Outside, little hands press against the glass door, shadowed by a half dozen faces watching the frantic waves and chops of the children inside. Then the hands are gone, tugged away by parents in a hurry.

Jesse Dixon, here to watch his two daughters practice their side kicks, blocks and punches at the Joe Lowe Karate Federation school on 4th Avenue, glances at the hopefuls outside. “The same kids pass here all the time. Their parents pull them away,” he says as his 9-year-old, Natasha, corrects another student’s fighting stance. “These parents don’t know. My kids, they’ve changed. My daughter used not to like to talk to anyone. Now she teaches the classes.”

The man who runs this space, Joe Lopez, is dressed like his students in a black robe and black pants. He walks among the sparring pairs, advising, motivating, correcting.

In most ways, Lopez’s martial arts studio looks like any other. Trophies won at student tournaments crowd a window, and the walls are lined with mirrors, framed Japanese characters and posters outlining the body’s strike zones. Look closer, though, and there are dozens of pictures of his students pasted on the wall: at home practicing, at tournaments holding awards, in class smiling. The studio has gone beyond teaching karate, ju jitsu and judo. It has become a local youth center.

Lopez has lived in Sunset Park for more than 20 years, gradually building a reputation here-first as a teacher, then a merchants’ association organizer and recently as the publisher of an underfinanced community newspaper called the Sunset Park Times. Lopez went so far as to give up his apartment for this latest venture. He moved into the karate studio’s finished basement, using the money that had gone to rent to pay the monthly printing bill. He says he doesn’t mind, though.

“In the martial arts, the idea of the Japanese samurai is they were originally servants of the people,” Lopez explains. This is his persona. He is “Jo Lo,” the martial arts master that his own karate teacher taught him to be. “People need encouragement and help. That’s what my calling is.”

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Growing up, Lopez and his family moved from Red Hook to East New York to Puerto Rico and back again. At age 8, he discovered karate, as many boys do, from Bruce Lee movies. But it wasn’t until he was nearing the end of adolescence that Lopez became a serious martial arts student. In 1991, he started working in the community as an instructor at the Center for Family Life. That same year he opened his school under the name Joe Lowe, an Anglicized version of the nickname his karate teacher gave him. Today, at 35, he is a seventh-degree black belt.

Lopez’s studio sits near the 50th Street corner of 4th Avenue along an unusually commercial stretch of this otherwise residential street. The school is close to 5th Avenue, where local kids spend their afternoons wandering in and out of the arcades and discount stores. Some kill time there. Others fall in with gangs, like the Nietas and Latin Kings, or drug dealers.

Lopez’s students are part of the mix. About half come from families of merchants and bureaucrats, with parents who are police officers, IRS agents, bodega owners. The other half are working class or poor, some getting scholarships from local businesses to help with the $40 a month in tuition. Most of the students are Latino, primarily Puerto Ricans and Dominicans, though a handful are African American.

The martial arts are about developing discipline, and Lopez believes that discipline must be intellectual as well as physical–he says he won’t advance a child who isn’t also improving in school. Rosie Chevres says that her daughter Samantha has been bringing home better grades–13 As and five Bs last year-since she started class. Last summer when Chevres had to go to Florida on business, she says she was astounded to find that she actually had to persuade her daughter to skip class to go to Disney World.

Chevres says she thought this was weird until recently, when she found herself passing up a trip to Atlanta so Lopez could train her and some other parents to be local tournament judges. She felt that this was a cause worth missing her trip for. Samantha and her classmates were being cheated at tournaments by combatants using illegal moves–and the other judges were allowing it. Lopez needed her in his corner, she says. “Joe is very fair, and most people take advantage of it.

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Standing outside of his studio on a wintry November afternoon, Lopez points to a deep dent in the fencing of the playground across 4th Avenue, the result of a collision between a car and an ambulance. There have been so many accidents along this four-lane boulevard that Lopez actively avoids the intersections. He finds it safer to give himself more reaction time by crossing a few feet inward.

Cars racing down the street are not his biggest worry, though. Lopez shares this block, and sometimes the loyalty of his students, with local drug dealers.

It was just a year ago October that his students walked out of the studio at the end of class and into the gunfire of a drug battle, sending kids and their parents screaming and crying into the night. The battle drove out the old group of drug dealers in front of the supermarket next door, but established a new guard on the end of the block. “After that shoot-out, if I didn’t do anything about it, I’m a hypocrite,” he says. “I’m saying [to my students], ‘Go out and change the world,’ and I can’t even change the community.”

So Lopez set about organizing the businesses and homeowners along the avenue. The first project for the group, 4th Avenue Businesses, was to paint the strip’s gates and doors green–an attempt to ward off the pervasive graffiti in the neighborhood.

The group was also able to engage the local police. “As soon as a fight breaks out, an unmarked car pulls up,” says Kathy DeRiso, who with her husband owns the DeRiso Funeral Home down the street from Lopez’s studio. “It’s like they’re already there, waiting.”

But an idea to pressure the city to plant trees along the partition that divides 4th Avenue hasn’t moved beyond talk. Plans to formalize the group have stalled, with the three dozen other members leaving it up to Lopez to raise a $500 incorporation fee. And silver graffiti tags have reappeared on the green doors on either side of Lopez’s storefront. To cover up similar scrawls on his own gates, Lopez has had to abandon the signature green for cheaper black paint.

Undaunted, Lopez had a second idea following the shoot-out: the Sunset Park Times. He says he found it amazing that the local papers ignored this violent event. Last February, he began publishing the eight-page monthly, printed and bound magazine-style. He and a handful of his students attend most of Sunset Park’s public meetings, writing them up in straightforward, albeit rough, prose. The front page says the paper costs a quarter, but Lopez usually hands it out at local organizations and shops for free.

To finance the $400 monthly printing bill, Lopez gave up an apartment on 50th Street. He now lives below his storefront in a Spartan yet cozy plywood-paneled room. “That’s okay,” he says. “I look at it as an investment in the community. If I don’t get the money back, I don’t care.”

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From the time he wakes up around 10 a.m. until he coaxes the students, parents and onlookers out of his studio 12 hours later, Lopez dedicates his waking hours to his self-imposed community obligations. He admits he can’t say no–especially if it looks like no one else will help. “I just say, it has to get done,” he explains, “and I get my eight hours of sleep.”

Word has gotten around about the guy on 4th Avenue who will help you with your problems. During the day his phone rings with calls from people seeking help for some new endeavor or another. The community group UPROSE down the street wants a new logo. A parent needs to spruce up a newsletter for the Hispanic Society, a police association. His former karate teacher calls to arrange the taping of a public-access show on martial arts they are producing.

The phone rings again. The Southwest Brooklyn Industrial Development Corporation needs classroom space for a youth boat-building project. On the phone, Lopez is non-committal. Once he hangs up, though, the acquiescence begins.

Lopez does have another room that might work.

He shakes his head, smiles, then starts figuring out how he might move out his weights and exercise equipment.