Isotonisen

Isotonisen is a Finnish term that denotes isotonic in English. It is used in Finnish-language science and medicine to describe substances, solutions, or biological conditions that have an osmotic pressure similar to that of bodily fluids, typically blood plasma. In practice, isotonic in this sense means that there is no net movement of water across cell membranes due to osmotic balance.

In physiology and medicine, an isotonic solution has an osmolarity close to that of the body's fluids. Such solutions are commonly used for intravenous administration because they are less likely to cause cellular swelling or shrinkage. Examples often cited in medical contexts include normal saline (0.9% sodium chloride) and lactated Ringer’s solution. These solutions are contrasted with hypertonic and hypotonic solutions, which can draw water out of or into cells, respectively.

In exercise science, isotonic (isotoninen in Finnish) refers to a type of muscle contraction or training where the muscle shortens or lengthens against a constant load, producing joint movement. This is commonly associated with resistance training using free weights or machines that provide a relatively steady resistance throughout the range of motion, as distinct from isometric or isokinetic modalities.

Etymology traces the term to Greek roots meaning equal and tension or tone, adapted into Finnish as isotoninen with the genitive form isotonisen. The English equivalent is isotonic, and the concept appears across physiology, medicine, and sport science.