Dominic Stone
The human body is an incredibly complex system, with over 600 muscles working in harmony to enable movement. While some muscles are the prime movers, or agonists, for certain actions, they are nearly always assisted by other muscles. These assisting muscles are called synergists, and they help by adding extra force or reducing unnecessary movement, thereby providing stability and support. For example, the biceps brachii is the prime mover in a bicep curl, but it is assisted by the brachialis and brachioradialis, which are the synergists. Understanding the relationship between agonists and synergists is crucial for designing safe and effective exercise routines and preventing injuries.
| Characteristics | Values |
|---|---|
| Number of muscles in the human body | Over 600 |
| Muscle movement | Flexion, extension, abduction, adduction, supination, pronation, elevation, depression |
| Muscle actions | Concentric, eccentric, isometric |
| Types of muscles | Agonist, Antagonist, Synergist |
| Agonist | The main muscle that does an action, also known as the "prime mover" |
| Antagonist | The muscle that resists a movement, often by doing the opposite of the action it is resisting |
| Synergist | Assists the agonist or prime mover in performing its action by adding extra force or reducing unnecessary movement |
| Examples of Agonists | Iliopsoas (hip flexion), biceps brachii (elbow flexion), triceps brachii (elbow extension) |
| Examples of Antagonists | Triceps (during a biceps curl), biceps brachii (during a triceps extension) |
| Examples of Synergists | Iliacus, psoas major, rectus femoris (hip flexion), brachialis and brachioradialis (elbow flexion), anconeus (elbow extension) |
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What You'll Learn
Agonist and antagonist muscles
The human body has over 600 muscles, and each muscle is attached to two bones. When a muscle contracts, it brings one bone closer to the other, creating movement. This movement is known as a muscle contraction, or a "flex".
When discussing movement at a joint, the muscle that performs the action is called the agonist, and the muscle on the other side of the joint with the potential to oppose the action is called the antagonist. These terms are a construct to help us talk about biomechanics. Agonist and antagonist muscles simply oppose each other's actions.
For example, when we flex the elbow, the bicep is the agonist because it causes the elbow to flex, and the tricep is the antagonist because it has the potential to oppose the elbow flexion. However, when we are talking about active elbow extension, the tricep is the agonist because it causes the action, and the bicep is the antagonist. This shows that one muscle can be the agonist for one movement and the antagonist for another.
In any pair, the agonist muscle contracts, while the antagonist muscle relaxes, allowing for the free movement of our joints and muscles. For example, when we flex our arm with a bicep curl, the bicep is contracted, making it the agonist muscle, and the tricep is relaxed, and therefore the antagonist muscle. When we bring our arm back to a natural position, our bicep is relaxed (the antagonist muscle), and the tricep is contracted, and is referred to as the agonist muscle.
In addition to their individual roles, agonist and antagonist muscles also work together as synergists to achieve an overall goal. For example, when holding a crimp on a climbing wall, the finger and wrist flexors are working to keep you on the hold.
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Synergist muscles
The human body has over 600 muscles. While we often have one main muscle responsible for an action, it is almost always assisted by other muscles. These assisting muscles are known as synergists.
The bicep is the agonist during a bicep curl, but the brachialis and brachioradialis are synergists as they assist with the motion. In this case, the synergist muscles help to stabilize the elbow joint. Similarly, during a bench press, the anterior deltoids and triceps are synergists as they assist the chest.
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Muscle attachments
There are two types of muscle attachments: direct and indirect. Direct attachments involve the muscle attaching directly to the bone, while indirect attachments utilise connective tissues such as tendons or aponeuroses. Aponeuroses are sheet-like structures of fibrous tissue that provide indirect attachments from muscles to bones or other connective tissues. The latissimus dorsi muscle, for instance, attaches indirectly to the bone via an aponeurosis.
The attachments of muscles to bones can be classified as origins or insertions, depending on their role in movement. An origin is a stationary attachment point, where the muscle attaches to a bone that remains immobile during an action. Conversely, an insertion is a movable attachment point, connecting the muscle to a bone that moves during the action. For instance, the triceps brachii have one insertion point on the ulna and three origin points on the scapula and humerus.
Understanding the attachments of muscles is crucial for predicting their actions and functions. By visualising the line of pull, which is an imaginary line connecting the attachment points, one can approximate the movement that occurs when the muscle contracts. This understanding of muscle attachments and their associated movements is fundamental for designing safe and effective exercise programs, as it helps predict and control the actions of various body parts during exercise.
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Muscle contractions
Types of Muscle Contractions
The three primary types of muscle contractions are concentric, eccentric, and isometric. These terms describe the changes in muscle length and tension during contraction.
Concentric Contraction
Concentric contraction occurs when a muscle actively shortens under tension. This type of contraction helps in lifting heavy objects. For example, when lifting a dumbbell, the bicep muscle tightens and contracts to lift the weight. The muscle tension is sufficient to overcome the load, resulting in a shortening of the muscle.
Eccentric Contraction
Eccentric contraction happens when a muscle lengthens under tension. This type of contraction assists in lowering heavy objects. Using the dumbbell example again, when lowering the weight, the bicep muscle remains contracted but lengthens as the weight is lowered. Walking is another example of eccentric contraction, as the quadriceps muscles lengthen and contract when the heel touches the ground and the knee bends or straightens in stride.
Isometric Contraction
Isometric contraction occurs when muscle tension changes without any corresponding change in muscle length. This type of contraction is observed when holding an object in a steady position, such as carrying something in your arms in front of you. The muscle is actively held at a set length, generating tension without changing its length.
Muscle Relationships
The relationship between muscles during contraction can be described using the terms agonist, antagonist, and synergist. The agonist, or prime mover, is the main muscle responsible for a specific action. For example, during a bicep curl, the bicep brachii acts as the agonist. The antagonist is the opposing muscle group that resists the movement of the agonist. In the case of the bicep curl, the triceps is the antagonist. Synergists are muscles that work together with the agonist to create a movement. During the bicep curl, the bicep brachii and brachioradialis are synergists, assisting the agonist in stabilising the elbow joint.
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Types of muscle movement
The human body has over 600 muscles that help us move, breathe, swallow, and stay alive. These muscles are made of thousands of small fibres woven together, and they work together to move our bodies.
There are three types of muscle tissue in the body: skeletal, smooth, and cardiac. Skeletal muscles are part of the musculoskeletal system and work with bones, tendons, and ligaments to support our weight and move us. Tendons attach skeletal muscles to bones, and these muscles are voluntary, moving when we think about moving a body part. Smooth muscles line our organs, and cardiac muscles make up the heart and the middle layers of the heart.
Muscles can be grouped by their location, such as chest, leg, or back muscles, or by the type of movement they perform, such as abductors, flexors, or extensors. The muscles surrounding synovial joints are responsible for moving the body in space, and these movements are often paired, like flexion and extension, or abduction and adduction.
Flexion and Extension
Flexion and extension are movements forward and backward from the body, such as nodding the head. Flexion decreases the angle between two bones (bending), while extension increases the angle and straightens the joint. For example, the triceps brachii and anconeus extend the elbow, while the biceps brachii, brachialis, and brachioradialis flex the elbow.
Abduction and Adduction
Abduction and adduction are side-to-side movements, such as moving the arm laterally when doing jumping jacks. Abduction is moving away from the body's midline, while adduction is moving towards it. The gluteus medius, gluteus minimus, tensor fasciae latae, and sartorius are abductors of the hip, while the pectineus, adductor longus, adductor brevis, adductor magnus, and gracilis adduct the hip.
Supination and Pronation
Supination and pronation refer to the rotation of the forearm. Pronation is when the forearm rotates so the palm faces backward or down, while supination is when the forearm rotates so the palm faces forward or up.
Elevation and Depression
Elevation and depression are up-and-down movements, such as chewing or shrugging the shoulders. Lowering the mandible to open the mouth is mandible depression, and moving the mandible back up is mandible elevation.
Circumduction
Circumduction is the movement of a body region in a circle, with one end remaining relatively stationary while the other end moves in a circular motion. This involves the sequential combination of flexion, adduction, extension, and abduction at a joint.
Superior and Inferior Rotation
Superior and inferior rotation are movements of the scapula, defined by the direction of movement of the glenoid cavity. Superior rotation of the scapula is required for full abduction of the upper limb.
Lateral Flexion
Lateral flexion is the bending of the neck or body towards the right or left side. In the limbs, flexion decreases the angle between the bones (bending of the joint), while extension increases the angle and straightens it.
In addition to these types of movements, muscles can also be described by their relationships with one another. The main muscle that performs an action is called the agonist or prime mover, and it is assisted by other muscles called synergists. The antagonist resists a movement or performs the opposite action to the agonist. For example, during a biceps curl, the triceps is the antagonist to the biceps brachii and brachioradialis, which are the synergists, while the brachialis is the agonist.
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Frequently asked questions
Assisting muscles, also known as synergists, work together with prime movers or agonists to create a movement. They add extra force or reduce unnecessary movement, providing stability and support during complex movements like lifting or running.
Some examples of assisting muscles include the brachialis and brachioradialis, which assist the biceps brachii during a bicep curl. The brachialis and brachioradialis also assist the biceps brachii in flexing the elbow joint.
An agonist, or prime mover, is the main muscle responsible for a particular action. For example, the iliopsoas is the agonist for hip flexion. An antagonist is a muscle that resists or opposes the movement of the agonist. During a bicep curl, the triceps is the antagonist to the biceps brachii.
Understanding assisting muscles, or synergists, is crucial for designing safe and effective exercise routines. By knowing which muscles work together, fitness professionals can optimize exercises and help prevent injuries by maintaining balanced muscle function.
