After collecting All-America honors in football and lacrosse at Syracuse, the bruising fullback made nine Pro Bowls in nine years with the Browns before his abrupt 1966 retirement. Brown retired with NFL records for single-season and career rushing yards.
2 of 50Mark Kauffman/SI
No athlete embodied toughness as a measure of courage more than Robinson. He weathered unthinkable acts of discrimination while breaking baseball's color barrier in 1947. But Robinson proved his fortitude in other venues, becoming the first athlete in UCLA history to letter in four sports (baseball, basketball, football and track) and serving as a second lieutenant in the Army from 1942 through 1944.
3 of 50Neil Leifer/SI
The man SI deemed "The Most Feared Man in the Game" made eight Pro Bowls and carved out a reputation as the greatest linebacker of his generation.The Chicago native played four years at Illinois before spending the entirety of his eight-year pro career with the Bears.
4 of 50AP
The all-time leader in World Championship Rodeo Cowboy awards made a career out of getting thrown to the dirt and picking himself up for more punishment. The Oklahoma native's laundry list of fractures includes an ankle, both arms (twice) and collar bone (on three occasions).
5 of 50Neil Leifer/SI
As the cornerstone of the Green Bay defense during the franchise's golden years, Nitschke won five league titles during a brilliant 15-year career. The linebacker collected MVP honors in the 1962 NFL championship game after recovering a pair of fumbles and deflecting a pass which was intercepted.
6 of 50Manny Millan/SI
Approaching the game with a physical toughness belying his modest 6-foot-1 frame, Thomas was a feisty competitor whose mental fortitude translated into a knack for taking over games at will. He won a pair of titles with the Pistons and retired as the franchise's all-time leader in points, assists, steals and games played.
7 of 50John G. Zimmerman/SI
A pair of hands that gnarled like tree roots told you everything you needed to know about Concrete Charlie, the last two-way player in the NFL and one of the great blue-collar heroes of the sport. Bednarik's infamous hit on Frank Gifford of the New York Giants sidelined the star running back for a year-and-a-half. He helped the Eagles to a pair of NFL titles in 1948 and 1960.
8 of 50AP
Following a decorated collegiate career under Eddie Robinson at Grambling State, the 6-foot-7, 287-pound defensive lineman astonished pro football fans with his durability over a 13-year career with the Chiefs. Buchanan played 182 games including 166 in a row -- no small feat given the brutal nature of life in the NFL trenches.
9 of 50Walter Iooss Jr./SI, Tony Tomsic/SI
A devastating hitter and masterclass intimidator, Lambert anchored the Steel Curtain defense from the middle linebacker position. The Ohio native and Kent State product won four Super Bowls in 11 years with Pittsburgh, amassing more than 1,400 tackles, 28 interceptions and 23.5 sacks.
10 of 50AP
Widely credited with coining the term "quarterback sack," Jones piled up 173.5 sacks with the Rams, Chargers and Redskins. But even more impressive was his freakish durability: Jones missed just six games during his 14-year career.
11 of 50John Iacono/SI, V.J. Lovero/SI
Yes, boxing's only four-time heavyweight champion has hung around a little too long, but there's no questioning the physical and mental tenacity he has exhibited throughout 24 years and 53 fights. From his controversial bronze medal in the 1984 Olympics to his classic battles Riddick Bowe, to his beating of Mike Tyson (and the equally memorable rematch), Holyfield's panoramic career cements his place among the all-time pugilistic legends.
12 of 50Mark Kauffman/SI, John G. Zimmerman/SI
Known as one of the most powerful punchers in the history of the sport, Liston overcame a checkered past to earn a shot at the heavyweight title against Floyd Paterson in 1962. The Big Bear scored a first-round knockout -- and another first-round KO in the rematch -- to secure a place among the sport's greats.
13 of 50Walter Iooss Jr./SI
The longtime Green Bay gunslinger retired in February with the NFL records for touchdown passes (442), passing yards (61,655), completions (5,377), attempts (8,758) and interceptions thrown (288). But no record testifies to Favre's toughness more effectively than his mark for most consecutive starts among NFL quarterbacks: an unfathomable 275 straight games including the playoffs.
14 of 50AP
Known as "The Assassin" for his bone-crunching hits out of the defensive backfield, Tatum played nine seasons in Oakland and helped the Raiders to the team's first Super Bowl victory.
15 of 50John Iacono/SI
Lott made 10 Pro Bowl appearances playing cornerback and safety during a 15-year career with the 49ers, Raiders, Jets and Chiefs. Between his versatility and knack for making demoralizing hits on opposing players, the USC product is widely considered the best secondary player in NFL history.
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He was known as the "Raging Bull" for his aggressive style and extraordinary ability to absorb punches. LaMotta won 83 of his 106 fights from 1941 through 1954 -- including the first defeat of Sugar Ray Robinson's career in the second of their legendary six-bout rivalry.
17 of 50AP (2), Dan Baliotti/SI
The six-time Hart Trophy winner assumed the nickname "Mr. Hockey" thanks to his scoring ability and extraordinary durability throughout a lengthy career. After making an appearance with the IHL's Detroit Vipers in 1997, the Saskatchewan native became the first player to skate in six different decades.
18 of 50Mike Powell/Getty Images
Messier established himself as one of hockey's all-time best during a 25-year career in the WHA and the NHL. The Edmonton native remains the lone pro athlete to captain two different teams (Oilers and Rangers) to Stanley Cup championships.
19 of 50Mike Powell/Getty Images, Bruno Fablet/Reuters, Jonas Karlsson/SI, Simon Bruty/SI
Doctors didn't give Armstrong much of a chance after testicular cancer metastasized to his brain and lungs. But after beating the disease thanks to surgery and extensive chemotherapy treatments, the Texas native rebounded to win seven straight Tour de Frances.
20 of 50John W. McDonough/SI, Lynn Johnson/SI
The Greco-Roman grappler remains best known for his unlikely gold medal victory in the 2000 Summer Olympics over Russian Alexander Karelin, who had gone 13 years without losing a match and six years without conceding a single point. Gardner's survival of a 2007 plane crash into Utah's Lake Powell -- he swam for an hour in near-freezing water and spent the night without shelter before rescue -- adds another memorable chapter to the Wyoming native's tough-guy mystique.
21 of 50Jerry Cooke/SI
The Puerto Rican-born Cordero, a fearless rider who kept himself in immaculate shape, rode 7,057 winners during his illustrious career, including three Kentucky Derbys (1974, '76, and '85), two Preakness (1980 and '84) and a Belmont Stakes win in 1976 (Bold Forbes). He was the leading rider at Saratoga for 13 years.
22 of 50Mark Kauffman/SI
How tough was the Brockton Blockbuster? He's the only heavyweight champion in the history to retire undefeated: Marciano won 49 fights in 49 tries with 43 coming by knockout.
23 of 50Manny Millan/SI (3), Tony Triolo/SI
Hagler retired in 1987 with a career record of 62 wins, three losses and two draws, establishing himself as history's greatest middleweight. But it was his triumph against Thomas Hearns -- in a brief and brutal 1985 fight known simply as "The War" -- which elevated Hagler from champion to legend.
24 of 50Jerry Cooke/SI
The Texas native's diminutive 4-foot-11 frame stands in contrast with his massive legend in horse racing. His career spanned six decades and he finished with 8,883 winners, including 11 Triple Crown race victories.
25 of 50George Tiedemann/GT Images/SI
Dale Earnhardt Sr.
Fueled by a near-obsessive competitive drive and the willingness to do anything to win, Earnhardt won 76 races and owns a record seven Winston Cup championships.
26 of 50Walter Iooss Jr./SI
The all-time hits leader won three World Series titles and made 17 All-Star appearances playing five different positions: first base, second base, third base, left field and right field. His reckless and aggressive approach to the game -- epitomized by his "Charlie Hustle" moniker -- proved infectious among his teammates and helped create a winning atmosphere for his teams.
27 of 50John Iacono/SI, Walter Iooss Jr./SI, Tony Triolo/SI, Hy Peskin/SI
Martin earned a reputation as a player for making clutch plays in big-time situations, like his game-saving catch of a quirky pop-up in Game 7 of the 1952 World Series. His aggressive managing resulted in dozens of legendary encounters with umpires.
28 of 50Ronald C. Modra/SI, AP
The only pitcher in history to face Roger Maris, Mark McGwire and Sammy Sosa, Ryan's incredible longevity -- he pitched a record 27 seasons -- punched the Texas native's Hall of Fame ticket on the first ballot. The right-hander played the part of ornery old man to perfection in a memorable 1993 on-mound fight with Robin Ventura, an opponent 20 years his junior: Ryan placed the charging Ventura in a headlock and landed six punishing blows to the head before other players could intervene.
29 of 50Walter Iooss Jr./SI
Known as the Russian Bear, Karelin competed for the Soviet Union and Russia throughout a remarkable career in the Greco-Roman discipline. He won three gold medals -- Seoul 1988, Barcelona 1992 and Atlanta 1996 -- while going undefeated in international competition from 1987 through 2000. In Sydney he suffere a shocking loss to Rulon Gardner and shortly retired.
30 of 50Michael O'Neill/SI, Illustration by C.F. Payne, Walter Iooss Jr./SI, Ronald C. Modra/SI
Cal Ripken, Jr.
Ripken's long chase of Lou Gehrig's 2,130 consecutive games played ended Sept. 6, 2005 when the Baltimore shortstop broke the mark in Baltimore before a national television audience. He ultimately exceeded Gehrig's record by 502 games before taking a seat on Sept. 20, 1998.
31 of 50AP
The durable and dependable Hall, a three-time winner of the Vezina Trophy, didn't miss a start for seven straight years from 1955 through 1962, playing a total of 502 consecutive regular-season games between the pipes without a mask. In 18 seasons with Detroit, Chicago and St. Louis, Hall finished with a losing record just four times.
32 of 50Manny Millan/SI (3), Carl Skalak/SI, John Biever/SI, Barry Gossage/NBAE/Getty Images
He played his best during crunch time -- he had 28 game-winning shots. And his well-documented ability to persevere though a variety of physical ailments -- including his performance overcoming food poisoning in Game 5 of the 1997 NBA Finals -- still inspires today.
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One of the largest players of his era, he served as a model for future bruising backs like Jim Brown, Larry Czonka and John Riggins. The brawny Canadian native enjoyed a second career as a pro wrestler following his retirement, winning the world heavyweight championship three times. Nagurski was the fullback on the NFL's 75th Anniversary All-Time Team.
34 of 50Charles M. Conlon/TSN/Icon SMI
Possibly the greatest player of the deadball era -- he was certainly the meanest-tempered -- Cobb still holds the record for highest career batting average (.367) and steals of home (54), along with dozens of others marks. The longtime Detroit outfielder intimidated teammates and opponents alike with his hyper-aggressive play.
35 of 50Gerry Cranham/SI
Billie Jean King
She won a dozen Grand Slam singles titles but as an advocate against sexism in sports, most memorably embodied by her "Battle of the Sexes" victory over Bobby Riggs in 1973, the California native brought visibility to a social issue once largely ignored by the public.
36 of 50John Iacono/SI
A freakish combination of size, speed, agility and manic energy, Taylor forever changed the linebacker position. "I don't like to just wrap a quarterback," the 10-time Pro Bowler once said. "I really try to make him see seven fingers when they hold up three. I'll drive my helmet into him, or, if I can, I'll bring my arm up over his head and try to axe the sonuvabitch in two. So long as the guy is holding the ball, I intend to hurt him."
37 of 50Sporting News Archives/Icon SMI
Gibson fanned 3,117 batters during a brilliant 16-year career with the Cardinals but remains best known for his legendary competitive streak and clutch pitching during the World Series.
38 of 50John Olson/SI
As the ringleader of the Broad Street Bullies, Clarke's hard-nosed work ethic reflected the blue-collar nature of Philadelphia; the city embraced the team accordingly. The Manitoba native spirited the Flyers to back-to-back Stanley Cups during the mid-`70s.
39 of 50Walter Iooss Jr./SI
Released by the Steelers before ever playing an NFL game, Unitas worked construction to support his family and played on a local semi-pro team as a quarterback and punter for the princely sum of six dollars a game. But the Pittsburgh native rebounded from rejection, latching on with the Baltimore Colts to win three NFL titles and make 10 Pro Bowls.
40 of 50Lou Capozzola/SI
During his 22-year career with the Capitals, Blues and Devils, Stevens turned the open-ice hit into an art form. If you don't believe us, just Eric Lindros, Paul Kariya, Joe Thornton or Slava Kozlov.
41 of 50AP
Perhaps the toughest fighter in league history, Ferguson played just eight seasons with the Montreal Canadiens but long enough to leave a lasting impression. Initially called upon to fill the role of "enforcer" to protect the team's smaller skill players, Ferguson got into a fight just 12 seconds into his first NHL game.
42 of 50AP
He turned pro in 1938, retired in 1963, and held the light heavyweight title for seven years during the 1950s and again in 1961. His record for knockouts (145) still stands today.
43 of 50James Drake/SI
Throughout a 20-year career with the Blackhawks, Rangers and Red Wings, Gadsby accumulated 640 stitches (by his wife's count). One vicious Gadsby check in a 1955 game broke Tim Horton's jaw and leg.
44 of 50AP
After becoming the youngest-ever PRCA All-Around Rodeo Cowboy, Murray won six consecutive World All-Around Rodeo Championships, from 1989 through 1994. A horrific series of injuries followed -- including a double knee reconstruction and a broken shoulder -- but a 1998 comeback ended with a record seventh All-Around title.
45 of 50Hy Peskin/SI
Maurice "Rocket" Richard
Growing up the son of a railroad machinist on the streets of Quebec, Richard would blossom into one of the game's most prolific goal-scorers and hardest-nosed talents. His fierce temper and dedication to his teammates, reflected in regular spats with opponents and officials, are celebrated as legend among the Quebecois.
46 of 50Manny Millan/SI
As the baddest of the Bad Boys, Mahorn was part of championship teams in 1989 and 1990. Undersized for the center position, the Hampton product compensated with his physical style of play and a consistently high energy level. The team's defense-oriented, hard-nosed style proved a perfect fit.
47 of 50David E. Klutho/SI
Known as Wayne Gretzky's bodyguard during the early-'80s, Semenko's circle of protection extended to Edmonton's other star players. Promoters didn't fail to cash in on the left wing's reputation as a fighter, signing Semenko up to fight Muhammad Ali in a 1983 exhibition.
48 of 50James Drake/SI
A five-time All-Star in the ABA/NBA, Lucas helped lead the Trail Blazers to the 1977 World Championship. , He was a power forward with an emphasis on power. His nickname in Rip City? The Enforcer.
49 of 50Bob Rosato/SI
A vital part of Pat Riley's great Knicks teams of the 1990s, Oakley, a rough-and-tumble power forward out of Virginia Union, played a massive role in the success of New York's defensive goals.
50 of 50John Iacono/SI
Considered one of the sport's most exciting fighters up until his 2007 retirement, Gatti held the junior lightweight and junior welterweight belts at various points during his 16-year career. But the fearless Garden State native is remembered for his willingness to take a couple shots in order to land a few of his own.
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