What does flexion mean?
Flexion is primarily an anatomical term describing the act of bending: movement at a joint that reduces the angle between two bones, as when the elbow bends to bring the hand toward the shoulder. It stands in direct opposition to extension, the straightening movement that increases that angle, and the two together describe much of the body's range of motion. Clinicians and physiotherapists routinely measure flexion — of the hip, knee, spine, or fingers — to assess injury, recovery, and mobility, with specialized terms such as plantar flexion describing downward bending of the foot. Historically, the word was also applied in grammar to describe how word forms change to signal tense, case, or number, though that sense has since been absorbed by 'inflection'. Rooted in Latin flectere, meaning 'to bend', flexion carries a precise, technical connotation and remains essential vocabulary in medicine, fitness, and biomechanics.
nounThe act of bending a joint or limb, in which the angle between two bones is decreased; also, in grammar, the variation of the form of a word to express different grammatical functions (a variant spelling of 'flection').
- In anatomy and physiology: the bending of a joint or limb that decreases the angle between two bones.
- In grammar: the variation of a word's form to express grammatical distinctions such as tense, case, or number; now more commonly written as 'inflection' or 'flection'.
"The physiotherapist measured the flexion of the patient's knee after surgery."
"After weeks of rehabilitation, she regained full flexion in her injured elbow."
"The linguist studied how flexion in Latin verbs encodes tense and person."
The plural 'flexions' is uncommon; in clinical contexts the phrase 'range of flexion' or repeated 'flexion movements' usually serves instead.
"The exercise routine consisted of twenty gentle spinal flexions."
Every time you curl a bicep, you're performing flexion — and the muscle doing it is literally named for this Latin word: the flexor.
Reviewed by Deb Chak, Editor. AI-assisted content curated by RJS Tech Solutions LLP.
Etymology of flexion
Flexion derives from the Latin verb flectere, meaning 'to bend' or 'to curve', from which English also drew flexible, deflect, reflect, and the anatomical term flexor. The word entered English in the early seventeenth century, borrowed via scholarly and medical writing. Its grammatical sense — the bending or modification of word forms — developed alongside the anatomical one, but over time 'inflection' became the preferred spelling in linguistics, leaving flexion dominant chiefly in anatomy and medicine.
Related word forms
How flexion is actually used
Flexion remains standard terminology in anatomy, physiotherapy, sports science, and medicine, while its grammatical use has largely been superseded by 'inflection'. It contrasts directly with 'extension', which straightens rather than bends a joint.
Easily confused with flexion
Inflection refers specifically to the grammatical modification of a word's form, whereas flexion (in anatomy) refers to bending a joint; historically 'flexion' was used for both meanings.