isotonic

(of two solutions) Having the same osmotic pressure.

(of a medical solution) Having the same concentration of solutes as human blood.

(of two muscles) Having equal tension.

(physiology) Of or involving muscular contraction against resistance in which the length of the muscle changes. Antonym is isometric. Isotonic movements are either concentric (working muscle shortens) or eccentric (working muscle lengthens). See also Wikipedia:Isotonic (exercise physiology)

(music) Having the same tone.