Fighting is a young man's game. That is a truism that has held legitimate for centuries. A young man is quicker, stronger and finds it much easier to get up after being knocked down.
The more years on a body clock make all of those attributes that much harder.
Time waits for no one and in MMA it appears to be extremely difficult for certain fighters to let go of past glories.
Hey, it has happened to the best of them. One of the biggest stars of the early UFC era was "The World's Most Dangerous Man" Ken Shamrock. Shamrock had it all; size, strength, agility, punching power and a devastating arsenal of submission skills.
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Shamrock dominated the competition in UFC and Pancrase, defeating names such as Bas Rutten (twice), Dan Severn, Kimo and Maurice Smith.
When he departed UFC to join the World Wrestling Federation in 1997, the desire to fight for real still burned within Shamrock and he signed on with Japanese promotion, Pride FC in early 2000. At the age of 36, he knew that he had a limited window in which to fulfill his ambitions.
Shamrock's return initially, was a successful one. He became the first man to stop Alexander Otsuka in regulation time via Knockout and in his final Pride bout of his contract in February 2002, he contested the fight of the year with Don Frye, in which he lost a very close split decision.
However, when he returned to UFC later in 2002 at UFC 40 versus Tito Ortiz, it was clear his best days were behind him. Multiple knee surgeries had hampered his ability to take opponents down, which was his best weapon and also sapped his mobility.
Ortiz outclassed the veteran three times in total as the Shamrock legend faded in front of the world. The UFC icon would continue to fight on and off until 2017; a full 20 years past his prime. Unfortunately, embarrassing defeats would become the norm, not the exception as he was soundly defeated by limited fighters such as Mike Bourke and Kimbo Slice.
More recently, the UFC has seen many of it's once, best fighter's legends vanish into the dust as they struggle to keep pace with younger combatants.
In this slideshow, SK looks at five UFC fighters who should hang up the gloves at the earliest opportunity.
#5 Gray Maynard
Gray Maynard's classic series of fights with Lightweight Champion, Frankie Edgar seem like a very long time ago now. Maynard was once one of the most dynamic and entertaining fighters in the entire UFC.
Maynard did not taste defeat for the first five years of his MMA career but when tat first loss came versus Edgar at UFC 136 on October 8, 2011, it spelt the beginning of a decline for Maynard in which he has never recovered.
Maynard won his next fight but then lost his next four in a row and was lucky not be cut by the promotion.
This once excellent fighter has only tasted victory in the Octagon twice in the past five years.
A return to Lightweight in his most recent fight at UFC 229 did not improve his fortunes as he was knocked out by veteran journeyman Nik Lentz.
Maynard should see nothing but retirement in his future.
#4 Andrei Arlovski
Andrei Arlovksi could feel himself unfortunate that he lost his most recent bout via Unanimous Decision versus Shamil Abdurakhimov. Arlovski seemingly had done enough to win but the judges did not agree.
In truth however, Arlovski's days as a top line fighter were over a decade gone. It was way back between 2005 and 2006, that the Russian reigned as UFC Heavyweight Champion.
After dropping the belt to Tim Sylvia, Arlovski defeated Marcio Cruz, Fabricio Werdum and Jake O'Brien before leaving UFC to compete for Affliction.
Following a six year absence, Arlovski returned to UFC in impressive form with four straight wins including victories over "Bigfoot" Silva and fellow, former UFC Heavyweight Champion, Frank Mir.
However, recent years have been less kind to this one great Champion. Arlovski has lost seven of his past nine bouts over the past two years and will never put himself back in contention for the Heavyweight title. Therefore, there is no need for this legend to continue to put himself through the punishment of elite fighting competition at 39 years old.
#3 Stefan Struve
Stefan Struve debuted in UFC with a 16-2 record and was seen as a legitimate contender in the Heavyweight division from day one.
In his debut fight, he was pitted against fellow contender, future Champion, Junior dos Santos at UFC 95.
Struve's first match didn't last long and didn't go well. He was knocked out inside a minute. However, the "Skyscraper" bounced back impressively with eight wins from his next 10 UFC bouts, which included victories over big names such as Paul Buentello and Stipe Miocic.
In perhaps his most impressive performances ever he won a Unanimous Decision over Brazilian legend, Antonio Rodrigo Nogueira, completely dominating his opponent with his long reach with stunningly effective combinations.
However, Struve ceased to be a serious Heavyweight contender when he dropped three consecutive fights to Alexander Volkov, long past his prime, Andrei Arlovski and Marcin Tybura.
Struve's best days are no longer in his future.
#2 Mark Hunt
New Zealander, Mark Hunt first burst on the MMA scene in Pride FC, wherein he defeated the legendary Wanderlei Silva and Mirko Cro Cop.
Those wins were enough to earn him a Heavyweight title bout with long reigning Champion, Fedor Emelianenko. Hunt lost that one but earned plaudits for his performance against the near unbeatable Russian.
Hunt lost his UFC debut fight versus the giant Sean McCorkle but rebounded to win four in a row which included huge wins over superstars Cheick Kongo and Stefan Struve.
Hunt's recent record has been mixed but did include notable victories over former UFC Heavyweight Champion, Frank Mir and contenders Roy Nelson and "Bigfoot" Silva.
However, Hunt's past two UFC bouts have ended in decisive losses, most recently at UFC Fight Night 136 when he was submitted by Alexey Oleinik.
Hunt, at 44 years of age has clearly seen better days. It's time for this legend to hang up his gloves for good.
#1 BJ Penn
BJ Penn last won an MMA fight on November 20, 2010, versus a past his prime, Matt Hughes in the latter's penultimate fight before retiring.
Since that bout, Penn has fought seven times, drawing one and losing his next six in a row.
It is fair to say that this once great fighter, a UFC Hall of Famer has nothing left in the tank.
Unbelievably, Penn is training for yet another ill-advised comeback and is scheduled to face Ryan Hall at UFC 232 on December 29, 2018. That fight is likely to end the same way as his previous six.
For a fighter who is renowned as one of the greatest Lightweight and Welterweight Champions of all time, these damaging defeats only sully the legacy of the man, whom most fighters declare as their idol.
Penn should retire immediately while he still has a legend to protect.