Though treadmills generally provide good surface cushioning, trail running offers the added bonus of forcing runners to vary their stride and change direction at a moment's notice, putting more muscles to work.
But running on uneven surfaces also improves stability by strengthening lower, peripheral leg muscles that get little use on flat pavement runs, according to trainers and podiatrists.
"Trail running helps improve balance and makes you a better athlete because you have to use the outer muscles of your legs," says Perry Julien, a veteran trail runner and team podiatrist for the NFL's Atlanta Falcons and the NHL's Atlanta Thrashers.
Steep hills, a staple of most trail runs, also force runners to swing their arms more, giving them a better upper body workout, and the changing terrain compels runners to take shorter, more efficient strides, eliminating the likelihood of overstriding, the tendency to take excessively long strides. Overstriding eats up too much running energy and can lead to injuries.
An outdoors boost
Despite the hills, rocks and branches, trail runners say the serene beauty of the backcountry trails -- the cool streams, green shade trees and towering canyons -- rejuvenates a stressed and overworked mind.
"It allows you to get away from the city and smog, the honking horns and crossing stop lights," says Pat Connelly, an avid trail runner and coach for the L.A. Roadrunners, the official training program for the Los Angeles Marathon. "It's peaceful and people can commune with the environment."
Charlie Brown, a Charlotte, N.C., psychologist who specializes in improving the performance of athletes, says part of the benefits of exercising outdoors comes from sunlight, a proven natural mood booster.
"Anyone who has been outdoors will tell you there is a centered and grounding feeling to being outside," he says.
Since the 1970s, surveys and medical journal reports by environmental psychologists have linked even brief exposures to the outdoors with a reduction in stress, brightened moods and improved mental clarity. One of the most often cited studies on the greening of moods is a 1984 report by behavioral scientist Roger Ulrich at Texas A&M University, who found that patients with bedside views of nature had briefer hospital stays and needed less medication than those with views of a brick wall.
More recently, studies have shown that the physical benefits of exercise and the mental health benefits of being outdoors are a "win-win" combination.
A 2002 study by researchers at Ithaca College compared the moods of college students who ran indoors with those who ran outdoors. The report, published at a conference of the American College of Sports Medicine, found the outdoor runners not only had brighter moods but ran faster than the indoor runners.
Research coordinator Sarah Hammel, one of the researchers in the Ithaca College study, says she expected to find improved moods among those who exercised outdoors but was surprised to find that they also ran faster. The faster speed, she says, may be a byproduct of the runners' improved mood.
A 2005 study by researchers in East Carolina University found that people who walked outdoors were more likely to continue such exercise in the future over people who walked indoors.
Eric Edwards, an attorney from Brentwood, has been running for 30 years and credits trail running with aiding his battle against work-related stress. "I just love getting out in the wilderness after a long day in the office or in the courtroom," he says after the Santa Ynez Canyon run.
Benefits outweigh risks
But trail running has its drawbacks, namely injuries caused by sharp rocks, protruding roots and perilous gopher holes that can wreak havoc on an unsuspecting runner.
During the Santa Ynez Canyon run, a veteran runner slipped while rock-hopping over a small stream, landing with a crash in the dirt. He suffered a minor knee scrape but was back on the trail quickly, barely losing pace with his colleagues. But other runners say the results can be more serious, including broken toes, cracked ribs and twisted ankles.
And while eyeing the trails for hazards, runners keep their heads and torsos pointed down, creating bad running posture. For maximum lung efficiency, trainers say, runners should look forward with their torso straight.
Still, the benefits far outweigh any hardship, runners say.
For proof, look to Stan Swartz, the director and founder of the Trail Runners Club.
Nearing the end of the 8.5-mile run, Swartz trots along a twisting dirt path, ducking under overhanging branches, stepping over roots and jagged rocks.