A joint axis is an imaginary line that passes through the center of a joint, acting as the pivot point around which a bone or limb rotates. Think of it like the hinge of a door: the door swings open and closed around an invisible line. Our body parts similarly move around these unseen lines within our joints, allowing for a wide range of motion.
Anatomical Planes and Axes
To precisely describe human movement, anatomy uses three primary reference planes that divide the body. The sagittal plane divides the body into left and right halves, running vertically from front to back. Movements in this plane occur around a mediolateral axis, which runs horizontally from side to side, like a skewer piercing the body from hip to hip.
The frontal plane divides the body into front and back portions. Movement within this plane occurs around an anterior-posterior axis. This axis runs horizontally from front to back, similar to an arrow shot through the body from navel to spine.
The transverse plane divides the body into upper and lower halves. Rotational movements in this plane occur around a longitudinal axis, which runs vertically from head to toe.
How Joints Move Around an Axis
Specific types of movements are consistently associated with these anatomical planes and their axes. Flexion, which decreases the angle at a joint, and extension, which increases it, both occur in the sagittal plane around a mediolateral axis. An example is bending and straightening the elbow or knee.
Abduction, moving a limb away from the midline of the body, and adduction, moving it towards the midline, happen in the frontal plane. These movements rotate around an anterior-posterior axis. Lifting your arm sideways away from your body exemplifies abduction, while bringing it back down is adduction.
Internal and external rotation, which involve twisting a limb around its own long axis, take place in the transverse plane around a longitudinal axis. Turning your head to look over your shoulder or rotating your arm inward at the shoulder are examples of these rotational movements.
Joints also vary in their “degrees of freedom.” Uniaxial joints, like the elbow, allow movement around a single axis. Biaxial joints, such as the wrist, move around two axes, enabling movements like flexion, extension, abduction, and adduction.
Multiaxial joints, including the shoulder and hip, offer the most extensive range of motion by moving around all three axes. The shoulder, as a ball-and-socket joint, can move in all three planes, providing three degrees of freedom.
Practical Applications of the Joint Axis
Understanding the joint axis is fundamental across various scientific and medical disciplines. In physical therapy, therapists assess a patient’s range of motion by observing how limbs move around their specific axes. This knowledge helps them design targeted rehabilitation programs to restore proper joint function and prevent future injuries.
Biomechanics and ergonomics also rely on this understanding to design safer and more efficient environments. By analyzing movement around joint axes, engineers can develop tools, equipment, and workspaces that minimize strain and reduce the risk of musculoskeletal disorders. This includes optimizing workstation layouts or designing athletic gear.
Orthopedic surgery, particularly in joint replacement procedures like total knee arthroplasty, requires precise alignment of artificial joints with the natural mechanical axes of the limb. Surgeons ensure the new joint components are positioned to replicate the body’s original movement patterns, which helps achieve optimal function and longevity of the implant.