympic sport, such as the predominant participation of men in international play, and the short three-week Olympic schedule. Large team sizes are an additional difficulty, due to the Olympics' set limit of 10,500 athletes and coaches. These issues are similar to those that rugby union faced prior to being admitted into the Olympics in the form of rugby sevens, a modified version of the sport, but football also has the issue of global visibility. Nigel Melville, the CEO of USA Rugby, noted that while rugby union has a major international presence through the International Rugby Board, "American football is recognized globally as a sport, but it's not played globally". In order to solve these concerns, major effort has been put into promoting flag football, a modified version of American football, at the international level.
Football is a full-contact sport, and injuries are relatively common. Most injuries occur during training sessions, particularly ones that involve contact between players. To try to prevent injuries, players are required to wear a set of equipment. At a minimum players must wear a football helmet and a set of shoulder pads, but individual leagues may require additional padding such as thigh pads and guards, knee pads, chest protectors, and mouthguards. Most injuries occur in the lower extremities, particularly in the knee, but a significant number also affect the upper extremities. The most common types of injuries are strains, sprains, bruises, fractures, dislocations, and concussions.
Concussions are particularly concerning, as repeated concussions can increase a person's risk in later life for chronic traumatic encephalopathy and mental health issues such as dementia, Parkinson's disease, and depression. Concussions are often caused by helmet-to-helmet or upper-body contact between opposing players, although helmets have prevented more serious injuries such as skull fractures. Various programs are aiming to reduce concussions by reducing the frequency of helmet-to-helmet hits; USA Football's "Heads Up Football" program is aiming to reduce concussions in youth football by teaching coaches and players about the signs of a concussion, the proper way to wear football equipment and ensure it fits, and proper tackling methods that avoid helmet-to-helmet contact.
Canadian football, the predominant form of football in Canada, is closely related to American football - both sports developed from rugby, and the two sports are considered to be the chief variants of gridiron football. Although Canadian football developed independently from American football, the two games share a similar set of rules, but there are several key rule differences: in the Canadian game, the field measures 150 yards (137 m) by 65 (59 m) yards, which includes two 20-yard end zones, teams have three downs instead of four, there are twelve players on each side instead of eleven, fair catches are not allowed, and a single point is scored if the offensive team kicks the ball out of the defense's end zone. The Canadian Football League (CFL) is the major Canadian league and is the second-most popular sporting league in Canada, behind only the National Hockey League.
A major variant of football is arena football, played by the Arena Football League (AFL). Arena football has eight-player teams and uses an indoor field 50 yards (46 m) in length, excluding end zones, and 28.3 yards (25.9 m) wide. Punting is illegal, and kickoffs are attempted from the goal line. Large overhead nets deflect forward passes and kicks that hit them, and deflected kicks are live balls that may be recovered by either team.
Below the Arena Football League are what New York Times writer Mike Tanier described as the "most minor of minor leagues:" indoor football leagues. Like in arena football, teams in indoor football leagues play in arenas, but games are only attended by a small number of fans, and most players are semi-professional athletes. Indoor football leagues are unstable, with franchises regularly moving from one league to another or merging with other teams, and teams or entire leagues dissolving entirely. The Indoor Football League, Southern Indoor Football League, Ultimate Indoor Football League, Continental Indoor Football League and American Professional Football League are examples of prominent indoor leagues.
Flag football is a variation of football in which players are not tackled - instead, plays end when a "flag" attached the waist of the runner is pulled off. Touch football is a similar variant in which a play ends when the runner is touched. A game of touch football may require that the player be touched with either one or two hands to be considered down, depending on the rules used.
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