Dominic Stone
The human body contains over 600 skeletal muscles, which are attached to bones by tendons. Muscles can only contract or pull; they cannot push. As a result, muscles generally operate in pairs. These are known as antagonistic muscle pairs, where one muscle contracts to move a body part, and the other contracts to return it to its original position. For example, when a footballer prepares to kick a football, their hamstrings contract to flex the knee while the quadriceps lengthen to allow the movement.
| Characteristics | Values |
|---|---|
| Definition | Muscles work in pairs, with one muscle contracting to move a body part and the other relaxing or lengthening to return the body part to its original position. |
| Other names | Antagonistic muscle pairs, agonist-antagonist paired muscles |
| Example | Biceps Brachii and Triceps Brachii |
| How they work | Through reciprocal inhibition, with one muscle contracting and the other relaxing. |
| Identification | The agonist is the muscle that is contracting and doing all the work, while the antagonist is the muscle that is relaxing or lengthening. |
| Fixators | Other muscles called fixators assist antagonistic pairs by supporting and stabilising the joint and the rest of the body. |
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What You'll Learn
Muscle contraction
- Offering stability to joints and connective tissues: Muscles lengthen and shorten, sometimes involuntarily, as the body needs them to.
- Producing heat to maintain body temperature: Around 40% of body temperature is converted into muscle work. Shivering is the body's response to feeling cold, and skeletal muscles activate to warm the body.
- Maintaining posture: Muscles help maintain positions like sitting or standing.
There are three types of muscles in mammals: skeletal, cardiac, and smooth. Skeletal muscles are attached to bones and give the body structure and strength. Skeletal muscle works in conjunction with the bones of the skeleton to create body movements. It is also associated with the diaphragmatic, oesophageal, and eye muscles. Skeletal muscle contracts primarily in response to a voluntary stimulus, although the contractions of smooth and cardiac muscles are myogenic, initiated by the muscle cells themselves.
The complex process leading to muscle contraction, called excitation-contraction coupling, begins when an action potential causes depolarisation in the myocyte membrane. This depolarisation spreads via the transverse (T) tubules, which help spread depolarisation signals to the entire muscle fibre. Depolarisation of the T tubules causes a conformational change in the dihydropyridine receptors, which causes the opening of nearby ryanodine receptors on the sarcoplasmic reticulum (SR), the storage site for calcium within muscle cells.
The sarcomere is the fundamental contractile unit of the myofibril. Z lines separate each sarcomere. The A bands, located at the centre of each sarcomere, contain the thick filaments, which may overlap with thin filaments. The thick filaments are made from the protein myosin, which has one pair of heavy chains and two pairs of light chains. The two heavy chains twist around each other helically, and each heavy chain is paired with two light chains, giving rise to two heads. The myosin heads have an actin-binding site that helps them attach to the thin filaments. The thin filaments are composed of actin, which combines with other actin globules to form two intertwined strands with positive and negative ends. The double-stranded actin filaments are covered by tropomyosin, which blocks the interaction between myosin and actin when the muscle is inactive.
Cross-bridge cycling begins when ATP binds to an ATP-binding domain on the myosin head. Myosin dissociates from the actin, breaking the cross-bridge. ATP is then hydrolysed into ADP and P, causing the myosin heads to change conformation and move toward the positive end of the actin, cocking the myosin head. The phosphate is released, and the ADP-bound myosin binds to a new location on the actin filament. ADP is then released, causing the myosin to return to its original position, pulling on the actin filament and causing the sarcomere (and, therefore, the muscle fibre) to contract.
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Agonist and antagonist
When muscles contract, they move our bones and body parts by pulling on them. However, a contracting muscle cannot push the bone back into its original position. This is where muscle pairs come into play. In an antagonistic muscle pair, one muscle contracts to move a body part, and the other muscle then contracts to return the body part to its original position. The muscle that contracts to produce movement is called the agonist or prime mover, and the muscle that relaxes or lengthens to allow the movement to occur is called the antagonist.
To illustrate this, consider the example of a bicep curl. The biceps contract to raise the forearm, acting as the agonist, while the triceps relax to allow this movement, acting as the antagonist. Now, if you want to lower your forearm, the triceps contract and become the agonist, while the biceps relax and become the antagonist. This dynamic can be observed in various muscle pairs throughout the body.
For instance, in the preparation phase of kicking a football, the hamstrings contract to flex the knee, acting as the agonist, while the quadriceps lengthen to enable this movement, acting as the antagonist. In the striking and recovery phase, the roles switch: the quadriceps contract to extend the knee, becoming the agonist, while the hamstrings lengthen to facilitate this movement, becoming the antagonist.
Antagonists play two crucial roles in muscle function. Firstly, they maintain body or limb positions, such as holding the arm out or standing erect. Secondly, they control rapid movements, like shadow boxing without landing a punch or checking the motion of a limb. Additionally, other muscles called fixators assist antagonistic pairs by providing support and stability to the joint and the body.
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Reciprocal inhibition
Muscle contraction moves our bones and associated body parts by pulling on them through tendons. However, muscle contraction cannot push the bone back into its original position. Therefore, muscles work in "antagonistic pairs", where one muscle of the pair contracts to move the body part, and the other muscle in the pair then contracts to return the body part to its original position.
For example, when the triceps brachii is stimulated, the biceps is reflexively inhibited. Similarly, during a press-up, the triceps are the agonist and contract eccentrically to control the extension of the elbow so that the forearm is lowered towards the floor. The biceps are the antagonist. During the upward phase, the biceps are the agonist and contract concentrically to flex the elbow, and the triceps are the antagonist.
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Fixators
Muscle pairs, also known as antagonistic muscle pairs, are groups of muscles that work together to move bones and body parts. In a muscle pair, one muscle contracts to move a body part, while the other muscle relaxes or lengthens to allow the movement. These muscles are called the agonist and the antagonist, respectively. The agonist is the muscle that is contracting and doing the work, while the antagonist is the muscle that relaxes or lengthens to return the body part to its original position.
Neutralisers are similar to fixators in that they also act to prevent unwanted movements. However, neutralisers differ in that they do not prevent the movement of a specific body part but instead pull against and cancel out unwanted lines of pull from the agonist or prime mover. An example of a neutraliser is the pronator teres, which cancels out the supination torque produced by the biceps brachii when flexing the elbow with a supinated forearm.
In summary, fixators are muscles that stabilise and reduce the movement of joints, working isometrically to prevent unwanted motions. They are distinct from synergists, which are muscles that assist the prime mover in a given motor task, and antagonists, which return the body part to its original position after the agonist has contracted. Fixators and neutralisers work together to ensure that only the desired movements occur, providing stability and control during joint actions.
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Isometric contraction
An isometric contraction is a type of muscle contraction that involves tension development within a muscle without any change in its fibre length. Isometric contractions are performed without joint motion, and the muscle length remains constant. This means that there is no observable movement in the angle of the joint. The term "isometric" comes from the Greek words "isos" (equal) and "-metria" (measuring), indicating that the length of the muscle and the angle of the joint remain equal or the same during these contractions.
Isometric exercises are a form of strength training that utilises isometric contractions. These exercises focus on improving the body's ability to apply power from a static position or maintain a position for an extended period. They are valuable in preparing the body for subsequent power movements. Isometric exercises can be categorised into three main types: isometric presses, pulls, and holds.
NASA has also researched the use of isometrics in preventing muscle atrophy in astronauts due to prolonged exposure to zero-gravity environments. While isometrics promoted muscle growth, it was found that this type of exercise did not effectively prevent a decrease in contractile proteins within muscle tissue, leading to muscle degradation at a molecular level. This finding suggests that isometrics may not be the optimal method for maintaining muscle tissue health in astronauts.
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Frequently asked questions
Muscle pairs, also known as antagonistic muscle pairs, are groups of two muscles that work together to move bones and body parts. One muscle in the pair contracts to move the body part, and the other muscle then contracts to return the body part to its original position.
The contracting muscle is called the agonist, or prime mover, and is the muscle doing all the work. The muscle that relaxes or lengthens is called the antagonist.
The biceps and triceps are an example of an agonist-antagonist muscle pair. When the biceps contracts and raises the forearm, the triceps relaxes.
