Kinesiology is the study of human movement. Most people attempting to pass a musculoskeletal anatomy or kinesiology class or a personal trainer exam must understand the basic joint movements of the human body.
Synovial joints, or those that are functionally classified as diarthroses, are the primary joints that have considerable movement compared to the sutures in the skull or the teeth fitting into the gums.
There are 6 subcategories of synovial joints including hinge joints, ball and socket joints, pivot joints, saddle, condyloid, and planar joints. Because most large or gross movements occur at the hinge and ball and socket joints, I’ll focus on these.
There are several common hinge joints worth considering. They are the elbow joint where the humerus (arm bone), radius and ulna (forearm bones) meet. The ankle or talocrural joint occurs where the distal ends of the fibula and tibia (leg bones) join with one of the bones of the foot, the talus. Another common hinge joint is the knee joint – the articulation (joint) where the tibia (shin bone) and the femur (thigh bone) come together.
All three of these hinge joints are capable of flexing and extending as their only actions. For example, the action of straightening the leg is considered to be knee extension. The opposite action, flexion, is signfied by someone bringing their heels toward their buttocks. In the elbow example, flexion is bringing the hand up toward the chest/shoulder area from a straight elbow position while extension is straightening the forearm at the elbow. The ankle joint is unique in its description of flexion and extension. When one stands in a position known commonly as “on the tippy toes”, s/he has plantar flexed (extension at other joints, but a special name for this action at the ankle). Moving the toes toward the shin, is dorsiflexion of the ankle joint.
Flexion is defined as decreasing the angle between the bones comprising the joint. Extension is described as increasing that joint angle. I consider, when trying to remember which is which, that flexion is always moving a body part “forward” while extension is just the opposite – moving it backwards. This “rule” holds true for every joint THAT IS CAPABLE of flexion and extension with THE EXCEPTION of the knee joint. At the knee joint, flexion is moving the heels BACK toward the buttocks while extension is the opposite.
The two ball and socket joints, the hip and the shoulder joints, are capable of moving in flexion and extension as well as medial (internal) and lateral (external) rotation and adduction and abduction. To flex the shoulder, one would move the arm as if reaching out to shake hands with someone. To extend the shoulder, a person would reach back as if they were accepting a baton in a track relay race. The act of punting a football would be flexing the hip, while moving the hip in the opposite direction, would be hip extension.
Due to the structure of the shoulder and hip joints, they are able to rotate. If a person stood erect with feet facing forward, arms to the side but palms facing forward (defined as the anatomical position), and then proceeded to rotate the arms so that the palms touched the sides of the thighs, this would be medial rotation of the shoulder joint. The opposite, rotating the arms back to the original position described above, would require lateral rotation of the shoulder joint.
Imagine a person standing erect BUT pigeon-toed or toes turned in. I liken this to having rotated the hips medially. The only way to have turned the toes in, since there is no rotary movement at the knee or ankle, would be to rotate the hips. I remember lateral rotation of the hip, as being opposite, where a person stands erect in the anatomical position BUT with the toes pointed OUT.
Finally, both shoulder and hip joints are capable of abduction or moving the arm (s) away from the body (assuming a starting position that is anatomical) and adduction or moving the arm (s) toward the body. Abduction at both hips and shoulders would be demonstrated by performing the first HALF of a jumping jack where thighs are spread and arms are over the head. Adduction would be evidenced when executing the second HALF of a jumping jack. The “spread” arms and thighs would return to the original starting position of the jumping jack – shoulder and hip adduction would have occurred.
By using the descriptions and definitions of joint actions for the hip and shoulder, the knee and ankle and elbow above, a person should be able to analyze basic human movements.