As Anthony Rendon gets more comfortable playing second base, he is also adjusting to the extra muscle he packed on during the offseason. (John McDonnell/The Washington Post)

In his three seasons at Rice University, before the Washington Nationals selected him with the sixth overall pick in the 2011 draft, Anthony Rendon built a reputation for possessing the sweetest swing among amateur baseball players. He was named national college player of the year as a sophomore, reached base more than 50 percent of the time and had a slugging percentage of .801 in one season. He accomplished all this with an unimposing 5-foot-11, 190-pound frame. He cared little about weight training and conditioning.

“I was just trying to put in the work and not really working out,” he said Saturday. “I had the weight, but it wasn’t good weight. I really didn’t buy into the weightlifting.”

It’s a regret Rendon, 23, holds to this day. Since then, however, he has dedicated himself to better preparing his body for professional baseball. After injuries throughout his career, he wants to stay as healthy as possible. He has added 18 pounds since the end of last season thanks to an intense offseason lifting program under Houston-based trainer Ben Fairchild, who marvels at Rendon’s cat-like reflexes and strength.

Rendon is 199 pounds, the heaviest he has played at, but his body fat is down to 8.5 percent, and he says he feels amazing. He played 134 games last season between the minor and major leagues, and his body felt the grind. Although he is competing for the Nationals’ starting second baseman’s job, he has prepared his body for the 162-game marathon of what may be his first full season in the majors.

“It was a big eye-opener when I came in and kept playing and playing and playing,” Rendon said of last year. “I’m like, ‘This is a long season!’ But that’s why you live and learn.”

Rendon’s rookie season was a whirlwind. He wowed coaches and teammates in spring training, but because he missed so much time the previous season with a fractured ankle, he began the year at Class AA Harrisburg. An April injury to Ryan Zimmerman forced the Nationals to call up Rendon to play third base for two weeks before returning him to the minors. An injury to the slumping Danny Espinosa forced the Nationals to call up Rendon from Class AAA Syracuse in June — but to play second base, a position he last played every day in Little League.

Just before his second call-up, Rendon’s wisdom teeth were removed, and he lost 15 pounds. Then came the heaviest workload of his career playing every day at a new position at the highest level. By the time he went home to Houston after the season, Rendon weighed 181 pounds.

The goal for Fairchild, who has worked with Rendon every offseason since he was drafted, was to add muscle to Rendon’s body. Rendon did a lot of training that required lifting weights slowly in specific movements. They focused on knees, hips and core strength, areas of the body heavily used in hitting, fielding and throwing.

Throughout the winter, Fairchild admired Rendon’s dedication and strength. Fairchild has trained baseball players in the past, including Craig Biggio and Lance Berkman, but Rendon stands out. He can deadlift 505 pounds on a trap bar for seven repetitions. The next strongest player at Fairchild’s facility can deadlift about 400 pounds, he said.

“What’s special is the hand strength,” Fairchild said.

The skills that made Rendon such a gifted hitter — exceptional hand-eye coordination and quick, strong wrists — make him a disproportionately strong weightlifter. Fairchild uses Rendon as an example with younger players he trains.

“We’ve got hitters who are 230 pounds who can’t lift the weight that he does at 190,” Fairchild said. “His nervous system can grasp every muscle in his body and get it all to fire in concert. I marvel at all the physiological tests we do with him.”

There’s a drill Rendon loved to do over the winter. Fairchild would toss a ball that had knobs and, as a result, would bounce unpredictably. Rendon would catch it with his left hand. To mimic playing in the creeping shadows of an afternoon baseball game, they did it with an open door to let sunlight in. As he shuffled his feet, Rendon kept his legs spread open as if fielding a grounder and nabbed every ball.

“I’ve been blessed with great hand-eye coordination,” Rendon said. “That’s just one of the drills I wanted him to push me on. I thrived and really pushed myself to do that because it’s more game-like activity.”

Drills like this prepare Rendon for the possibility of playing second base nearly every day. He may not have the level of defensive skill or experience at second base that Espinosa does, but Rendon has improved at the position and can hit. He posted a .265 batting average with a .329 on-base percentage and a .396 slugging percentage last season with seven home runs. He might also play third base on days Zimmerman plays first base. Rendon has played both positions this spring, but having a full camp in which to work out at second has helped.

“It’s been awesome,” he said. “Just trying to get as many reps as possible and trying to learn from all the guys around me. And even the minor leaguers that come up. ‘You’ve been playing second base your whole life. What do you like doing in this? Or that?’ ”

Rendon feels prepared for the season and vows to do his best to keep his weight up with tips he learned from teammates last season. Like many baseball players, he knows he will lose weight over the next six months, playing in the summer heat.

“I feel great,” he said. “I’m just trying to get used to the weight and moving my body around as quick as I can but still staying stable in my movement. Just gonna try to keep my weight on the whole year and see how it goes.” Then he paused. “I haven’t lost any since I’ve been in spring training,” he added, smiling. “Still 198, 199.”