For decades fighters have been looked at as great athletes and finely tuned machines. We all remember seeing Rocky Balboa run the stairs at the Philadelphia Art Museum and thinking, man to be a fighter you have to be in perfect shape, and we have the same thoughts when we see guys like Conor McGregor dominate the UFC. The world of professional fighting has always been littered with great heavyweight fighters, but today we will take a look at the 15 fighters who truly defined the word heavyweight.

Sure, having the extra weight will help a guy carry more power from his strikes, but what about all the other intricacies of fighting? What about the speed you need, endurance, stamina, agility and durability? Some guys were able to have that along with their extra weight, but at some point, cardiovascular endurance will catch up.

We've heard about the extreme measures fighters have had to take to cut weight before a fight, but these guys sure never had to put their body through such extreme adjustments, or at least they didn't show it.

Some of these guys were actually very disciplined and talented fighters at one time or another, but all of them could have used a little more discipline at the dinner table.

For the purposes of this list, it will not be limited to boxing, MMA or sumo wrestling but for all contact sports. It will feature not necessarily the heaviest fighters ever, but fighters who were simply too big for their role in the sport.

15 15. George Foreman

George Foreman is a two-time WBC Heavyweight Champion and holds claim to being the oldest heavyweight champion in boxing history. For his last fight against Shannon Briggs, Foreman weighed in at a career high 260 lbs. He went on to lose that fight and would never box again. While Foreman made a heroic comeback at 45, this time it was clear it was time to retire.

Foreman is now well known for his "lean-mean fat grilling machine" which Foreman may have used one too many times in his day.

14 14. Eric Pele

Eric Pele is a 6-foot-2 retired MMA fighter. In his fighting days Pele weighed in at 340 lbs. He was listed as a super heavyweight during his fighting career, a class not typically occupied by fighters under 6-foot-6. Pele retired from fighting in 2008 with a respectable 11-5 record, including a win over MMA legend Antonio "Bigfoot" Silva. Pele is now an accomplished tattoo artist and has appeared on reality shows "Inked" and "Tattoo Nightmare".

13 13. Bob Sapp

Bob Sapp played college football at the University of Washington and had a brief stint in the NFL with the Chicago Bears and the Minnesota Vikings. Sapp was unable to live his dream of being a professional football player, so he turned to the world of MMA, joining promotions PRIDE, and K-1 fighting. Sapp is a legend in the organized fighting circles of Japan, he even had an action figure made of to his likeness. Sapp weighed in at 350 pounds, standing at 6-foot-5 during his time as a professional fighter. While a lot of it was initially muscle, his definition began to slip later on.

12 12. Roy Nelson

Listed at six feet and 268 pounds, Roy Nelson was the winner of the popular Spike TV show "The Ultimate Fighter." Nelson is still an active fighter within the UFC, and as of December 2015 he was ranked as the #11 heavyweight fighter in the UFC. Nelson has beaten some of the top UFC fighters in his career including Brendan Schuab and Matt Mitrione. Nelson recently snapped a three-fight losing streak by beating Jared Rosholt on UFC Fight Night.

11 11. Zuluzhino

Zuluzhino is an MMA fighter who has participated in fights at Cage Warriors Strike Force 2 in England and also with the Pride Fighting Championships. He stands at a staggering 6-foot-7 and weighs in at 390 pounds. He has a career record of 18-6 (unofficially) with his best win coming against Ikuhisa "The Punk" Minowa. Zuluzhino also fought Butterbean in 1994, where he lost by submission.

10 10. Alexandru Lungu

Alexandra Lungu is an accomplished MMA, Kickboxer, K-1, and PRIDE fighter. The Romanian native has been active in MMA for over 10 years now.  He comes in at husky six tall and 364 pounds. Lungu is still an active fighter, and his last win came in June 2015. He has a career MMA record of 11-5, with his career kickboxing record is 5-1, including 10 career knock outs. If you have yet to see this large man in action, he is worth finding.

9 9. Riddick Bowe

Riddick Bowe was a great American boxer and had many memorable matches against the likes of Evander Holyfield and Andrew Golata. Bowe, however, let himself go towards the tail end of his career, including his last couple of fights where he weighed in at 280 pounds. When Bowe fought Evander Holyfield for the second time in his career he weighed just 245 pounds. Bowe also tried his hand at MMA, with very limited success.

8 8. Buster Douglas

We all remember Buster Douglas as the young strong underdog who was the first person to ever defeat Mike Tyson. Buster completely let himself go after that unfathomable victory. It is reported that Buster let himself balloon up to 320 pounds, after weighing 226 pounds when he pulled off the upset against Tyson. The knockout of Tyson brought Douglas incredible fame and it appears he was ill prepared to handle the attention and temptations that a heavyweight title can bring.

7 7. Francois Botha

Francois Botha is a South African born boxer and kick boxer. In 1995 Botha defeated Axle Schulz to become the IBF heavyweight champion, though he was later stripped of that title due to his positive test for steroids. Botha was a top flight boxer during his prime with fights against some of the best in the sport, including Lennox Lewis and Wladimire Klitschko, unfortunately for Botha he lost to both of those fighters.

6 6. Buster Mathis Sr.

Buster Mathis was a legitimate heavyweight contender in his day. He took on some of the greats in the sport, including Joe Frazier and Muhammad Ali. Mathis is the father of boxer Buster Mathis Jr. who famously was knocked out by Mike Tyson. Mathis Sr. retired from boxing in 1972 and went on to work in the trucking business. It has been reported that Mathis ballooned up to 550 pounds after his boxing career ended, Mathis died in 1995 due to a severe heart attack.

5 5. Gabe Brown

Gabe Brown was a heavyweight boxer through the 2000s. He is a monster of a man weighing in well over 350 pounds. Brown retired in 2011 with a career record of 18-17-4. Brown was never able to get a signature win, or even a signature fight for that matter. The biggest fight he participated in was in 2003 when he lost pitifully to Sherman Williams, Williams would later go on to fight Evander Holyfield.

4 4. Dustin Nichols

Dustin Nichols is a retired American boxer, who now resides in his home state of Alabama. Nichols had a pathetic 4-5 record in his career, including a fight against Deontey Wilder where he was knocked out in the first round against a man weighing 200 fewer pounds than him. Nichols has since retired and has not been involved boxing in any way. It's all for the best, because that's not where he belongs.

3 3. Akebono Taro

Akebono started out as a sumo wrestler, but in 2003 he turned to the K-1 fighting scene and became an MMA fighter. He had 13 total fights in K-1, going 1-12 in those bouts. He became a joke amongst Japanese fight fans and he ultimately turned to the WWE for employment. Akebono was listed at 6-foot-8 and 514 pounds in his last fight. As a sumo wrestler, that'd be fine, but him joining K-1 is what gets him here.

2 2. Eric "Butterbean" Esch

Butterbean was an American boxer, kickboxer, and MMA heavyweight fighter. Esch transitioned to boxing in 1994 after a successful career in the Toughman Contest. Once he began boxing he found many successes including winning the WAA heavyweight title and also the IBA super heavyweight championship. In 2003 Esch began fighting in the K-1 world and also the Pride Fighting Championships. Butterbean's combined fight record is an impressive 97–24–5 with 66 knockouts and 10 submissions

1 1. Emmanuel Yarborough

Emmanuel Yarborough was an MMA fighter who participated in the third ever UFC competition. He is still the heaviest participant in the UFC history at just north of 600 pounds. Yarborough did not have much luck in his MMA career, going 1-2 with the losses coming via TKO and tap out. Yarborough stood at a whopping 6-foot-8 and at his 600 pounds weight he was immovable in the ring, which often times played to his detriment as opponents would run around him until they were able to knock him off of his feet, rendering him helpless. Yarborough died in December of 2015 but the cause of death was never confirmed.