When the Cooper Street YMCA Family Center celebrates its 10th anniversary Birthday Bash on Saturday, Helen Maddox will join in the celebration.Like many other well-wishers, she marvels that the 41,400-square-foot-center, which opened in 2003, serves an average of 19,500 people a month in Arlington and Mansfield. Maddox, however, is looking at the milestone from the perspective of a founding member of the YMCA.It was 1958 when Maddox, who will be 99 years old next month, decided to get involved in founding the city’s first YMCA. When she and her late husband, John, donated a new desk and chair for the office of the fledgling YMCA, they never dreamed how much it would grow to 60,000 memberships across the city with three branches offering scores of programs and services.“When my husband and I decided to help start the YMCA in Arlington all those years ago, not many people in town knew much about the Y and what it stood for,” said Maddox. “This was a little before the time that getting exercise and being physically fit got so much attention.”“Now the YMCA is known as a place to improve your health and keep kids involved in healthy activities.”Saturday’s event will celebrate with a family-style hot dog picnic lunch and birthday cake. The public is invited to participate in field-day-themed games and swim in the indoor pool.4,100 membershipsSince its opening, the Cooper Street Family Center has grown to 4,100 membership units with family memberships making up 65 percent of the enrollment. A Silver Sneakers program for older adults at the facility has more than 2,500 participants.“We are grateful to the donors, the volunteers, and the staff that worked so hard to make the South Cooper Street YMCA a reality 10 years ago. The support from the community has been incredible from the moment we opened the doors,” said President and CEO Roberto Aguirre. Saturday’s event will also feature the unveiling of a half-mile nature trail – the Cooper Outback Trail. Nestled behind the facility in a natural area not typically expected in an urban setting, the 12-acre trail has four exercise stations sprinkled along the outdoor walk. Each station has exercise equipment made from recycled plastic with exercise instructions provided for users.Participants can choose simple activities such as a heel flex, hip flex, knee grip and arm stretch to more strenuous exercises such as push offs, body pull, sit ups, arm walk, and others.“Families can go on a walk together along our trail, and stop at one or more of the stations, and do multiple exercises at each station. They can exercise or just walk outside together in a natural setting,” said Y spokeswoman Pamela Webb.A grant from the Arlington Tomorrow Foundation funded the equipment and signage, and volunteers did the construction work. Volunteers from Lockheed Martin, Lowe’s Home Improvement store and JP Morgan Chase helped in clearing the land and installing mulch.Outdoor chapelLocal Eagle Scouts built a chapel in a clearing in the nature area that features a redwood archway, podium, benches and a large cross.In 2008 a 4,400-square-foot expansion increased the size of the fitness center and added office space. The facility now includes an indoor pool, a two-court gymnasium, fitness center, aerobic and cycling studios, multipurpose and conference rooms, an indoor child care center (Kid Zone) and an outdoor playground. Original plans that called for adding an outdoor pool and sand volleyball and basketball courts were put on hold as the Y re-assessed community needs.“We encourage the community to be a part of our efforts to strengthen our community and visit the Cooper Street YMCA Family Center, or any of our other two facilities, and learn about all of the great programs available to you and your family,” said Aguirre.