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Chapter 15: The Muscular System Back To Chapter

15.3: Muscle Coordination and Action

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Muscle Coordination and Action

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15.2: Fascicle Arrangement in Skeletal Muscles

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15.4: Naming Skeletal Muscles

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The skeletal muscles can coordinate, oppose, or assist each other to facilitate voluntary body movements.

Depending on their function, a muscle or group of muscles are termed agonists, antagonists, synergists, or fixators.

An agonist muscle produces an action at a joint. For example, during elbow flexion, the biceps brachii is the agonist, pulling the radius towards the body.

In the same movement, the brachialis muscle acts as a synergist. It helps stabilize the elbow joint, allowing the biceps brachii to flex the arm efficiently.

The four rotator cuff muscles — the subscapularis, supraspinatus, infraspinatus, and teres minor — stabilize the humeral head in the glenoid cavity. They act as fixators and stabilize the origins of the biceps brachii, the agonist during arm flexion.

Lastly, antagonists are functionally opposite to the agonist, usually located on opposite sides of the bone. They reverse the movements facilitated by agonists.

For example, while the triceps brachii acts as an antagonist to the biceps brachii during elbow flexion, their role reverses during elbow extension.

15.3: Muscle Coordination and Action

Muscle coordination is a complex and finely tuned process essential for smooth and purposeful movements like flexion, extension, adduction, abduction, and rotation. The human body orchestrates the actions of various muscles working in concert, each with a specific role. Four functional types describe how muscles work together: agonist, antagonist, synergist, and fixator.

Agonists

Agonist muscles, often called prime movers, are the primary muscles responsible for producing a specific movement. For instance, during a bicep curl, the biceps brachii acts as the agonist, contracting to lift the forearm. Agonists are the focal point of any movement, creating the necessary force to change the position of bones and joints.

Antagonists

Antagonist muscles serve as the counterforce to agonists. While the agonist muscle contracts, the antagonist muscle relaxes and lengthens to provide balance and control. This relaxation is crucial for smooth motion and preventing injury. In the bicep curl example, the triceps brachii acts as the antagonist, elongating as the biceps contract. When the movement is reversed and the arm is extended, the roles switch: the triceps become the agonist and the biceps become the antagonist.

Synergists

Synergists aid an agonist by providing additional pull or stabilizing the agonist's origin, their importance varying throughout the movement. Synergists ensure that the motion is smooth and coordinated. For example, during a bicep curl, muscles like the brachialis and brachioradialis work synergistically, providing additional force and ensuring the elbow joint moves correctly.

Fixators

Fixators, or stabilizers, are muscles that hold a part of the body steady, providing a firm base for the agonist to act upon. These muscles might not be directly involved in the movement but are crucial for preventing unwanted motions and maintaining posture. In the bicep curl example, muscles in the shoulder and the upper back act as fixators, keeping the shoulder stable and allowing the biceps to perform the curl efficiently.

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