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.