OK, let's take a closer look at that stride

STEPHANIE Felix didn't realize running could be so complicated.

The high school junior took up the sport a few years ago and quickly emerged as a top distance runner at La Mirada High School. But then her coach had team members' gaits assessed.

The analysis, in which Stephanie was videotaped while running on a treadmill, showed a litany of problems. She shifted her weight to the inner foot, which could lead to ankle and foot injuries. Her arms swung too much, which could cause misalignment in her lower body. And she had a heel strike, meaning the heel of her foot hit the surface first, placing more stress on the heel and slowing her down.

"I was really surprised by what I saw," says Stephanie, 16. "I learned a lot about my form that I didn't know -- that it matters how your heel hits, where your arms are. When you have good form, you're a better runner."

And perhaps a less injured one too.

Long used in medical settings to help people with movement disorders, such as cerebral palsy, learn to move more efficiently and with more stability, gait analysis is increasingly offered in running and footwear stores.

Casual joggers and walkers are using the analysis to find comfortable -- and productive -- footwear, to prevent injury and assess chronic injuries or soreness.

In general, gait analysis is the study of foot motion during normal running and walking. The process evaluates the foot's anatomical structure, muscle flexibility and strength -- assessing how those factors influence the way a person runs or walks.

Weak or inflexible muscles, if stressed by a lot of running over time, can lead to injury. And even a simple movement such as an excessive arm swing can make a difference in the performance of a competitive runner, adding a precious second or two to running times.

"It shows you things from a biomechanical standpoint: What are you doing when you run? What small improvements can you make?" says Reed Ferber, director of the running injury clinic at the University of Calgary in Alberta, Canada. "But it also suggests things on the clinical side, such as where someone is inflexible or weak in a specific muscle."