Isometric exercises, also known as static strength training, involve muscular actions in which the length of the muscle does not change and there is no visible movement at the joint (1).
The term 'static contraction training' is sometimes used to describe isometrics. However, 'contraction' signifies a change in length (shortening) of the muscle belly, which does not occur during static strength training. The term 'static action' is preferred to static contraction.
Isometric exercises can be used for general strength conditioning and for rehabilitation where strengthening the muscles without placing undue stress on the joint is warranted.
Some actions within a wide variety of sports require isometric or static strength. Examples include climbing, mountain biking and motocross (grip and upper body strength), Judo, wrestling, alpine skiing (static strength required to stabilize the upper and lower body), shooting, gymnastics and horseback riding.
Isometric exercises can be completed with submaximal muscle action - such as holding a weight steady, out to the side. The force used to hold the weight still is not maximal as this would lift the weight further causing movement and a change in the muscle length and joint angle. Static strength training can also involve maximal muscle actions and examples here include pushing against an immoveable object such as a wall or heavy weight.
Both submaximal and maximal isometric muscle actions can increase isometric strength (2,3,4) and induce muscular hypertrophy (5,6). In practice, maximal isometric exercises are used for strength and conditioning and submaximal exercises are used for rehabilitation (1).
Although isometric exercises can increase strength they are not the most suitable form of resistance training for dynamic actions such as sprinting and jumping. Most sports and athletic movements are dynamic in nature, performed at maximal speed against little or no external resistance. Isometric exercises do not increase the limb's maximal velocity and only strengthen the muscle at the angle at which it is trained (see below).
Isometric exercises can raise blood pressure significantly for the duration of the exercise. While it will return to a resting level soon after, this can be dangerous for people with hypertension or any form of cardiovascular disease. Even if you don't suffer from high blood pressure it is important to breath continuously throughout the exercises. Breath holding will only compound any increases in blood pressure.
As with all forms of exercise you should warm up thoroughly first. Muscles are under tension for a longer period of time and although that tension is more constant compared to a dynamic contraction, tears can still occur. Finally, try to maintain some degree of tension in the abdominal region during all exercises. This will help to maintain a correct posture and will help to develop core stability.
Number & Duration of Muscle Actions
Volume for a classic strength training routine is prescribed based on the number of sets and repetitions. The equivalent in isometric exercises is the length of time each action is held for and the number actions in total. Research has measured both longer duration actions (i.e. 10 seconds or above) and fewer repetitions, and shorter duration actions (i.e. 2-3 seconds) with more repetitions (6,7,8). Both approaches seem to increase static strength.
The general consensus is that in healthy individuals training to improve strength (as opposed to rehabilitation of an injury), the most efficient use of isometric exercises is 15-20 maximal voluntary actions held for 3 to 5 seconds (1). Three sessions per week is required (2) and results can be seen in as little as 2 weeks. However, when submaximal loads are used (such as bodyweight) it may be more suitable to increase the duration and reduce the number of repetitions.
This number and duration of contractions is required for each muscle group. As with traditional dynamic strength training, exercise selection should be based on a needs analysis of the athlete. Multi joint isometric exercises such as static leg presses may be more suitable then isolating the quadriceps, hamstrings and other hip flexors / extensors.
Isometric exercises strengthen the muscle at or near to the joint angle at which the exercise is performed. For example a static bicep exercise held with the joint at 25o only increases the athlete's strength at that specific angle (9) and there is no gain in strength when the elbow is held at other angles. However, at particular joint angles (and it varies from muscle group to muscle group) there is some cross-transference of strength to other joint angles. An isometric bicep curl performed at 80o for example also increases strength at other angles to a lesser extent (9). The same phenomenon is true for the knee (10) and plantar flexors (5).
Essentially, training at only one joint angle does not increase strength throughout the full range of motion (1). In order to improve dynamic power, isometric exercises would have to be performed at multiple joint angles for the same muscle group. This becomes time consuming and enervating for an athlete who may already be spending considerable time on other training modalities.
If static strength training is used to increase strength throughout the entire range of motion, isometric exercises should be performed at every 10 to 30 degree increments. If this is too time consuming, it is better to perform exercises at an extended joint angle (rather than a flexed joint angle) as this leads to greater cross-transference of strength at other angles (1).
The following isometric exercises use submaximal contractions i.e. bodyweight or a light free weights.
Hundred Breaths Exercise
This isometric exercise is taken from Pilates and is excellent for developing static strength in the core region.
Isometric Push Ups
Isometric Shoulder Raises
Isometric Calf Raises
Isometric Leg Extensions
Isometric Hip Extensions
Isometric Hip Abductions