Wearing black crocs accented with two Fast U buttons, Pittsburgh Steelers running back Najee Harris shuffles through the doors of O Athletik just after 9 a.m. Muhammad Ali's likeness stretches across the back of his black T-shirt. A black Nike boxing backpack is slung over a shoulder.

Inside the cavernous downtown Houston gym -- co-owned by NFL players Adrian Peterson and Trent Williams with trainer James Cooper -- Harris fist-bumps his way to the back, where a small group of NFL players, boxers and local athletes have gathered to stretch before the morning workout.

Once in the back corner, Harris takes a seat on the black rubberized floor and trades his Crocs for a pair of sunset-colored Nike Air Max 270s.

In a day with three distinct types of workouts, Harris' footwear changes are checkpoints. Over the next 12 hours, the 24-year-old runs through a gauntlet of workouts in mid-July as he prepares for his second NFL season.

"Najee's like, if we have to lift a truck up over our head, then that's what we're doing," said Cooper, who has worked with more than 700 professional athletes. "He's not eager to do it like he's going to hurt himself. He's eager to do it to learn it, to understand the movement and get better at it."

After leading all rookies in touches a year ago, Harris is primed for another big season, though his training camp has been truncated after an offensive lineman stepped on his left foot. Head coach Mike Tomlin said the Steelers need Harris to remain a dominant, lead running back, and while offensive coordinator Matt Canada said Harris' volume of carries probably won't increase, his effectiveness will be a bellwether for the offense.

To prepare, Harris spent the offseason doing an intense regimen of workouts to fine-tune his body. It also served to strengthen him mentally, bolstering his confidence to handle the challenges ahead, including filling the leadership void created by Ben Roethlisberger's retirement.

"I think he'd be prepared to take a significant step in terms of embracing leadership and responsibility, regardless of who else is in the huddle with him," Tomlin told ESPN. "That's just where he is, and what he's done and where he desires to go."

On this day, Harris defined that leadership by finishing his workout schedule as the sun set on an empty high school football stadium near Houston.