Once a fox-hunting horse, Beau suffered a leg injury that forced him to embark on a second career — as a therapist.

The therapy comes from the gentle rocking motion the 17-year-old flea-bitten Arabian gray makes as it ambles through a paddock, a stone’s throw from the Mississippi River at Ride On St. Louis, a therapeutic riding center in Kimmswick. Nathan Collier, an 8-year-old with cerebral palsy, struggles to remain upright on Beau as he lifts a purple swimming pool noodle over his head. But he’s all smiles.

“The children don’t know they’re getting therapy,” said physical therapist Anne Cochran. “They’re just having fun.”

A small but growing number of scientific studies are showing how hippotherapy — from the Greek word “hippos,” for horse — can help children with cerebral palsy, a neurological disability that keeps them from smoothly controlling their muscles. The theory: The rocking motion of a horse creates more than 100 small shoves a minute. A child would therefore strengthen core muscles and neural connections about 3,000 times in a 30-minute therapy session.

“The people that do it, know and see the results,” said Ride On St. Louis director Rick Wassman. “But the insurance companies don’t recognize that.”

Hippotherapy has been practiced for decades, and anecdotal evidence for it is strong. But the science is just beginning to catch up. Researchers at St. Louis University are trying to add to the slim body of scientific evidence — and perhaps legitimize hippotherapy in the eyes of insurance companies — with a new study that will quantify a child’s disability before and after a 12-week course of weekly hippotherapy sessions.

Biomechanist Jack Engsberg has used his motion analysis laboratory to study the gaits of amputees and the post-surgery posture of people with scoliosis. Now, he’s using six special cameras to capture the movements of children with cerebral palsy.

He and occupational therapist Tim Shurtleff will stick 21 reflective balls on the children’s bodies, concentrating on the spine. The cameras capture the movement of the balls, and a computer transforms the images into a 3-D model of how the child moves over time.

Before and after the 12-week course, the children will ride a mechanical horse built by Shurtleff. The horse was the culmination of his doctorate in occupational therapy he finished last year at Washington University, where Shurtleff, 58, says he was the program’s first-ever grandfather.

Ingredients for the mechanical horse are two cheap skateboards bought at Wal-Mart, an 18-gallon plastic drum and an electric motor with a quarter of Beau’s power.

In a pilot study of six children, the researchers found that hippotherapy significantly improved head and trunk stability. Now, they want more statistical power and are trying to recruit 12 children. They have found four so far. They also want to see if the hippotherapy “sticks,” by testing the children on the mechanical horse a third time, three months after the therapy has finished.

Using the motion analysis laboratory to study hippotherapy is a novel idea — and needed, said Dr. John A. Sterba, a physician and physiologist who does pediatric rehabilitation research at the Women and Children’s Hospital of Buffalo, N.Y.

“There’s very little research because it’s extremely hard to do it,” he said.

In a 2007 review paper, Sterba found 11 published studies of the effects of horse therapy on children with cerebral palsy. All but one showed the therapy improved the children’s motor skills.

Insurance companies are somewhat suspicious. A recent policy bulletin from Aetna said hippotherapy was “experimental” because the science was still “insufficient.”

Is hippotherapy better, per dollar or per hour, than traditional forms of physical therapy? Sterba said no one has yet made that comparison. But it is an important question, because cerebral palsy is one of the world’s most costly congenital conditions, with $11.5 billion in lifetime costs for everyone that currently has the disability, according to a 2003 estimate from the Centers for Disease Control and Prevention.

Costs of therapy

Hippotherapy is by no means cheap. In addition to a horse, it requires four people: a leader for the horse, two side-walkers to keep the child from falling and a physical therapist to lead the program. (That’s a distinction some make between hippotherapy, where a physical therapist uses the horse to direct the child in exercises, and therapeutic horse riding, where the child rides the horse and gains incidental therapeutic benefits.)

At Ride On St. Louis, Wassman says the total cost of hippotherapy runs about $400 per hour, though foundation grants allow families to pay 10 percent of that.

Costs don’t concern Nathan much, but he did seem a bit wary last week as he readied himself to get on a horse for the first session. Beau nosed between Nathan’s knobby knees, looking for carrots.

Nathan was born 16 weeks premature. He was 11 inches long and weighed 1 pound 5 ounces. His lungs collapsed twice. He stayed in an intensive care unit for five months. His cerebral palsy was set at birth by a stroke that caused brain damage.

While Nathan has struggled with his health, different physical therapy regimens have improved his motor skills, said his mother, Connie Collier, of south St. Louis. “Every goal they make, he meets,” she said.

Collier leaned on the white fence bordering the paddock. The dirt is soft. The day is warm. Trees are in bloom.

It’s also a springtime of sorts for Nathan, who is riding backward on Beau. Collier grins at her son and waves. Nathan can’t wave — he and Beau are busy working — but he grins right back, showing off a right front tooth that’s poking its way through his gums.