Why does the weight on your knees suddenly 'double' when going downstairs?

Because when you go downstairs, your knees have to withstand immense 'eccentric contraction' and dynamic impact forces! When walking on flat ground, your knees bear about 1 to 2 times your body weight. But when 'going downstairs,' this number instantly skyrockets to 3 to 4 times your body weight (and can even reach 7 times when going downhill)! This is because, when descending stairs, the supporting leg must bend and slowly relax against gravity (known as eccentric contraction in muscle science). At this time, the body's gravitational acceleration and the impact force of stepping down are concentrated on the patella (kneecap) and cartilage of the knee joint. If the 'quadriceps' muscles in the front of your thighs lack sufficient strength to act like 'a car's brake shock absorber' in time, this massive impact force, several times your body weight, will directly slam into your knee joint cartilage!