That’s Louie on the right, and every location on him where bones come together and can be articulated (moved) is a joint. Working in tandem with the joints are ligaments and tendons.
In Latin, ligamentum means bandage, and in our bodies, ligaments connect bone to bone. Ligaments do not heal on their own and tend to need surgery if there is a problem.
Tendon comes from the Latin tendo, meaning “I stretch out”, and in our bodies tendons connect muscle to bone. Think of tendon as the cable that allows your muscles and bones to stretch and elongate. If a tendon tears, it tends to heal by scaring.
The vast majority of joints that allow us to move are known as synovial joints, of which there are multiple types. Thanks to a regular reader for a previous comment containing a link to a synovial joint video. Minus narration but with crystal clear visuals, below is another video describing the various types of synovial joints.
Each synovial joint is encapsulated by fibrous tissue, with ligaments around the capsule. The inner lining of the capsule produces lubricating synovial fluid, which is kept in place by the synovial membrane.
Thanks to our synovial joints, we can move in multiple directions. Flexion refers to bending, like bending over at your waist or bending your arm at the elbow or flexing your foot. Now imagine a mid-line splitting your body down the middle from the crown of your head, through your nose and belly button. Adduction refers to movement that comes towards this mid-line (ADDing to the body), and Abduction refers to movement that goes away from this mid-line (ABDUCTing, or taking away).
Extension refers to lengthening, such as holding your arms out in front of, or behind you. While standing in anatomical position, you are standing in full extension. Rotation refers to turning in a circular direction, such as the rotation of the shoulder when a baseball pitcher winds up to throw a pitch.
Our mechanical system for movement is just one of the many systems that comprise our bodies. On the last day of FAMI, Dr Laitman gave a talk that put our bodies in evolutionary and comparative context. As he explained:
Consider that some 60 million years ago, our earliest ancestors were moving around on four feet that also functioned as four hands! Along the way, humans made some “progressive improvement and refinement of walking”. While other animals rely on their muscles for movement, we depend on our skeletal structure and our ligaments, thus allowing us to conserve some energy that goes into our cortical functions. It is our cortex, ability to speak, and our musculoskeletal system that specializes us as humans.