Terms of Motion

Terms of Motion refer to how the motion is described relative to the body parts. There are many terms of motion, the most common of which are abduction, adduction, mastication, eversion, inversion, extension, flexion, protraction, retraction, plantarflexion, dorsiflexion, depressor, levator, tensor, and opposition. Like the terms of spatial orientation, the terms of motion usually come in pairs, with one term directly opposing the other.

The process where two structures are separated from each other or are moved away from the median plane is called abduction. Meanwhile, the process where two structures are brought together or are moved toward the median plane is called adduction. Abduction and adduction are visible in the folding of the vocal folds. The vocal folds “abduct” when they move away from each other, while they “adduct” when they move towards each other.

When a body part moves in a circular direction, the movement is known as the circumduction. The eyeballs “circumduct” when they move in circular direction (such as when people roll their eyes to indicate sarcasm or annoyance).

Mastication simply refers to the process of chewing. Le Reverend, et. al. (2014) stated that food consistency affects the orofacial development of humans and other mammals. This suggests that eating food with harder textures supports the muscle and bone growth, which indirectly leads to mastication efficiency, the efficiency of crushing the food between the teeth and turning the food into swallowable food bolus.

Flexion simply means bending. A good example of this is the flexion of the forearm toward the shoulder by contracting the biceps. Extension means stretching out a body part. A good example of this is straightening the forearm. A research from Noyes et al. (1989) emphasizes that the knee also has a flexion-extension mechanism with its axis located in the femur, and its orientation, relative to the femur, does not change.

Eversion means turning outward (an example of which is turning the sole of the foot away from the body’s midline), while inversion means turning inward (an example of which is turning the sole of the foot medially).

Protraction means sliding or moving forward, while retraction means sliding or moving backward. Plantarflexion means moving the foot inferiorly like tiptoeing, while dorsiflexion means moving the foot superiorly.

Depressor is the process of lowering a structure, while levator is the process of raising a structure. Fun fact: Remember the popsicle sticks the pediatricians use to move the tongue down when we were babies? Those sticks are actually called “tongue depressors” since they aid in lowering the tongue!

Tensor refers to the process of contracting and making a structure rigid. Opposition refers to moving a structure toward another structure.

Agonist muscles help move the muscles while antagonist muscles make them move in the opposite direction.

References:

Le Révérend, B. J., Edelson, L. R., & Loret, C. (2014). Anatomical, functional, physiological and behavioural aspects of the development of mastication in early childhood. The British journal of nutrition, 111(3), 403–414. https://doi.org/10.1017/S0007114513002699

Noyes, F. R., Grood, E. S., & Torzilli, P. A. (1989). Current concepts review. The definitions of terms for motion and position of the knee and injuries of the ligaments. The Journal of Bone & Joint Surgery, 71(3), 465–472. https://doi.org/10.2106/00004623-198971030-00027