Many area martial artists were numb yesterday as they learned of the death of Robert Massaroni, a champion Tang Soo Do instructor who collapsed and died moments after he finished the 10-mile Broad Street Run on Sunday.

Massaroni, 29, of Holland, Bucks County, was chief instructor at the Bensalem branch of Nate Gordon's Black Belt Academy, where he was idolized by his "little dragons," the younger students who trailed after him.

"He was the first guy to show up to help, and one of the last guys to leave," said Louis Castelli, who trained Massaroni when he was a boy and later taught with him. "This was his life. The kids loved him."

Massaroni excelled at the Korean discipline known for explosive kicks and punches. He was the Regional Black Belt Men's Grand Champion in 1997, 1998 and 1999, and won a men's world championship in 2002, according to the school.

Gordon said in an e-mail that he was lecturing in Singapore but was cutting his trip short.

"Rob was an incredible individual," he wrote. "He came from a very tight Italian family - was the 'baby,' fell in love with his wife, Val, who came from an even closer Italian family. [He] was married less than a year."

Gordon said Massaroni was "loved by the students and had a special knack for teaching, especially little ones. . . . A wonderful person, son, husband, teacher and friend. He has touched many lives in his short time on Earth."

Massaroni was also a seventh-grade teacher at Shafer Middle School in Bensalem, where he took an interest in youth fitness. Last year, a reporter who visited the school described Massaroni helping students to learn yoga, aerobics and karate.

Efforts to reach school officials were unsuccessful.

Massaroni earned a master's degree in education from Temple University.

The Bensalem Black Belt Academy was closed yesterday in Massaroni's honor. Staff members posted a Web tribute to the chief instructor, whom they remembered as "a man that loved. He loved Tang Soo Do, he loved his family, he loved Val, his wife."

Castelli said Massaroni had trained for his third Broad Street Run. He came in 3,934th among approximately 15,000 who finished the race, running the course in 1:24:34 on the cool and windy morning.

"He was an athlete," Castelli said. "That's why this is so crazy. . . . It's devastating."

Massaroni collapsed just after completing a point-to-point run that Runner's World magazine has named one of the fastest 10-mile courses in the nation. The race started at Central High School at 8:30 a.m. and finished at the end of Broad Street in South Philadelphia.

Temple University, Hahnemann University and Methodist Hospitals stand along the course. Massaroni was taken to Methodist, where he was pronounced dead. No cause of death was immediately available.

Sudden death among young, competitive athletes is rare, but not as singular as it may seem. The well-known basketball players Hank Gathers of Loyola Marymount University and Reggie Lewis of the Boston Celtics both died on the court. A 1991 study published in the Journal of the American Medical Association examined the sudden deaths of 158 trained athletes between 1985 and 1995. It found that death can be due to congenital heart defects or a variety of cardiovascular diseases, and that medical pre-screenings were of limited value in revealing abnormalities.

Massaroni had recently completed the requirements to become a fourth-degree master, which typically requires at least 16 years of training. He was to receive his belt this fall.

"He wasn't the most talented kid when he came to me, but he became a world champion," Castelli said. "He had the drive. He was a good role model, too. If everyone in the world was as good as this guy, the world would be a better place."