1828, from Greek isotonos "of level pitch; equally stretched," from iso- (see iso-) + tonos (see tenet).
isotonic i·so·ton·ic (ī'sə-tŏn'ĭk)adj.
Of equal tension.
Having the same concentration of solutes as the blood.
Of or involving muscular contraction in which the muscle remains under relatively constant tension while its length changes.