Eccentric training

The new rules - Jonathan Chang, a sports-medicine orthopedic surgeon in Monterey Park, CA. "Recent studies have shown that you spend three times as much energy walking downhill as walking up," he says. "It just doesn't feel as tiring." To wit, a 2007 study in the Journal of Sports Sciences showed that 3 days after one 30-minute downhill run, subjects had shorter strides, less range of motion, and muscle damage.

Here's why: Each descending step requires a so-called eccentric contraction, meaning you're actually lengthening your quad muscles as you contract them (weightlifters call such movements "negatives"). Ignore the eccentric side of things and you'll get that telltale thigh burn–and that morning-after-the-morning-after soreness. Luckily, eccentric training may be even better for you than previously believed. A study in the Scandinavian Journal of Medicine and Science in Sports found that soccer players who did eccentric exercises for their hamstrings rather than traditional leg curls saw increases in both their eccentric and overall hamstring strength.

The fix - You can get the same benefit for your quads, says Bill Roberts, medical director for the Twin Cities Marathon and an avid backpacker. "I always work step-downs into my routine before I leave on a trip. They do wonders for soreness." Find a 6- to 8-inch step, box, or stair. (You can work up to 10 inches, but start smaller. Wear a pack for a bonus workout.) Stand with both feet on the step, facing "downhill." Keep your arms at your sides. Balancing on your left foot, bend your left knee and lower yourself until your right heel touches the floor, then push back up. Alternate sides, doing 3 sets of 15 reps per side.

Simple solutions to protect 3 trouble spots

Sore knees You know lunges are good for you, but they can do a number on bum knees. No need to give them up: Dwight Daub, an assistant coach with the Seattle SuperSonics, often has his players do lunges with their front foot on an elevated surface–an aerobic step or small box–which strengthens the muscles around the knee without overly stressing it.

Shin splints "The shin muscle's sole purpose is to raise your foot upward every time you take a stride," says Soika. If you suddenly increase mileage after a winter of watching reruns of That '70s Show, you're in danger of stressing your shins. Soika's easy fix? Walk on your heels with your toes pointed up as high as you can for 90 seconds or longer; do 3 sets a day.

Weak ankles According to Justin Price, ankle sprains are directly related to the health of your feet–namely, the arch and the plantar fascia, the band of tissue that runs along the sole. Once those are strong and supple, they'll help your foot adapt to unstable surfaces and prevent rolls. Start by resting one foot on a golf ball, wearing socks, and rolling your foot over the ball for 1 to 2 minutes. Over time, you can start doing this "massage" by standing on the ball (just be sure to transfer your weight gradually). Switch sides.

The fix Try these two dynamic warmups for a few minutes each before shouldering your pack.

Skips with self-hug Do a standard high skip, bringing knees up to 90 degrees with each skip. At the same time, extend your arms out to the side (parallel to the ground) and then bring them in to hug your chest. Repeat both motions simultaneously.

Frankenstein walk With back upright, arms out in front of you, and palms down, walk forward straightlegged and try to touch your shins with your hands by bringing your legs up, not by bending over.

Extra Credit: The roll-up
Your core–the band of muscles around your midsection, including your abs and obliques–is the foundation for a proper body position in any sport. "You should always keep a neutral, tall pelvis, almost like a dog tucking its tail," says Dieffenbach. Once your middle gets tired (after, say, 6 hours on the AT), your posture will fall apart, and your body actually moves into new positions and angles–stressing your legs in abnormal ways. Dieffenbach suggests borrowing a move from Pilates: Lie flat on your back with your legs straight, arms parallel to the floor above your head. Inhale as you contract your abs, then exhale as you slowly roll your torso up, reaching for your toes. Hold for a few seconds, then slowly roll back to the floor. Do 10 reps.