Classification of Skeletal Muscles

To illustrate the interactions between skeletal muscles, muscles in different states are classified into distinct categories.

They are primarily divided into two major categories: stabilizer muscles and mover muscles:

1. ‌Stabilizer Muscles‌: These muscles are responsible for maintaining stability in the limbs and torso. Located in the deeper layers, stabilizer muscles typically span a single joint and are composed primarily of slow-twitch muscle fibers. Although they have a limited range of motion, this is sufficient to fulfill their role in stabilizing joints."

‌2. Mover Muscles‌: These muscles primarily generate movement in the limbs and torso through contraction. Located in the more superficial layers, mover muscles are mainly composed of fast-twitch muscle fibers. They have a broader range of motion and are responsible for executing primary movements, often the focus of gym-based strength training. Depending on their functional roles, mover muscles are further categorized into distinct types:

‌(1) Agonist (Prime Mover)‌: The muscle directly responsible for initiating a specific movement, such as flexion or abduction at a joint. For example, the deltoid muscle (particularly the middle deltoid) is the agonist for shoulder abduction, as its contraction lifts the arm away from the body.

‌(2) Antagonist‌: The muscle that opposes or reverses a specific movement. When the agonist contracts, the antagonist is stretched or relaxed in the opposite direction. The relationship between agonist and antagonist mirrors the traditional concept of yin and yang—a dynamic interplay of mutual growth and decline. This antagonistic balance is critical for maintaining postural stability, controlling movement speed, and ensuring coordinated motion.

‌(3) Synergist‌: These muscles assist the agonist in completing a movement. For instance, during shoulder abduction by the deltoid, the supraspinatus muscle acts as a synergist to stabilize and enhance the motion. In many cases, it can be challenging to strictly distinguish between the agonist and synergist, as they work cooperatively to achieve the same action. Thus, they are described as mutually synergistic.