The term isometric has been abused over a great period of time. At one point the term tonic contraction was used interchangeably with isometric contraction. This trend seems to be decreasing. Isometric contraction and isometric exercise are again two interchangeable terms. To elaborate on this it seems people describe an isometric contraction as performing isometric exercise. Hislop and Perrine (1967) described isometric exercise as muscular contractions against a load which is fixed or immovable or is simply too much to overcome. Two German physiologists (Muller and Hettinger, 1954) performed a study which claimed that one six second isometric contraction at two-thirds maximum performed once each day for five days was sufficient for 5% strength gains per week. This received a disproportionate amount of publicity from which it would appear that the medical community has never recovered.
Although it has been shown that strength gains are possible from isometric contraction these strength gains are very minimal and almost all studies since have shown that the gains in pure muscular strength are only at the specific angle at which the exercise is performed. Hence, to make isometric exercise effective at increasing functional strength it must be repeated at many different joint angles. Isometric improvements have also been shown to be rate specific (Morrissey, Harman and Johnson 1995), this means that isometric strength gains can be best utilized only at particular speeds. These improvements are seen mostly in slower movements which are not functional and of little use to people wishing to return to any kind of physical activity.
Isometric exercise does not, contrary to popular opinion, increase muscular endurance or functional capacity in real world situations.
The extreme effort involved with isometric exercises causes considerable internal pressure both within the muscles themselves and in the abdominal and thoracic cavities. Isometric exercise can increase blood pressure and heart rate to levels that would be dangerous for anyone with undiagnosed cardiac problems (Nagle, Seals and Hanson 1988, White and Carrington 1993 and Baum et al. 1995), whilst also increasing intra abdominal pressure to dangerously high levels (Williams and Lind 1987).
For those in good health this is not problematic however, for those who have suffered muscular or tendonus injuries the consequences can be dire. Isometric exercises are, however, extremely good for strengthening muscle groups around an injured joint as the joint surfaces actually distract from one another during isometric contraction. However, following isometric exercise there is a decrease of muscle power by up to 60-70% (Tidas and Shoemaker 1995), this can last for up to 96hrs (4 days). During this time the associated joints are exposed to much higher than normal impact and sheer forces as they have lost one of their most vital protective mechanisms. This could lead to discomfort as demonstrated by Melchionda et al. (1984) which is not experienced with isokinetic concentric contractions (Dvir 1995). In reality electrical stimulation of a muscle is more effective at increasing muscular strength than isometric exercise as has been shown by Draper and Ballard (1991).
Isotonic literally means equal tension. In exercise science isotonic contraction is a contraction in which the tension remains constant as the muscle shortens or lengthens. 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). I will continue to use isotonic 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).
This form of exercise is in contrast to the other forms in that it permits maximum muscle contraction throughout the full range of joint movement. You will often seen it referred to as ‘accommodating resistance exercise’ (a term first introduced by Hislop and Perrine, 1967). The resistance is now variable in proportion to the change in muscular capability at every point in the range of motion. The variation is controlled so that at all times it equals the product of the muscular strength.
There are two major types of machine available to offer isokinetic exercise.
Type 1 - an active dynamometer (the most familiar to clinicians) In this type of exercise the speed is controlled whilst resistance is variable according to the amount of force throughout the range of movement. No matter how much force the individual applies the speed does not change. Thus the motion of the body segment is kept at a predetermined velocity.
Type 2 - utilizes a change in the moment arm of the selected resistance to coincide with the change in moment arm of the muscle effort and the change in tension due to muscle length change. The resistance thus accommodates the continuous changes in muscle force which occur throughout the range of movement.
The changes in the resistance arm on the machine are determined and pre-set to coincide with the average strength curves for various joint actions (obviously this involves a different machine for each movement).A comparison of the different types of exercises can be seen here.
