BOX: Exercise - a double-edged sword Exercise is vital to strengthening muscles that support the knee, but if done incorrectly or to excess, it can damage the knees. Some doctors advise against jogging, which puts triple the pressure on the knees than walking. Joggers with seemingly healthy knees should vary activity, by swimming on alternate days or by biking to strengthen quadriceps, for example. There are also at-home exercises that can strengthen quadriceps and stretch the back of the leg. Swimming and water exercises are excellent for strengthening knees, as long as you avoid the breast stroke, since the whip-kick unduly stresses the knees. Biking with sore knees is not recommended. But if you bike before pain is gone, avoid hills and raise the seat so that you can pedal comfortably without bending your knee more than halfway to a right angle. Skating and roller-blading done with proper technique can also build knee support. Once pain is gone, a stair-climbing machine, which does not require as much lift as real stairs, can also help. Stretching exercises for muscles and tendons along the back of the leg are very important for your knees. Use common sense. Avoid positions like squatting that put undue stress on the knees. If you kneel, use a cushion. If your knees hurt, see a professional. SEQN: 9112070417