The term isometric (n) comes from the Greek "ios" which means the same, and "metric" which means distance. i·so·met·ric (s-mtrk) Combined together they make a static contraction where there is no perceivable motion. There are two types of isometric resistance:-
  • An immovable force (overcoming isometric)
  • An opposing resistance (yielding isometric) which is what isokinetics machines produce

Isometric is variable effort against an immovable force

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 is used to describe both a type of contraction and a type of exercise!

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 (NASA 2004). 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 was origionally made popular by Charles Atlas (born Angelo Siciliano 1892-1972). He developed a bodybuilding routine best known for it's 'bully kicks sand in face' advertising. The course was extremley popular but was in fact a mixture of isometric and isotonic exercises. Joe Weider commented in his recent book that he felt Atlas had in fact used progressive resistance training to develop his physique rather than simply isometric exercises.

Isometric exercise does not, contrary to popular opinion, increase muscular endurance or functional capacity in real world situations. They are, however, very effective if a muscle shows weakness at  particular point in range. This is most commonly seen in rehabilitation especially after surgery. Here there will often be points through range where strength is limited. Training at these points using isometric exercise will in fact restore the strength deficiency and is the fastest way to address these perturbations in the curves. 

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).
 
Advantages:
  • Does not aggravate sensitive joint surfaces.
  • Easy to perform and remember.
  • Convenient.
  • Reproducible.
  • Easy to measure.
  • Cost effective.
 
Disadvantages:
 
  • Not Functional.
  • Any improvements are speed and angle specific.
  • Many Contraindications.
  • Poor efficiency for strength enhancements.
  • No endurance enhancements.