Scientists, using mathematical calculations too complicated to explain here, estimate that the average person will walk what amounts to three trips around the Earth in a lifetime.

Nobody, however, has ever called Chad Worthen average.

One of Sacramento's top marathoners, the 35-year-old Worthen routinely runs far more than 100 miles a week, rarely taking a day off. Extrapolate that over his lifetime and … well, it's got to add up to a lot of miles.

Is it any wonder, then, that Worthen's feet once staged a rebellion so prolonged and painful that he almost gave up the sport he loves?

Worthen, like an estimated 10 percent of the adult population, had suffered through a nasty bout of plantar fasciitis – what podiatrists say is the most common of myriad ailments that can develop in the intricate 26-bone, 33-joint, 100-tendon design of the foot.

The plantar fascia is a fibrous tissue that runs from the heel to the ball of the foot, providing cushioning and support to the arch and serving as a connective buffer between the foot and the Achilles tendon. When it gets stressed, microtears and inflammation cause sharp pain and can make weight-bearing and flexion unbearable.

The Achilles tendon may have had Greek mythology to boost its profile, but plantar fasciitis can be equally painful and debilitating.

Just ask Worthen, who tried numerous treatments over 14 months before custom-made orthotics – insoles of hard plastic and leather specifically molded to a person's foot – eased the condition. He since has won the 2008 San Francisco Marathon and placed 22nd in the 2008 California International Marathon.

"I tried everything," recalls Worthen, who first started experiencing heel pain in 2006 after a marathon in Kentucky. "I rolled my foot over a frozen water bottle. I tried rolling a golf ball over it. Massage. Stretching. I even got cortisone injections, which were extremely painful.

"I'd take, like, a month off of running, then start again, and it was back. I guess you can treat the symptoms as much as you want. But until you treat the actual problem, find out what's causing (plantar fasciitis), it's not going to go away."

In many respects, Worthen was lucky he felt pain for only 14 months.

Orthopedic foot surgeons and podiatrists say plantar fasciitis can be difficult to treat and unpredictable to cure, not to mention nearly impossible to prevent. The underlying cause remains a matter of debate among specialists.

Liz Racin, a 62-year-old Citrus Heights resident, started feeling heel pain several years ago and wrote it off to "spurs." Not a runner, Racin nonetheless spends a lot of time on her feet doing flower arranging.

"My pain was like somebody poking knives into my arches," she says. "It wasn't until recently I found out I had plantar fasciitis."

Nothing helped – until Racin was fitted for custom orthotics. Her pain has eased and "my mood is much better, I tell you."

The majority of plantar fasciitis sufferers are not endurance runners such as Worthen but simply aging baby boomers who perhaps have packed on some pounds, stand for long periods in their occupation and perhaps have structural abnormalities such as flat feet or high arches.

Or not.

Foot experts can only speculate.

"When inflammation occurs, it doesn't always have a rhyme or a reason," says Dr. Eric Giza, chief of foot and ankle surgery at UC Davis Medical Center. "Most of the time, it's associated with overuse, which sets off an inflammatory pathway. Incidence of (plantar fasciitis) in kids is almost nonexistent. So we know, in some way, it's associated with degenerative tissue process. That's all I can tell you. It's very vague."

Researchers posit that repetitive trauma to the fascia can lead to irritation that, in time, leads to degeneration of the tissue itself. Podiatrists such as Kevin Kirby, who treated Worthen's case, says degeneration is the biggest obstacle to overcoming the condition.

"When you're 10 years old, (the fascia) are like rubber bands, they stretch and go back," Kirby says. "With an older person, it's like paper. You pull on them and they rip. Maybe not that dramatic, but that's what happens. When (fascia) are put under a load and stretched repeatedly for years and years, they are more likely to get these little tears."

Call The Bee's Sam McManis, (916) 321-1145. Read his postings on the "Sacramento Health & Fitness" blog at sacbee.com/blogs.