At UFC 70 in April 2007, Gabriel Gonzaga gave Mirko "Cro Cop" Filipovic a dose of his own medicine when a lancinating kick to the head sent the heavily favored Croatian into momentary oblivion.
Four months later at UFC 74, Randy Couture rendered Gonzaga a similar fate, when a flurry of unanswered punches prompted veteran referee Herb Dean to stop the fight.
Dean, who refereed both contests and who has officiated thousands of bouts, noted the Filipovic knockout was the worst he has ever seen because of the way Filipovic went down.
"[Filipovic] got knocked out and fell in such a way that his limbs were mangled," Dean said.
A knockout or technical knockout from strikes to the head results in an alteration in alertness or consciousness. The brain shuts down from direct compression against the skull; cerebral circulation halts and muscle tone is lost. The shutdown is a protective mechanism. Once the brain reboots, consciousness is regained.
The force, duration, type of shot and delivery site (jaw, chin or temple) determine the blow's significance.
When Chuck Liddell knocked out Couture at UFC 52, it was from a devastating right cross that rotated Couture's head. The shot compressed Couture's carotid arteries and cut out the circulation running along the side of his neck.
That type of strike is a classic example of acceleration blows, in which a stationary head is struck by a rapidly moving leg or hand. It was the first time Couture was knocked unconscious.
A deceleration blow is what resulted when Carlos Newton controversially lost his welterweight title at UFC 34. Newton had a triangle choke around the neck of Matt Hughes before he was rendered unconscious when Hughes slammed Newton against the mat. Newton's choke might have rendered Hughes unconscious before he fell to the canvas with Newton locked onto him. Newton was knocked out when his head crashed into the mat.
It's a bit more simple in boxing to determine if a fighter has recovered from head shots. He must either remain standing, or get up and walk toward the referee after an eight count.
In MMA, though, there is no count by the referee; the fighter is either able or unable to continue.
"I look to see if the fighter's limbs are in a defensive position. He can't be sprawled out," said Dean, himself a former mixed martial artist. "They can do anything that lets me know they are in the game. If on the ground, they need to regain a guard, show head movement or buck their hips if they're getting hit. I'll tell them, 'Fight back; I need to see movement.'"
And what if a fighter is out of guard but still conscious?
"If semiconscious, I allow two to three blows before I stop it, as the fighter can't defend himself," Dean said. "You know you are OK if you can stand, but on the ground, as a fighter, you don't have a point of reference, and when you wake up you don't know what has happened."
Between rounds, the doctor will evaluate a fighter who has taken punishment to determine if they are able to continue, and make a recommendation to the referee. The doctor's role becomes front and center once a fight is stopped on head shots.
In New Jersey, California and Nevada, there are at least four doctors assigned to an event: two ringside and two that follow the fighters to their dressing rooms. If a knockout or technical knockout occurs, the doctors enter the cage to make certain the fighter has a clear airway, is breathing and has regained consciousness. If not, the athlete is immediately transported to a hospital for a brain scan.
The fighter is allowed to leave the hospital if the scan is normal and there is no evidence of cognitive injury like confusion, memory loss, headache or dizziness.
Fighters stopped by head shots receive a medical suspension from the commission that is upheld nationally. During this time, he cannot spar or schedule another match.
When Couture stopped Gonzaga in August 2007, Nevada suspended Gonzaga for up to six months for a fractured nose. He was also given two months for suffering a technical knockout.
Suspensions are meant to give a fighter rest. The brain needs time to heal just like any other part of the body. Unlike sports like football and hockey, U.S.-regulated mixed martial arts events have tight restrictions in providing the athlete significant time off.
While certain fighters are more prone to knockouts than others, there are protective factors to help avoid being knocked out.
Fighters strengthen their neck muscles to absorb the effects of blows. A good chin tuck lessens head movement with a punch. A well-made mouth guard stabilizes jaw alignment and stability. It absorbs much of the force of a blow to the jaw and temple.
Lastly, good gym habits, maintaining weight and solid preparation are perhaps most important to preventing a knockout.
Lightweight contender Kenny Florian, has never been knocked out in a fight or in the gym. Florian typically walks around at 168 pounds. "I don't sweat out more than two pounds," he said. "I have a nutritionist, am smart with my diet and there is no offseason."
Dr. Margaret Goodman, a former Nevada State Athletic Commission Medical Advisory Board chairman and chief ringside physician, contributes regularly to The Ring magazine.