The role of the fullback is a sometimes thankless and overlooked job. Responsible for taking on linebackers and clearing out hulking pass rushers looking to take the quarterback's head off, it's one of the most physically demanding positions on the field, and it takes an athlete who has little regard for their own physical well-being to succeed at the spot.
"The fullback has to be a physical guy out there, and it starts with us," Vainuku said. "If I can't block the linebackers and be physical with them to get the back through, that's my fault. So I just need to know my role and know that I need to be a physical fullback out there -- put fear in people's eyes and cause some mind games for them."
In Vainuku, who was the team's primary starter last fall, the Trojans have a virtual wrecking ball at the spot, with the redshirt sophomore possessing a 6-foot, 250-pound frame and deceptive speed. More than that, he's also one of the team's most tireless workers, and he's customarily the last player to leave Brian Kennedy/Howard Jones Field each day after practice.
Pinner, a sophomore Vainuku took under his wing as a first-year contributor in 2012, is a sturdy 225-pounder who showed great promise as a freshman, factoring more and more into the rotation as the season progressed. Not surprisingly, he's taken the lead of his counterpart and developed quite a work ethic of his own.
Together they form an imposing tandem, and one that is continually moving forward thanks to a friendly competition that has developed between the two.
"We're always going to compete on the field, but me and him, we're the best of friends off the field," Pinner said. "Last year, he basically had to mentor me. He helps me out, I help him out, so there isn't any feud or anything -- it's just competition."
With that kind of teamwork, Vainuku and Pinner have helped each other continually raise the level of their games, but when it comes to their production in terms of offensive numbers, there is certainly still room for growth. With the two accounting for just 26 yards on the ground and eight receptions for 50 yards this past season -- all by Vainuku -- Kiffin is hoping the pair can develop into bigger threats in 2013.
"Although we don't have much depth at fullback, we would like to get that position more involved in the offense," Kiffin said. "The two fullbacks are getting better, and they need to. We got no production from them really last year, which was a big difference from the year before."
With the likes of Rhett Ellison in 2011, and prior to that, Stanley Havili, serving as examples of effective playmakers who have lined up at the position in recent seasons, Vainuku and Pinner are well aware of the fact that the potential certainly exists for the fullback to fill up the stat sheet in the USC offense. But in order to earn the added responsibility, they first have to prove themselves this spring.
"I think that's just predicated on how we do in practice as far as showing them what we can do," Pinner said. "If we're out there catching passes and doing everything that we're supposed to right, then they're going to show us some love and get us more involved in the offense, get us some touches, maybe get us a couple of touchdowns this year. It's definitely on us."
But it's not as if both players haven't found success with the ball in their hands before. As a junior at Orange County high school-power Mission Viejo (Calif.) in 2010, Pinner amassed more than 1,000 yards on the ground before gaining more than 500 yards as a senior despite being hampered by an ankle injury for much of the season. Vainuku, on the other hand, racked up a staggering 2,202 yards and 38 touchdowns his senior season at Eureka (Calif.) High School in 2009 as the team's featured back.
Under the tutelage of newly hired running backs coach Tommie Robinson, Vainuku and Pinner have been putting in more time and effort than ever before. And while they've only taken instruction from him for a total of three practices so far, both players already like what they've seen from the veteran with 26 years of coaching experience under his belt -- the past three spent with the Arizona Cardinals.
"He's always coming in with energy and everything, which is what we need as an offense." Vainuku said.
"He's a great coach," added Pinner. "You can't go wrong coming from the NFL back down to college, for me, from a player's standpoint. He's been where I want to go, so I definitely want to listen to everything that he has to say so that it can help me improve upon the skill set that I have now."
With a new position coach and a general feeling on the team that every player is starting with a clean slate, both players understand the importance of this spring. For Vainuku, who has a year up on Pinner when it comes to playing in the offense, it's more a time for working on the smaller things than anything else.
"I've got the playbook down pretty well now to where I know my assignments and I know the formations and everything, so I just want to perfect my game... knowing the offense, knowing the defense, who's out there, what coverages they're in and just being able to play faster," Vainuku said.
Pinner, meanwhile, sees this March and April as a time to build off of the momentum that he gained at the tail end of last season, and possibly make a case for more playing time.
"I've felt really good this spring," Pinner said. "Since last year, I know a lot more, so coming in now it's easier…the flow of the game, the practice -- the energy is way different. It's always time to evolve, but definitely now, it's time to show the coaches what I can do."
And with Vainuku always there to provide a boost for Pinner, and vice versa, it's safe to say that when it comes to the evolution of both players, they're not even close to being done.
"Nothing is locked down, it's the spring, so it's definitely a competition out here," Vainuku said. "We're both out here working to just get better."