Byrd, 34, has added a martial arts workout called Muay Thai (pronounced "moy tie") to his offseason program. The intense sessions, which he does three to four times a week, plus a dramatic change in his diet has helped the Cubs center fielder go from 255 pounds, which he weighed at the end of last season, to 215 now. That's the same weight he was his senior year in high school.
"Now, he's a lean, mean, fighting machine," said Robert Cole, Byrd's instructor at L.A. Boxing in Chicago and a retired national champion from England.
The first step in the transformation regarded Byrd's diet, and he saw New York nutritionist Robert Pastore in New York on the recommendation of Raul Ibanez and Jayson Werth. Tests revealed Byrd was allergic to milk and wheat, and very close to having celiac disease. His wife, Andrea, had the same allergies. Pastore advised the Byrds to change their diet and both saw instant results.
"The fat started melting away," Byrd said Wednesday. "No more bloating, no more food sensitivities. My body just kicked into high gear and I was able to keep it revved up."
He used to box when he was younger, and also when he was in Philadelphia at Joe Hand Gym. But this offseason, Byrd was looking for more.
"I wanted a change of pace," said Byrd, who spent the winter in Chicago. "I did the boxing and I wanted to throw the kicks in. I heard about 'Muay Thai' training and I thought it was very intense."
It is. Muay Thai evolved from hand to hand tactics of the Thai army. A form of martial arts, it features punches, kicks, elbows, knees, standing grappling, and head-butts to wear down the opponent.
The workout begins with jumping rope. Then, Byrd and either Cole or Aaron Swenson, a three-time U.S. national champion, will do some sparring in the ring. It's three minutes of kicking and punching, then one minute of rest, and another three-minute workout, a pace they keep up for one hour. That session can burn about 800 calories.
But they're not done. There's more work on technique, some "clenches" which involve trying to hold the other man's head. Then, Byrd and his instructor add shin guards and mouthpieces, and do another, more intense, more violent round of sparring.
"I'm thinking he should use his legs for batting," Cole said.
They finish with more kicks and punches to a heavy bag. If a fighter goes from a workout with the bag into the ring, it has almost the same effect as a batter using a weighted donut on the bat and then stepping into the batter's box.
"Guys in the game are using this to get in shape," Byrd said of martial arts. "I don't know how many guys are taking it to this level but I love it."
Byrd isn't learning kicks and punches for the game. His training provides other benefits.
"If you look at throwing a punch or a kick, it's the same [as baseball]," Cole said. "You have to turn your whole body. When you're hitting, you have to go through the ball with your body. It's the same kind of physics."
It's just a part of his daily drill. By the time he arrives for his Muay Thai workout, Byrd has already done some weight lifting, and he follows his sessions with Cole and Swenson with hitting and throwing.
"I'm 34 years old now and I'm not getting any younger," Byrd said, "but at the same time, when people watch me play, they don't know my age. I have to keep up with these young studs who are coming up. Brett Jackson is full of energy, and Tony Campana, he keeps me young. These guys work hard and I'm trying to keep up with them."
Byrd got positive feedback in early January when he attended a mini camp in Mesa, Ariz.
"Everything is working in conjunction now," Byrd said. "You're thinking, 'Check a kick, throw a punch,' or 'throw an elbow,' or 'throw a kick and follow with an elbow, then the knee' [in Muay Thai]. In baseball, you have all these moving parts."
Watching him deliver right crosses or kicks, one can see how improved flexibility, balance and strength should have positive results.
"All around, it makes me a better athlete," Byrd said.
The Cubs can only hope everyone on the team is training as hard for 2012. Campana has added some good weight working out in Mesa, as did second baseman Darwin Barney, who put on 18 pounds of muscle. Byrd, entering the final year of his three-year contract with the Cubs, is hoping his changes affect the others.
"If I'm out there and they see me slow and sluggish and a little bit behind them, they're not going to listen to me at all," he said. "When I'm the guy out front and when I'm the guy pushing them, that shows leadership.
"It's like I'm passing the torch," he said. "These guys are going to be the future of this organization and by the time I leave this organization, whenever that is -- a year from now, three years from now, four years from now hopefully -- this organization will be better."
Part of the reason for the extra effort to improve his stamina is because of all the day games the Cubs play, which can wear down players.
"My energy coming in, I always had that energy," Byrd said. "But am I going to have the same amount of energy in August and September?
"Talking to Theo [Epstein], talking to Dale [Sveum], talking to Rudy [Jaramillo], we have to figure out a way to turn day games into a home field advantage," he said. "When guys come in here, they should not be beating us on our turf. The 1:20 [p.m.] games, noon games, those should be 'W's' all the time. This will get me ready."
Epstein, the Cubs president of baseball operations, plus Sveum, the new manager, and Jaramillo, the hitting coach, should all be happy to hear that.
Byrd wants to play all 162 games. That was his goal last season before he was hit in the face by a pitch in May by Boston's Alfredo Aceves.
"I'm very, very honored that I'm the oldest center fielder in baseball," he said. "Somehow I have to keep [center field] before they kick me out and move me to the corners. As long as I'm out there, I'll be running around like a wild man and this gets me ready to go all out, every day, for nine innings."
Martial arts is all in the family for Byrd. His 4-year-old son, Marlon Jr., is learning Jiu Jitsu, and Andrea also has been learning Muay Thai.
Cole, 39, admitted he didn't know Byrd was a professional baseball player when he first met him.
"He's a lighter, fitter, stronger, faster man now," Cole said. "Hopefully, he'll be knocking out ball after ball."
Carrie Muskat is a reporter for MLB.com. She writes a blog, Muskat Ramblings, and you can follow her on Twitter@CarrieMuskat. This story was not subject to the approval of Major League Baseball or its clubs.