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Muscle contraction is the tightening, shortening, or lengthening of muscles during activity. Muscle contraction is often followed by muscle relaxation, where contracted muscles return to their normal state. Muscle contractions can be described in terms of two variables: length and tension. When a muscle contracts, it can either shorten or lengthen, and this movement is referred to as isotonic contraction. If the muscle shortens, it is called a concentric contraction, and if it lengthens, it is called an eccentric contraction. Concentric contractions occur when there is sufficient muscle tension to overcome the load, causing the muscle to contract and shorten. Eccentric contractions occur when the muscle works to decelerate a joint at the end of a movement, acting as a braking force to protect the joints. Isometric contractions, on the other hand, are when muscle tension changes without any change in muscle length.
| Characteristics | Values |
|---|---|
| Type of contraction | Concentric |
| Muscle tension | Sufficient to overcome the load |
| Muscle length | Shortens |
| Muscle fibres | Tighten up and shorten |
| Muscle movement | Contract |
| Muscle relaxation | Follows contraction |
| Muscle stability | Offer stability to joints and connective tissues |
| Muscle heat | Produce heat to maintain body temperature |
| Muscle posture | Help maintain posture |
| Adaptive shortening | Chronic tight muscles from excessive sitting |
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What You'll Learn
Muscle contraction types: Isotonic and isometric
Muscle contractions are defined by the changes in the length of the muscle during contraction. They can be broadly classified into two types: isotonic and isometric.
Isotonic Contractions
Isotonic contractions are further divided into concentric and eccentric contractions. Concentric contractions occur when the muscle shortens, generating force. This type of contraction happens when the muscle's maximal force of contraction exceeds the total load on the muscle. An example of a concentric contraction is the motion during a bicep curl when the load is raised in an arc toward the body. The velocity of muscle shortening depends on the load acting on the muscle.
Eccentric contractions, on the other hand, cause the muscle to elongate in response to a greater opposing force. They act as ""brakes" to concentric contractions, decelerating the muscle joints and altering the position of the load force. An example of an eccentric contraction is downhill walking, where the muscle works to resist gravity.
Isometric Contractions
Isometric contractions are contractions in which there is no change in the length of the muscle. No joint or limb motion occurs, and the muscle generates force without changing its length. This type of contraction is typical of muscles found in the hands and forearms, where the joints do not move, but the muscles generate sufficient force to grip an object. An example of an isometric contraction is holding an object in your arms in front of you. You are not trying to raise or lower the object but are keeping it at a steady position.
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Muscle tension and load
Muscle contraction can be described as isometric if the muscle tension changes but the muscle length remains the same. This type of contraction is common in muscles in the hands and forearms responsible for grip. For example, when holding a heavy weight steady, neither raising nor lowering it. An isometric contraction generates force without changing the length of the muscle.
In contrast, a muscle contraction is described as isotonic if muscle tension remains the same throughout the contraction. In an isotonic contraction, the muscle length can either shorten to produce a concentric contraction or lengthen to produce an eccentric contraction. An isotonic contraction maintains constant tension in the muscle as the muscle changes length. This can occur only when a muscle's maximal force of contraction exceeds the total load on the muscle.
Concentric contraction occurs when there is sufficient muscle tension to overcome the load, and the muscle contracts and shortens. This type of contraction happens when the muscle is actively shortened, such as when lifting something heavier than usual. During a concentric contraction, the muscle generates force at the origin and insertion, causing the muscle to shorten and changing the angle of the joint. For example, a biceps curl causes the arm to bend at the elbow as the hand moves towards the shoulder.
Eccentric contraction occurs when the muscle lengthens in response to a greater opposing force. This type of contraction acts as a braking force in opposition to a concentric contraction to protect joints from damage. For example, a voluntary eccentric contraction would be the controlled lowering of a heavy weight.
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Adaptive shortening
The effects of adaptive shortening can be mitigated through interventions such as physical therapy or targeted stretching and mobility routines. However, it is important to note that simple stretches may not be sufficient to lengthen the muscles and that a more comprehensive approach may be required.
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Muscle relaxation
There are several types of muscle contractions, including isometric, isotonic, concentric, and eccentric. Isometric contractions occur when a muscle is held at a set length, for example, when carrying something in front of you. Isotonic contractions maintain constant tension in the muscle as it changes length, and can be either concentric or eccentric. Concentric contractions occur when a muscle shortens and generates force, for example, when doing a bicep curl. Eccentric contractions occur when a muscle lengthens under tension, acting as a braking force to concentric contractions, for example, when walking downhill.
Progressive muscle relaxation (PMR) is a technique that can be used to relieve muscle tension. PMR involves tightening and relaxing muscle groups one at a time in a specific pattern, usually starting with the lower extremities and ending with the face, abdomen, and chest. This technique helps to release tension from the muscles and promote mental calmness. It is often used to relieve stress, anxiety, insomnia, and chronic pain.
When practising PMR, it is recommended to tense each muscle group and hold for 5 seconds, then exhale and relax for 10-20 seconds before moving on to the next group. It is best to practise in a quiet, comfortable place, wearing loose clothing, and without distractions. PMR can be learned by anyone and typically requires only 10-20 minutes per day to practise.
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Muscle structure
Muscle contraction is based on two variables: length and tension. When a muscle shortens, it is typically due to a concentric contraction, which occurs when there is sufficient muscle tension to overcome the load, and the muscle contracts and shortens. This type of contraction stimulates the muscle to contract according to the sliding filament theory.
The structure of striated muscles, which are responsible for voluntary movements, plays a crucial role in understanding muscle contraction. These muscles are composed of numerous individual muscle fibres, called myofibrils, which contain parallel thin and thick filaments. The thick filaments are made of the protein myosin, which is responsible for muscle contractions. Actin, another globular protein, forms double-stranded filaments that interact with myosin during contraction. This interaction is facilitated by the troponin and tropomyosin complexes, which regulate the binding of actin and myosin.
During a concentric contraction, the muscle fibres shorten, generating force and changing the angle of the joint. For example, a concentric contraction of the biceps causes the arm to bend at the elbow, lifting a weight towards the shoulder. This type of contraction is essential for activities such as lifting heavy objects or performing a biceps curl.
It is important to note that muscle shortening can also occur through adaptive shortening, a physiological process where muscles and connective tissues shorten or become "tight" due to being chronically held in shortened positions. This is commonly observed in individuals who sit for extended periods, leading to shortened muscles in the hips, shoulders, neck, and back.
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Frequently asked questions
Muscle contraction is the tightening, shortening, or lengthening of muscles when you do some activity. It can happen when you hold or pick something up, stretch, or exercise with weights.
Adaptive shortening refers to the physiological process where muscles and connective tissues generally shorten their length or become "tight" over time as a result of being chronically held in shortened positions. This typically occurs due to prolonged sitting or repetitive postural positions.
There are three types of muscle contractions: isometric, isotonic, and isokinetic. Isometric contractions generate force without changing the length of the muscle. Isotonic contractions can be concentric (muscle shortens) or eccentric (muscle lengthens) and involve changes in muscle length. Isokinetic contractions are a type of isotonic contraction where the speed of contraction remains constant.
A concentric contraction occurs when muscle tension is sufficient to overcome the load, and the muscle shortens as it contracts. For example, a biceps curl causes a concentric contraction of the biceps, bending the arm at the elbow and lifting the weight towards the shoulder.
