Know your knee, and know where it stands today.
Understanding your knee has two halves. The first is knowing what the joint is and how it works. The second is knowing the current state of your own: how it feels, what it can do, and how that compares with last year. Most people skip both until something hurts.
The anatomy in brief
Femur above, tibia below, patella at the front. Articular cartilage covers the bone ends and gives the joint a surface smoother than ice on ice. Two C-shaped menisci sit between the bones, spreading load. The ACL and PCL cross in the centre and control front-to-back movement and rotation; the MCL and LCL guard the sides. Muscles power it all, tendons transmit the force, and the whole joint is sealed in a capsule of synovial fluid that lubricates and nourishes the cartilage.
The biomechanics
From outside the knee looks like a hinge. It isn’t. It rolls, glides and rotates as it bends, which is what lets you squat, land, cut and climb. It also means the knee never works alone: the hip above and the ankle below decide how force arrives at the joint. This is why so much knee rehabilitation is really about the rest of the leg.