MILWAUKEE -- It's not enough in mixed martial arts to be strong, though strength is a requirement.
It's not enough to be in tremendous physical condition, though that, too, is mandatory.
It's not enough to be technically proficient in a number of combat disciplines -- boxing, kickboxing, wrestling, Brazilian jiu-jitsu, taekwondo -- though a weakness in any is likely to be exposed.
If you want to reach the top of the heap, the Ultimate Fighting Championship, you must have the strength of a weightlifter, the cardio of a marathoner, the flexibility of a yoga instructor and the hand-to-hand fighting skills of a Navy SEAL.
And that's only when you train.
Once you step into the octagon, bare-footed and wearing nothing but shorts and lightweight open-fingered gloves, you've got to be something else:
"It's the hardest sport I've ever done," said Dan "The Outlaw" Hardy, who faces Chris "Lights Out" Lytle in the co-main event of UFC Live on Versus 5 at the Bradley Center on Sunday night.
"I just think it's one of the hardest sports out there," said Duke Roufus, a nationally known MMA trainer based in Milwaukee.
Whether or not you appreciate mixed martial arts, a sport that has exploded in popularity -- UFC.com boasts more than 6 million unique visitors per month and president Dana White has 1.6 million followers on Twitter -- there is no denying that UFC athletes are among the toughest, best-conditioned and well-rounded in the world.
Mixed martial arts has attracted former NCAA champion wrestlers such as Hartland's Ben Askren and ex-National Football League players such as Herschel Walker, but they are at a disadvantage until they master a variety of fighting skills.
"In football you can be a great quarterback or a great linebacker," Roufus said. "In MMA, you have to be a great quarterback, a great linebacker, a great running back, a great kicker and a great special teams player. Then you've got to hone your skills athletically and stay on top of your strength and conditioning.
"It's like marrying a sprinter and a marathoner. It's like trying to be an awesome decathlete. You've got to be fast, you've got to be strong, you've got to be smart, you've got to be in great shape.
"It's like playing chess with your body."
The training regimen for top MMA stars is beyond rigorous. Most are in the gym twice and often three times a day. One session might be devoted to cardio, another to pad work and a third to improving technical skills.
It's a Spartan lifestyle, one that requires daily sacrifice and constant discipline.
"There are guys in the UFC I don't like," Hardy said. "I don't want to have conversations with them. I don't want to be in the same room with them. But at the same time I have respect for them because I know what they go through on a daily basis for their sport."
In his final sparring session last week, Hardy went four five-minute rounds with a fresh sparring partner jumping in each minute.
"They give you everything they've got for one minute," he said. "I'm trying to match their pace and their cardio."
That kind of training is meant to prepare athletes for exhausting five-minute rounds but in truth the actual fight experience -- the crowd, the lights, the TV cameras, the adrenaline -- cannot be duplicated in a gym.
In the heat of battle, a fighter's heart rate can spike at 200 beats a minute, according to Matt Gifford, a sports performance coach who trains MMA athletes at NX Level in Waukesha.
"You've got to condition your body to get your heart rate up and then back down to recover for the next round," said UFC star Anthony "Showtime" Pettis of Milwaukee. "Even when you can do it in the gym, it's different when the lights are on and the TV cameras are in your face and people are screaming your name."
Pettis augments his MMA workouts at the Roufusport Martial Arts Academy with grueling fitness training at NX Level. In intense "hurricane training," he flips tires, sprints while pulling a sled and swings a sledgehammer dozens of times to build strength, stamina and explosive power.
"The big thing with these guys, classically they're all over-trained," Gifford said. "You've got to know when to back off with them. They're willing to run through a wall every day."
It's hard to imagine a more athletic and explosive move than the kick Pettis used to beat Ben Henderson last December.
Dog-tired in the fifth round, Pettis somehow summoned the strength to jump laterally onto the cage, then sprang off it with his right foot, made a shearing movement with his left leg and kicked Henderson in the face with his right foot.
Henderson, who faces Jim Miller in the other co-main event Sunday, said he momentarily froze when Pettis leaped off the cage because he had never seen such a move. The "kick heard around the world" has been viewed more than 3 million times on YouTube.
That kind of creativity, aggression and athleticism is what appeals to fans of mixed martial arts. In the UFC, the top rung on the MMA ladder, there's never a dull moment. UFC fighters are trained to attack and show no mercy until their opponents are knocked out or tap out.
Roufus calls it "controlled chaos."
"It's really hard to describe," Pettis said. "It's just crazy."