Joints can handle heavy pressure. For example, knees handle a force of three to four times a persons total body weight on average just talking a walk. The force of a deep knee bend during a squat can increase to nine times the body weight.
So just imagine multiplying weight of more than 150 pounds times a minimum of three or four, and then even more. That can sure add up to a lot of heavy work on knee joints over time.
Now for the science of this scenario. Where two bones meet, called the joint, the bone ends are covered with cartilage, also known as gristle. This cartilage is sturdy, elastic and spongy or compressible, and keeps the bones from moving against each other at the joint. The cells of this cartilage, called chondrocytes, are thought to be the longest living cells of the body.
Surrounding the bones and cartilage is strong, fibrous capsule lined with synovium, a thin membrane that lubricates the joint area with fluid. The end result is less friction or smoother rubbing together of the bones.
This fluid also feds the cartilage cells, keeping them healthy, and is pumped into them during joint movement. Thus lack of movement (activity / exercise) can be unhealthy.
Other parts of the body features involved with this arthritic scenario include muscles, tendons, ligaments, bursea and mental activity. Muscles, attached to bones with tendons and ligaments, move bones by contracting. They also cushion movement, absorbing impact or shock.
Throughout the muscle and tendon areas are bursae or sacs filled with fluid. These also help cushion movement. And throughout all the coordination of these parts during movement, the brain is a part. The brain communicates via nerves throughout the body, in particular the muscles for this scenario, to prepare joints for activity.
The exact science of what actually causes arthritis is still being researched. For most of the 100-plus forms of arthritis, the causes are unknown. Injury, overuse of joints and mechanical issues with joints (like skeletal abnormalities, worn out joint muscles) can lead to arthritis. And many point to issues relating to bacteria and germs as some of the problem.
Heredity, stress, drugs, food allergies and viruses have also been linked to some forms of arthritis. So have diet, poor circulation and lack of movement.