WASHINGTON (AP) - A jog on a treadmill is insufficient preparation for a hike in the hills, and this might come as a shock to those who count on gym workouts to get them ready for a vacation outdoors, experts say.

``I suspect almost everybody will have a surprise,'' said Dr. Benjamin D. Levine, director of the Institute for Exercise and Environmental Medicine at Presbyterian Hospital in Dallas. Health clubs can help a person prepare, but they are not set up for pre-trek training, so hikers have to find other ways to go the extra mile, he said.

Part of the reason is that hiking, like other exercises, uses specific muscle fibers in specific ways. The physiological principle of specificity says that the body learns to do exactly what it is called upon to do. Veteran hikers know that the best training for a hike is a practice hike. But there may not be time to hit the trail three times a week, and a health club is closer.

Athletes know about specificity from their other activities, Levine said: ``If you are training as a runner, you might not be as good a biker, and you certainly would not be as good a weight lifter.''

For hikers, this means simply running on a treadmill is not what the doctor ordered, Levine said.

On a flat treadmill, your legs are simply keeping you moving at a particular speed.

To train to go uphill, a hiker has to find something that mimics a hill.

A treadmill can help, but the grade has to be high, said Dr. Colin Grissom of LDS Hospital in Salt Lake City. ``Some go to a 15 percent incline, which is pretty steep, but that can give you a pretty good idea of what it's like,'' he said.

That grade, amounting to one foot of vertical gain for every 15 feet of trail, is sharp even for a car, Grissom said. ``A steep road for a vehicle is six percent,'' he said.

A stair climber machine also can mimic an uphill hike, although steps are more regular than what a person finds on a trail, which makes a stair climber less hiking-specific.

However, the big problem with health club equipment is not getting ready to go up - it's coming back down.

Downhills stress muscles differently than do uphills.

An uphill hike is closer to a standard gym-style movement, in which muscles shorten in what are called concentric contractions. In downhills, muscles do what are called eccentric contractions, stretching and tightening in tandem to control the descent. Going uphill, the muscle acts as an engine, providing power. Going downhill, the muscle also acts as a brake.

Treadmills and stair climbers work on the uphill motion, but there is little in a health club that works on the downhill. The closest a hiker might find won't be in the aerobics area but in the weights, Levine said. He suggested doing leg exercises such as presses and squats, concentrating especially on lowering the weights slowly back down.

Weight training has other advantages for hiking because hiking in hiking in many ways is like weight training, Levine said. Each step that pushes the body against gravity is, in effect, weight lifting, he said. And, under the principle of specificity, weight training gives a workout that treadmills and stair climbers don't.

The treadmills and stair climber can build an aerobic base, but the ability to get oxygen to muscle cells is only part of the hiking requirement, Levine said.

Hiking also requires power. And weights let the body strengthen the fast-twitch fibers that contract quickly to provide power. These are the ones that help a hiker climb over the next boulder.

Aerobic activities work on the slow-twitch fibers, which contract more slowly, but are more valuable for endurance. Slow-twitch fibers handle the bulk of the trek, keeping the hiker moving. But even endurance fibers can wear down with hours on the trail, and the fast-twitch muscles may be called upon for assistance, Levine said.

``Hiking is generally done at a lower relative intensity, but the muscular work, the weight traininglike work designed to lift you up a mountain, is of a much higher intensity,'' Levine said.

Copyright 2001 The Associated Press. All rights reserved.