Victor Forge
The human body is home to more than 600 muscles, which are arranged in pairs based on their functions. These muscles work in antagonistic 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. For example, the biceps and triceps are an antagonistic pair, with the biceps contracting to produce movement and the triceps relaxing to allow the movement to occur. Another example is the quadriceps and hamstrings, which are agonists and antagonists, respectively, during knee extension and flexion. These muscle pairs work together with fixator muscles to support and stabilise the body during movement.
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What You'll Learn
Agonist and antagonist muscles
The human body has more than 600 muscles that help us perform a wide range of actions, from moving our body to breathing and staying alive. Muscles work in pairs called "antagonistic muscle pairs", where one muscle contracts to move a body part, and the other muscle relaxes or lengthens to return the body part to its original position. The muscle that contracts is called the agonist, and the muscle that relaxes or lengthens is called the antagonist.
To illustrate this, let's consider the example of a bicep curl. When performing this movement, the biceps contract to lift the weight, making them the agonist. Simultaneously, the triceps relax to allow the movement to occur, so they are the antagonist. It's worth noting that the roles can reverse depending on the movement; for instance, during active elbow extension, the triceps become the agonist as they contract to straighten the arm, while the biceps become the antagonist as they relax to enable this action.
Another example is the dynamic between the quadriceps and hamstrings. When a footballer prepares to kick a ball, the hamstrings contract to flex the knee, acting as the agonist, while the quadriceps lengthen to accommodate this movement, functioning as the antagonist. Conversely, during the contact and recovery phase, the quadriceps contract to extend the knee and become the agonist, while the hamstrings lengthen and act as the antagonist.
It's important to understand the interplay between agonist and antagonist muscles for activities like resistance training. Weak antagonist muscles can limit the speed of movement, but strengthening them can lead to an increase in agonist muscle movement speed. This knowledge can be applied to improve performance in various sports and activities.
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Bicep curls
The standard bicep curl is performed by holding a dumbbell in each hand with palms facing forward, keeping the upper arms stable and shoulders relaxed, and then bending at the elbow to lift the weights towards the shoulders. It is important to focus on proper form, lifting the weights with a smooth motion, and taking as much time to lower the weight as to lift it.
There are several variations of the bicep curl that can be performed to target different muscle groups and keep workouts interesting. For example, the hammer curl is performed with a neutral grip so that the palms face each other, targeting the "outer head" of the biceps and the forearms. The concentration curl involves sitting on a bench with the back of the upper left arm resting on the inside of the thigh, isolating the biceps muscle. Preacher curls are performed on a preacher bench with an underhand grip, eliminating any momentum from swinging or twisting and putting the focus directly on the biceps.
Another variation is the EZ bar curl, which is similar to a standard barbell curl but with a different grip. The lifter places their arms on the indents of the bar, which can help with wrist pain. This variation does, however, take the wrist slightly out of supination, engaging the brachialis muscle.
Resistance bands can also be used to perform bicep curls. By standing on the middle of the band with each hand holding an end, the resistance will be easiest at the bottom of the movement and get harder as you rise, helping to match the strength curve of the movement.
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Elbow flexion
The elbow joint is a hinge joint, allowing movement in only one plane. Elbow flexion involves decreasing the angle between the forearm and the upper arm. This movement is performed by two groups of muscles in the arm: the anterior compartment and the posterior compartment of the arm. Most of the muscles producing flexion are found in the anterior compartment of the arm.
The prime movers of elbow flexion are the biceps brachii, the brachialis, and the brachioradialis. The biceps brachii is the main flexor of the elbow joint and acts as the prime mover during forearm flexion, such as when lifting a cup. The brachialis and brachioradialis are synergists that aid in this movement. The pronator teres is considered a secondary elbow flexor.
During a press-up, the biceps are the agonist during the downward phase, contracting eccentrically to control the flexion of the elbow and lower the body towards the floor. The triceps are the antagonist during this phase. During the upward phase, the triceps become the agonist, contracting concentrically to extend the elbow, while the biceps are the antagonist.
The brachioradialis has a lengthy origin along the lateral supracondylar bony column, separating the lateral head of the triceps and the brachialis muscle. It progresses distally to insert into the base of the radial styloid. The muscle is innervated by the radial nerve and has the greatest mechanical advantage of any elbow flexor.
The extensor digitorum communis, originating from the anterior distal aspect of the lateral epicondyle, can assist in elbow flexion when the forearm is pronated.
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Knee extension
The rectus femoris is unique among the quadriceps muscles as it crosses both the hip and knee joints, allowing it to assist in hip flexion in addition to its role in knee extension. It contributes to knee extension through synergistic action with the other three muscles in the quadriceps group. The vastus lateralis is the largest component of the quadriceps group and plays a primary role in knee extension. The vastus medialis works alongside the other muscles to stabilise the knee during movement.
The vastus intermedius, located beneath the rectus femoris, also has a vital role in knee extension. It originates from the anterior and lateral surfaces of the femur and inserts into the common quadriceps tendon, connecting to the patella. This muscle is lesser-known among the quadriceps group but is essential for extending the knee joint. The coordinated contraction of these four muscles enables the leg to straighten from a flexed position, providing the necessary force for activities such as walking, running, jumping, and rising from a seated position.
In addition to the quadriceps group, the iliotibial tract (ITB) and tensor fascia latae (TFL) also play a role in supporting knee extension, especially when the knee is between 20 to 30 degrees of flexion. The effectiveness of knee extension can be influenced by factors such as muscle tightness and core strength. Knee extension strength is a significant factor in determining performance on balance tests and overall quality of life, especially in older individuals.
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Synergistic muscles
The human body has over 600 muscles, which help us do everything from moving our bodies to breathing and staying alive. During any movement, the principal muscle involved is called the prime mover or agonist. The agonist produces the most force and is primarily responsible for creating the necessary movement at a joint. For example, the bicep is the agonist during a bicep curl.
Synergist muscles assist the prime mover or agonist in some way, such as by helping to increase power output or promoting stability and reducing the risk of unwanted movement. For instance, during a bicep curl, the brachialis and brachioradialis assist with the motion and are synergists. In a squat, synergist muscles would be calves, glutes, adductors, abs, spinal erectors, and even the hamstrings. These muscles assist the quadriceps, which is the agonist in a squat, by directly producing force for a more powerful movement.
A pair of synergists consists of two muscles that simultaneously assist a prime mover. For example, the anterior deltoids and triceps are a pair of synergists that help the chest during a bench press. The chest is the prime mover, and the synergists assist by stabilising the scapula, allowing for a more powerful press.
The role of synergist muscles is to make movement more fluid, increase power and strength output, and reduce instability. Synergists can also be fixators that stabilise the muscle's origin. For example, the abdominals act as fixators to stabilise the body for hip and knee movements.
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Frequently asked questions
Antagonistic muscle pairs refer to how muscles work in pairs to move body parts. One muscle of the pair contracts to move a body part, and the other muscle relaxes or lengthens to return the body part to its original position.
One example of an antagonistic muscle pair is the biceps and triceps. During a bicep curl, the biceps contract to produce movement, and the triceps relax to allow the movement to occur. The agonist is the muscle that is contracting and in "agony" when you are doing the movement. The antagonist is the muscle that is relaxing or lengthening.
Some muscles called fixators assist antagonistic pairs by supporting and stabilising the joint and the rest of the body. For example, the abdominals act as fixators to stabilise the body for hip and knee movements. Synergists are muscles that stabilise a joint around which movement is occurring, performing the same action as the agonist. For instance, the brachialis and brachioradialis aid the biceps brachii in flexing the lower arm.
