Don't be fooled by the hot pink handlebars, the leopard-print wheelchair or the group of seventh-grade girls prone to bouts of giggling. The contraption sitting in 13-year-old Melissa Phelan's garage could turn out to be a valuable tool to help seniors retain muscle function. Melissa, Julianna Mola, Sarah Nocito and Christine Rocchio of Saddle Brook Middle School jokingly refer to themselves as the "Legally Blonde Inventors," but these girls are far from airheads.

Using their grandparents as guinea pigs, they did physics calculations and consulted doctors who specialize in sports medicine to create the Tred-Med, a multifunctional exercise machine for wheelchair-bound seniors. The invention has landed them in the finals for the Christopher Columbus Awards in a national science program that challenges middle-school students to explore opportunities for positive change in their communities.

The students, along with their coach, Marilyn Ryan, are headed to the awards finals in June at Walt Disney World, where the winning team will get a $25,000 grant to either improve its product or patent it. There are eight teams from across the country vying for the top prize.

The invention was part of a yearlong project in Ryan's gifted-and-talented class, which required teams to identify a problem in their community and come up with a solution. Ryan's curriculum, which follows the Christopher Columbus Awards model, seeks to use the scientific process to solve real-world problems and cultivate an interest in science at a young age.

The girls fall into a sobering demographic. Studies show that women make up just 25 percent of the workforce in science, technology, engineering or math fields.

"We weren't that interested in science before this project, but then we realized we can really help people," Sarah said.

When looking for a problem to solve, the girls turned to their grandparents.

"Me and Sarah both have grandparents who have had hip replacements, and we wanted to find a way to help them regain mobility or just help them get exercise because when you don't use your legs, the muscle fiber breaks down," Christine said.

The Tred-Med is essentially a manual treadmill with custom-built handlebars that has been lowered to the ground for wheelchair access. Seniors who have limited mobility can wheel up to the machine, grab the handlebars and exercise their legs while remaining seated. Others who can stand up with assistance can walk at their own pace while holding on to the bars.

The final prototype will include detachable resistance bands, a pedometer, a heart-rate monitor and a portable stereo with timed playlists, "because according to our research, seniors do better when they can stop exercising when the music stops, instead of having to time themselves," Julianna said.

Part of their research was a series of consultations on proper fitness techniques for the elderly with Robert DeStefano, a Bergen County-based chiropractor who works with the New York Giants. They also applied the Borg scale (a method of determining physical activity intensity) to help seniors figure out how much of a workout they're getting while on the Tred-Med.

Nearly a dozen seniors - grandparents and friends - tested the machine, prompting several changes, including switching from wood to PVC piping as handlebars because it is better suited to arthritic hands.

"These girls are so fabulous because they all have different personalities and different activities going on, but they were committed to the project and showed a lot of passion," said Ryan, who led a winning team to the finals two years ago.

"They applied the scientific method: identifying a problem, coming up with a solution, testing it and making changes when necessary."

Working on a $100 budget, the girls found wood and pipes by sifting through garbage or finding discarded parts on construction sites. They found an old manual treadmill for $40 on Craigslist, split the cost four ways and broke up the machine, discarding everything but the conveyor belt.

In the end, they met their deadlines, came in under budget and became best friends.

"We work really well together because we all bring something different to the team," said Melissa.

They also want to make sure the judges know they've got a lot of spunk.

As part of a skit each team performs for a panel of judges, the girls - most of whom are brunettes - are planning to spoof the Reese Witherspoon flick "Legally Blonde," which is now a Broadway musical.

"The main theme is girl power. You can be a girl and still have brains," Julianna said, calling the team "Scientifically Blonde."

Before they display their invention - which has been painted pink and features a leopard-print wheelchair cover - they plan to perform a modified number from the musical.