At the command of ex-SEAL and Libertyville High alumnus Rob Stella, the Wildcats did countless pushups, jumping jacks and flutter kicks. They rolled through dirt and crawled through human tunnels.
They sweated. They groaned. They pushed themselves beyond what they thought were their limits.
And through it all, Stella coached and coaxed them to work together. He urged them to overcome the blazing heat, the frustration of difficult physical challenges and the growing pain in their bodies.
"It's all in your mind," Stella told the nearly 80 football players as they did calisthenics on the practice field behind the school. "Sacrifice for your teammates."
Stella, who was accompanied by fellow retired SEAL Nick Hays, has helped train Wildcat football players the Navy way once before, in 2012. They run programs like this through a company called Acumen Performance Group.
Clients include groups from Northwestern University, the Miami Heat and Air Canada.
The goal Wednesday, Stella said, was to teach the teens how to maximize their performance.
"We only want you guys to get 1 percent better every day," Stella said. "That's doable."
Varsity offensive lineman Thomas Fenlon was among the boys who participated in the program.
"I was hoping (it would help me) get into better shape for football," he said. Fenlon admitted the session was harder than he expected.
Not every football player took part. Wide receiver Jack Morris watched from the side because of a knee injury.
As hard as the exercises were, Morris said he wished he could participate.
"This is a once-in-a-lifetime kind of thing," he said.
The day also included indoor discussions about mental toughness, mentoring and other topics.
Varsity head coach Mike Jones said he hopes the teens learned to work through adversity, and not just on the gridiron.
"It's in the classroom. It's in life," Jones said. "They can be more than they think they are."