The human knee is an extremely important joint. It holds the weight of the entire body, provides the strength needed to lift heavy objects, and bends and flexes to give us mobility. Since this crucial joint is under stress at almost all times (except when sitting or lying down), it is susceptible to wear over time. This is especially true for the cartilage which holds together all the bones in the knee. As we age, the knee ages and the cartilage can develop a number of ailments that could potentially prohibit mobility. A new study presented at the annual meeting of the Radiological Society of North America found that both too much and too little physical activity can accelerate the degeneration of knee cartilage in middle-aged adults.