Etymology
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lean (n.2)
c. 1200, "lean animals or persons," from lean (adj.). Meaning "lean part of anything, muscle without fat, lean meat" is mid-15c.
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abductor (n.)

1610s, in physiology, a muscle that moves (a limb) away from the axis of the body, from Latin abductor, agent noun from abducere "to lead away," from ab "off, away from" (see ab-) + ducere "to lead" (from PIE root *deuk- "to lead").

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tensor (n.)
muscle that stretches or tightens a part, 1704, Modern Latin agent noun from tens-, past participle stem of Latin tendere "to stretch," from PIE root *ten- "to stretch."
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aponeurosis (n.)
"fascia, fascia-like tendon, white fibrous membrane of the body (often connecting a muscle with a tendon)," 1670s, from Latin, from Greek aponeurosis, from aponeuroein, from apo "change into" (see apo-) + neuron "sinew" (see neuro-).
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sarcomere (n.)

"structural unit of a muscle," 1891, from sarco-, Latinized combining form of Greek sarx "flesh" (see sarcasm) + -mere, from Greek meros "part," from PIE root *(s)mer- (2) "to get a share of something."

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clonus (n.)

"violent muscular spasms, rapidly alternating contraction and relaxation of a muscle," 1817, from Modern Latin, from Greek klonos "turmoil, any violent motion; confusion, tumult, press of battle," a word of uncertain origin. Related: Clonicity.

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constrictor (n.)

"that which constricts," 1735, originally "a muscle which draws parts together," agent noun in Latin form from constrict. Meaning "a large serpent which envelops and crushes its prey in its coils" is from 1790.

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popliteal (adj.)

"of or pertaining to the back of the knee," 1786, with -al (1) + Modern Latin popliteus (n.) "flat, triangular muscle at the back of the knee-joint," 1704, short for popliteus (musculus), from Latin poples "ham (of the leg)," which is of unknown origin.

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pectoral (adj.)

1570s, "of or pertaining to the breast or chest," from Latin pectoralis "of the breast," from pectus (genitive pectoris) "breast, chest," a word of unknown origin. De Vaan considers Old Irish ucht "breast, chest" as "a likely cognate, if it reflects earler *pektu-." Pectoral muscle is attested from 1610s.

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extensor (n.)
"muscle which serves to straighten or extend any part of the body," 1713, short for medical Latin musculus extensor, from Late Latin extensor "stretcher," agent noun from Latin extendere "spread out, spread" (see extend).
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