Many hikers shuffle their feet, keep their knees bent, or hunch over as they slog up the trail. But these and other stride alterations can interfere with your body's ability to absorb shock, leading to knee and leg strains. You're also more likely to stumble or even sprain an ankle when walking with that tired stride, according to Peter Quesada, Ph.D., a professor in the mechanical engineering department at the University of Louisville (Kentucky).
Dr. Quesada has analyzed how the body responds to fatigue during backpacking. To avoid injury on the trail, he recommends building muscle endurance before the trip with a regimen of hilly hikes around the neighborhood (wearing a loaded pack), plus a few specific strength exercises, such as leg extensions and squats. Plan your mileage conservatively (hike per day no more than double the distance you walk or run during your workouts at home) and be sure to take plenty of rest stops. (For more leg-strengthening exercises, see "Survival Of The Fittest," April 2001.)