# Biomechanics

Foot motion is a complex topic that we will try to put in simple words. Each joint functions differently depending on the size, shape, and contour of the joint. For the easiest explanation think of three axis and planes of motion.

If a rod or axis was placed straight up and down through the leg and the foot could rotate around this rod from side to side. Motion around this joint is within the transverse or horizontal plane. The motion where the foot rotates inward toward the other foot is termed adduction, and the motion where the foot rotates outward away from the body is termed abduction. Muscles are sometimes named for there function. An abductor will pull the bones outward and an adductor will pull inward. A bunion is also termed hallux abducto valgus meaning the hallux (big toe) is abducted (rotated outward).

Next if a rod or axis was placed down the foot from toe to ankle in a straightline and the foot is rotated around this axis in the frontal plane the motion is termed inversion and eversion. An inversion rotation is where the foot is rotated inward around this axis. An eversion rotation is where the foot is rotated outward around this axis.

Now think of a rod going straight through the ankle from one side to the next, this is the axis, and the plane of motion is considered sagittal plane motion. If the foot is lifted upward and rotated along this axis the motion is termed dorsiflexion since dorsal means top. Plantarflexion is where the foot is rotated downward along this axis since plantar means bottom in a sense.

Foot motion would be fairly straightforward if all joint axis were perfect rods and placed in strictly these three motions, but in fact no joint acts perfectly. Every joint has its own joint axis, this imaginary rod may be almost perfectly straight up and down, but tilted slightly forward or inward. Most all joints will have some motion on each plane described above, thus will have some dorsiflexion/plantarflesion, abduction/adduction, and some eversion/inversion component.

The terms pronation and supination are used to describe motion along all three body planes at the same time. Pronation is a motion where the bone is rotated at the joint through dorsiflexion, eversion, and abduction movements. Supination is where the motion at the joint produces plantarflexion, inversion, and adduction. Commonly a person that is flatfooted is said to have excessive pronation, meaning there arch is collapsing because the joints involved are allowing the foot to dorsiflex, abduct, and evert in relation to the lower limb. Most foot motion does not involve just one joint and you can easily see how complex biomechanics can be.