Isometric contraction, unlike dynamic contraction, is a static form of muscle work. It plays the crucial role in all demands where stability is required.
What is isometric contraction?
Isometric contraction is a form of muscle work in which the tension increases while the length of the muscle remains the same. Isometric contraction is a form of muscle work in which the tension increases while the length of the muscle remains the same. Therefore, no movement occurs in the joints involved. The build-up of tension takes place in the smallest functional units of the muscle cells, the sarcomeres. In each muscle cell, thousands of these elements are connected in series. Incoming nerve impulses activate a certain number of sarcomeres, depending on their strength, but they never all contract simultaneously. The sum of the actions gives the state of tension in the muscle as a whole. The core of the sarcomeres is the actin-myosin complex. These two protein chains interact with each other during contraction. The actin filaments are connected to the boundaries of the sarcomere, called Z-strips. Myosin lies between the actin filaments and attaches to it with heads. A stimulus causes the myosin heads to flip over. During concentric muscle work, this mechanism causes the Z-strands to be pulled toward the center by the actin molecules. The sarcomere and, in sum, the entire muscle shorten. In isometric contraction, the length does not change; only the tension is increased by the flipping.
Function and task
Mechanically, the function of isometric contractions is to perform holding work. Joints, joint chains and entire areas of the body are thus stabilized and protected from unfavorable loads and damage. This form of muscle work is particularly important when external forces are also acting. Adverse stresses occur due to unfavorable lever ratios. Various structures can be severely stressed as a result. A typical example is bending and lifting that is not back-friendly. If the upper body is bent far forward because the legs are not used, a large load moment is created on the spine. The result is high compressive stress, especially for the intervertebral discs. The load moments become even more unfavorable if the back is bent in the process. The load distribution is then even more punctual. The load can be significantly reduced by trained straightening and good stabilization of the spine with isometric contractions of the supporting muscles. Muscles with different movement functions often cooperate in the joint-stabilizing function by simultaneously performing isometric muscle work. A very concise example of this is the stabilization of the knee joint while standing in a flexed position, for example in a downhill squat while skiing. The knee extensors basically hold the knee in position and prevent uncontrolled deviation. At the same time, the knee flexors, together with the ligaments, act as joint stabilizers by bringing the joint partners into a central position relative to each other to ensure an even distribution of pressure. An example of how the moving and stabilizing functions complement each other is the shoulder joint. In all movements of the arm, the rotator cuff is active as a stabilizer. The 4 muscles ensure that the humeral head always sits centrally in the socket, no matter what movements are performed. Isometric contraction is an important component of this. The fixation of a joint or an area of the body through holding work is also an important prerequisite for the execution of controlled movements. The moving parts are given a firm counterhold. Another important function of isometric contraction is the protection of internal organs. Together with fascia and fatty tissue, the tension of the muscles ensures that they are embedded in a protective sheath. In the case of inflammation or irritation, the protective tension is increased even more significantly to keep the mechanical stress as low as possible.
Diseases and complaints
Isometric contraction, like the other forms of contraction, can be affected by various diseases of the musculature and nervous system. Nerve lesions caused by spinal cord damage or damage to individual peripheral nerves result in flaccid paralysis of the affected muscles.This has a particularly dramatic effect in the case of a cross-section at the level of the cervical or upper thoracic spine. In addition to the arms and legs, the trunk can then also neither be moved nor stabilized. The consequence is usually wheelchair dependence. The muscular dystrophies are a group of hereditary muscle diseases. In their course, there is a progressive degeneration of the musculature. This affects the entire skeletal musculature as well as the musculature of the internal organs. This has consequences for isometric contractions at a very early stage, which is particularly noticeable in trunk stabilization. Amyotrophic lateral sclerosis has a similar effect. This is a degenerative disease of the nervous system in which only the motor part is affected. Severe neurological diseases such as a stroke or multiple sclerosis lead to changes in muscle tone, in addition to other symptoms. Often a variable appearance develops in which there are muscles with increased and decreased tension. The consequences for stability are often striking. Trunk stability in particular is affected. The holding function of isometric muscle work is impaired in many people due to a so-called muscular imbalance. Due to unfavorable postural and behavioral habits, certain muscles are not used sufficiently and atrophy. This has negative consequences, especially for joint stabilization. A typical example is the insufficiency of the deep layers of the back muscles, which are responsible for segmental stability of the spine. Many training programs do not address these muscles, or address them inadequately, but instead focus solely on the large superficial systems. For this reason, even well-trained people can still experience back problems.
