To break through tackles and push through mounds of men just to gain a few yards, one's toughness mustn't be questioned.
It's usually the running backs who are the storytellers, the guys who have reason to be beasts on the field. Typically, they're the player a fan fights for because he's been fighting all his life.
And then sometimes it only takes pure talent to captivate us. No braggadocio or backstory needed. Just a raw, physical ability that makes jaws drop and eyes hard to close.
Carlisle has no reason to be defiant.
His parents have been married more than 14 years, he's a devout Christian and his dad has a successful job as the director of sports performance at Purdue University.
So what makes this 5-10, 18 year-old kid so tough? What makes him so talented?
"Amir is standing out, continues to every day. Amir continues to show things," Trojan head coach Lane Kiffin said.
Those things include a shiftiness when evading defenders that are bigger than he's ever seen. Or displaying things like resilience when breaking through tackles that are stronger than anything he's ever known.
"Obviously he's not a big guy but to see him try to run and - you've been around when a freshman starts to shine a little bit - [the opposing side is] going to take their shots at him and you saw that today. You say guys kind of, really take shots at him and saw him bounce right back up and go," Kiffin said.
The 4.0 GPA student was a decommit from Stanford last year, opting for the Trojans after then-coach Jim Harbaugh's departure to the NFL.
It was a Reggie Bush run that first attracted Carlisle to USC. But unlike Bush, Carlisle doesn't play for the fame or money. He plays for God.
That mindset is what keeps him in the Bible and the playbook instead of a pocketbook.
In a short time, Carlisle has impressed both players and coaches alike. Fellow tailback Marc Tyler called him "explosive." Barkley said Carlisle has impressed him.
His off-the-field resume is pretty impressive, too. Carlisle gets stellar grades, is a role model to his two younger brothers, acts in theater performances as a hobby, and spends time socializing to make sure he's a good friend, although he said he's never had a glass of alcohol.
It's uncertain whether this group of backs feels threatened by a newbie like Carlisle coming in and making the noise he has. But perhaps they should. Because according to Kiffin, age doesn't matter.
"We don't have any issues about playing freshmen if they're the best guy," Kiffin said.
The best backs will emerge soon. And Amir Carlisle might just be among them.