Watch Rashaad Penny power through an arm tackle or stiff arm a defender to the turf this season, and you’d never guess what the San Diego State star’s biggest problem once was.

Believe it or not, he used to be afraid of getting hit.

When Penny played backyard football with his older brothers and their friends, he was typically among the smallest kids in those games. He’d compensate for his lack of size and strength by watching YouTube videos of Reggie Bush and emulating the ex-USC star’s famous cross-field runs, jukes and jump cuts.

Dynamic speed and rare elusiveness helped Penny make an instant impact during his first two seasons of organized football in high school, but at barely 160 pounds, he was uncomfortable fighting through contact for hard yards or running between the tackles. He preferred to bounce runs outside, where he could make defenders miss in open space or get safely out of bounds without sustaining punishment.

“When he’d get the ball, he wasn’t trying to get touched,” oldest brother Robert Penny said. “He’d run east-west across the field instead of north-south. It was hard for him running up the middle at first because he wasn’t used to reading holes, so I took some time out to teach him. I told him, ‘Once you get a little bit bigger and actually learn how to play the position, no one will be able to stop you.’ ”

That prediction appears prescient now that Penny is striking fear into opposing defenses instead of the other way around. The same kid who once shuddered at the thought of enduring hits in high school is now college football’s leading rusher, a long-shot Heisman candidate and a legitimate NFL prospect.

In his first season no longer playing second fiddle to Division I’s all-time leading rusher Donnel Pumphrey, Penny has piled up a national-best 588 rushing yards and four touchdowns while leading unbeaten San Diego State (3-0) to victories over Pac-12 foes Stanford and Arizona State. The multifaceted senior has also caught nine passes out of the backfield and taken a kickoff back 99 yards for a touchdown.

Whereas the 5-foot-8, 176-pound Pumphrey atoned for limited size and strength with razor-sharp cuts and remarkable quickness, the 5-foot-11, 220-pound Penny is a prototypical downhill slasher. He patiently waits for a hole to emerge, accelerates through the smallest crevice and unleashes his full array of speed and power in the open field, lowering his shoulder through some would-be tacklers and leaving others grasping at air.

“He’s explosive, he runs through tackles and he’s such a strong runner,” San Diego State offensive coordinator Jeff Horton said. “People underestimate how fast he is. He’s not as sudden or quick as Pumphrey was, but you don’t run back six kickoffs for touchdowns without being able to really go. He’s by you so fast that you can’t bring him down.”

Penny’s emergence has been perfectly timed for a San Diego State program trying to make a leap in relevance not only nationally but also in its own market. Days after the Chargers departed for Los Angeles last January, San Diego State unveiled a new “One City. One Team.” slogan in an effort to take advantage of the hole in the hearts of football fans throughout the region.

Mountain West football is no substitute for the NFL, but the Aztecs are doing their best to fill the void. Season ticket sales are up 26 percent this season for a program that has been to seven straight bowl games, captured two straight Mountain West titles and won more than two-thirds of its games under head coach Rocky Long.

“When the Chargers left, we sensed that this could be our time,” Penny said. “We want to come together as a team for this city. I’ve been in San Diego for four years and the entire time San Diego has been starved for a winning team. I feel like we have that winning culture here. We can do some things that have never been done at San Diego State this season, but we have to take it week by week.”

While Penny’s athletic success comes as no surprise to his family and friends, there was a time they probably thought he’d excel more on the diamond instead of the gridiron. Like his two older brothers, Penny focused primarily on baseball as a kid growing up in the Los Angeles suburb of Norwalk before picking up football in high school.

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