Isotonic (n) literally means equal tension. In exercise science isotonic contraction is a contraction in which the tension remains constant as the muscle shortens or lengthens. i·so·ton·ic (adj.) (s-tnk)
Isotonic is fixed resistance variable speed movement.
Although 'isotonic' is the term used most frequently to describe fixed resistance variable speed exercise, 'isoinertial' is a more accurate description of this type of movement (Abernethy et al. 1995). In this text isotonic will be used throughout. In reality it takes a very complicated piece of equipment, like an active dynamometer, to create pure isotonic exercise. People generally think of isotonic exercise as that seen in the gymnasium. The simplest example of this is where a dumbbell is lifted from the ground and used to perform an exercise. The tension generated by the dumbbell is now the constant, or in other words if you pick up a 2 kilo dumbbell it weighs 2 kilos whatever you do with it.The elaborate cam systems seen on most modern weight training equipment allow for a more isotonic movement to be performed. The key to isotonic exercise is that although the weight is constant the speed of movement associated with the exercise is variable. Think again of the dumbbell curl. The dumbbell always weighs 2 kilos but you can decide how quickly to move it.
Although the reliability of isotonic exercise is generally good, controlling the inertial forces that develop with different lifting techniques make it inappropriate for the study of musculoskeletal performance in humans (Sapega 1990). This method of testing should be limited to active dynamometers. Even then isotonic movement should only be used for the assessment of speed production at a given resistance.
Exercise programs have been proven to be most effective when the movements performed match those experienced most frequently by the person in question (Morrissey, Harman and Johnson 1995). For humans the closest form of exercise to normal movement is isotonic. So it would not be surprising to find that isotonic exercise increases muscle strength at double the speed of isometric exercise in the untrained population according to Connelly and Vandervoort (1995).