My knee hurts and is kind of stiff when i walk and play basketball. it hurts to bend and i have to straighten it slowly. what could it be?

Cartilage: The cartilage or meniscus cushions the knee and leg bones. When it tears, through injury or degeneration the leg can lockup, catch and have decreased stability. See a doctor for exam, treatment and imaging tests.

Knee overuse issues: Strengthening the hips, knees, and ankles will often make it easier to play without pain. A good thing to do is see one's doctor, who may refer one to a physical therapist if needed. A patient can also get exercises off the internet. Be sure to have well fitting shoes, proper arch support, and exercise on a flat surface. Anterior knee pains and knee stiffness happen to many athletic teens.