Ride the NordicTrack Ski machine. To avoid lower back strain, use proper form while exercising. Keep your abdominal muscles tight, position your shoulders over your hips, and avoid the tendency to lean forward. Try to take full, long strides but don't bring your feet in front of your body. Concentrate on an extending the back stroke of your stride. If you find yourself sliding backwards on the skier the tension is too low, increase the tension. I like a tension of about 3.5 for a distance of 2 miles at a speed of about 7 miles per hour. Increase tension as you improve your fitness level.