isotonics

Isotonicity is a term used in several fields, notably exercise science and clinical medicine. In exercise science, isotonic exercises describe dynamic contractions in which the resistance remains roughly constant while the muscle changes length. These are distinguished from isometric contractions, where muscle length is static, and from isokinetic contractions, where contraction speed is controlled. In practice, true isotonic contraction is an idealization, since resistance and leverage cause varying tension and joint torque throughout a movement. Isotonic training is commonly used to build muscle strength and endurance and is a staple of rehabilitation and general fitness programs.

In physiology and medicine, isotonic solutions have osmolality similar to body fluids, producing no net movement of water across cell membranes when in contact with cells. Normal saline (0.9% sodium chloride) is the classic isotonic fluid; other examples include lactated Ringer's solution and various balanced crystalloids. Isotonic fluids are used for fluid resuscitation, to expand circulating volume, and to dilute medications. By contrast, hypertonic solutions draw water out of cells, while hypotonic solutions can cause cells to swell. Some infusions labeled isotonic, such as 5% dextrose in water, are isotonic at administration but become hypotonic as the dextrose is metabolized.

Overall, isotonicity describes a condition where osmotic pressure is matched to that of body fluids, minimizing water movement across membranes and producing predictable physiological responses. The term has distinct meanings in exercise contexts versus medical contexts, but both rely on balancing forces to achieve stable, controlled effects.