The Knee Under Load

For Young Learners

Function

Stand still on both feet and each knee carries about half of you. That is the easy bit.

Start walking and the number goes up. Each step lands with a thump, and your knee has to absorb that landing as well as hold you up. Walking on the flat puts roughly three times your body weight through the joint.

Go up stairs and it climbs further. Squat down low and further still. Land from a jump and, for a fraction of a second, the force can be many times your weight.

That sounds alarming until you realise your knee handles it quietly, thousands of times a day, for decades.

Clinical Relevance

Two things make those numbers manageable.

The first is spreading. The menisci and the smooth cartilage spread the force over a wide area instead of letting it land on one spot. Pressure is force divided by area, so a bigger area means less pressure everywhere.

The second is bending. Watch someone land from a jump and their knees bend as they touch down. That bending stretches the landing out over a longer moment. The same force arriving slowly is far easier to handle than the same force arriving all at once. It is why you bend your knees to land and why landing stiff legged hurts.

Your muscles do the bending, and they do it by lengthening while still pulling, like lowering a heavy bag slowly instead of dropping it. That is work, and it is why strong legs protect knees.

There is a practical point here. Jumping is not bad for your knees. Jumping without landing well is harder on them, and landing well is something you can learn.