By 2025, it is estimated that there will be nearly 2 million total knee or total hip replacements performed every year. 
A number of factors can contribute to such meteoric figures, but sedentary lifestyles are sure to be a significant link. Why?
A certain type of cartilage covers the articulating surface of bones in areas like the hips and knees. Articular cartilage (aka hyaline cartilage) creates a smooth, gliding surface within the joint space. A unique feature of articular cartilage is its lack of a blood supply. Yet, there are still cells called chondrocytes within the joint that are responsible for creating and maintaining healthy cartilage. Like other cells, these chondrocytes still need oxygen and other nutrients. [2,3]
But without a blood supply, how do these cells get the fuel they need?
In order to keep those cells alive and well, essential nutrients like oxygen (which normally diffuse from blood), have to use an alternative pathway.
Instead of from blood, these nutrients are transferred from synovial fluid. Synovial fluid is the liquid within a joint that helps ensure a cushiony, smooth motion during movement.
When pressure is applied in a joint (i.e. a knee during a squat), the nutrients from synovial fluid are pushed into the articular cartilage. In this way, chondrocytes and cartilage maintain their respective health and thickness. [2,3]
What happens when you stop moving?
When joints are immobilized or unused, oxygen and other nutrients fail to diffuse from the synovial fluid into the articular cartilage. Without these essential compounds, the cells in that joint begin to die off. After a period of time, cartilage thins (bad) and the components of the cartilage itself are degraded (also bad).
When cartilage is worn down, joints are at a greater risk of osteoarthritis and bone-on-bone friction.
Move your body and subject your joints to external loads (i.e. weighted squats, deadlifts, or bench variations). Furthermore, improving your mobility with controlled articular rotations or other full-range movements can help maintain complete joint health. [2,3]
- Singh, Jasvinder A.; Shaohua Yu, Lang Chen, John D. Cleveland. Rates of Total Joint Replacement in the United States: Future Projections to 2020-2040 Using the National Inpatient Sample. The Journal of Rheumatology. 2019.
- Staff, Peer H. The Effects of Physical Activity on Joints, Cartilage, Tendons, and Ligaments. Scandinavian Journal of Social Medicine. 1982. 29:59-63.
- Vanwanseele B, Lucchinetti E, Stüssi E. The effects of immobilization on the characteristics of articular cartilage: current concepts and future directions. Osteoarthritis Cartilage. 2002 May;10(5):408-19