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The human body has over 600 muscles, which are pieces of soft tissue that help us move, breathe, swallow, and survive. They can be categorised into skeletal muscles, which are attached to bones, and prime movers, or agonists, which are muscles that provide the primary force driving an action. Antagonists, on the other hand, work in opposition to agonists, providing resistance or reversing a movement. To determine muscle action, one must identify the type of movement, the effect of external forces, the type of muscle action, and the muscles involved. This can be done using a muscle control formula, which involves six steps, including identifying the joint movement, the plane of movement, and the axis of rotation.
| Characteristics | Values |
|---|---|
| Number of muscles in the human body | Over 600 |
| Types of movements | Voluntary, Involuntary |
| Types of muscle actions | Concentric, Eccentric, Isometric |
| Types of muscle tissue | Skeletal, Cardiac, Smooth |
| Muscle attachment | Origin, Insertion |
| Prime mover | Agonist |
| Antagonist | In opposition to prime mover |
| Synergists | Assist prime mover |
| Stabilizers | Keep bones immobile |
| Forearm supination | Rotating the forearm so the palm is facing forward or up |
| Forearm pronation | Rotating the forearm so the palm is facing backward or down |
| Dorsiflexion | Bringing your foot upward toward your shin |
| Plantar flexion | Depressing your foot |
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What You'll Learn
Muscle terminology
Studying the muscular system involves learning a lot of terminology beyond the names of the muscles themselves. This includes learning how to describe the locations where muscles attach to bones, the different kinds of muscle actions that move the body, and the roles muscles play in different movements.
The first step is to identify the joint movement or position. This could be flexion, extension, abduction, or adduction. For example, the triceps brachii have three bellies with varying origins (scapula and humerus) and one insertion (ulna). It is a prime mover of elbow extension.
The next step is to identify the effect of the external force on the joint movement or position. This can be done by asking: "What movement would the external force produce in the absence of muscle action?".
The third step is to identify the type of muscle action (concentric, eccentric, or isometric). If the answers to the first two steps are in opposite directions, then the muscles are actively shortening in a concentric action. If the answers are in the same direction, then the speed of movement is the determining factor. If the movement is faster than what the external force would produce by itself, then the muscles are actively shortening in a concentric action. If the movement is slower, then the muscles are actively lengthening in an eccentric action. If no movement is occurring, then the muscles are performing an isometric action.
The fourth step is to identify the plane of movement (frontal, sagittal, or transverse) and the axis of rotation. This helps to identify which side of the joint the muscles controlling the movement cross. For example, flexors cross one side of a joint, while extensors cross the opposite side.
The fifth step is to identify on which side of the joint axis the muscles are lengthening and on which side they are shortening during the movement.
The sixth and final step is to combine the information from steps three and five to determine which muscles are producing or controlling the movement or position. For example, if a concentric (shortening) action is required and the muscles on the anterior side of the joint are shortening, then the anterior muscles are actively producing the movement.
Some key terms related to muscle actions include:
- Agonist or prime mover: The muscle that does the bulk of the work during a muscle action. For example, the biceps brachii is a prime mover in elbow flexion.
- Antagonist: The muscle that works in opposition to the agonist. It helps maintain the position of the body or limb during muscle actions and keeps muscle movements controlled. For example, the triceps brachii is the antagonist for elbow flexion but the agonist for elbow extension.
- Synergists: Muscles that assist the prime mover in its role. For example, the anconeus acts as a synergist in elbow extension.
- Stabilizers: Muscles that act to keep bones immobile when needed.
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Types of muscle action
The human body has over 600 muscles, which can be categorised into three types: skeletal, cardiac, and smooth. Skeletal muscles are attached to bones and give the body structure and strength. Cardiac muscles comprise the walls of the heart, allowing blood to be pumped through the vasculature. Smooth muscles are found throughout the blood vessels, gastrointestinal tract, bronchioles, uterus, and bladder.
Muscles perform two types of movements: voluntary and involuntary. Voluntary movements are actions you control, such as sprinting or scrolling through articles on your phone. Involuntary movements happen automatically without conscious thought, such as breathing or digestion.
Muscles can act in three modes: isometric, concentric, and eccentric. Isometric contractions generate force without changing the length of the muscle, such as when maintaining a grip on a heavy object or maintaining posture. Concentric contractions result in the shortening of a muscle while generating force, such as lifting a heavy weight. Eccentric contractions result in the elongation of a muscle while still generating force, such as the controlled lowering of a heavy weight.
Muscle actions are often paired, such as flexion and extension or abduction and adduction. Flexion and extension refer to movements forward and backward from the body, such as nodding the head. Flexion decreases the angle between two bones, while extension increases it. Abduction and adduction refer to movements away from and towards the body, respectively.
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Muscle control formula
The muscle control formula is a six-step procedure for determining the muscles involved in a joint movement and their action. It is a fundamental aspect of movement analysis, which aims to identify the specific muscles active in producing and controlling movement at a particular joint.
The formula begins with a statement of the problem: Given a specific joint movement, identify the name of the movement, the plane of movement, the effect of the external force acting on the system, the type of muscle action, and the muscles involved.
The six steps of the formula are as follows:
- Identify the joint movement or position (e.g., flexion, abduction).
- Identify the effect of the external force (e.g., gravity) on the joint movement by asking: "What movement would the external force produce in the absence of muscle action?"
- Identify the type of muscle action (concentric, eccentric, isometric) based on the answers to steps 1 and 2. If steps 1 and 2 are in opposite directions, the muscles are actively shortening in a concentric action. If they are in the same direction, the speed of movement becomes a factor. If the movement is faster than what the external force would produce, the muscles are shortening concentrically. If it is slower, the muscles are lengthening eccentrically. If no movement is occurring, the muscles are performing an isometric action.
- Identify the plane of movement (frontal, sagittal, transverse) and the axis of rotation (i.e., the line about which the joint is rotating). This helps determine which side of the joint the muscles controlling the movement cross.
- Ask: "On which side of the joint axis are muscles lengthening, and on which side are they shortening during the movement?"
- Combine the information from steps 3 and 5 to determine which muscles are producing or controlling the movement. For example, if a concentric action is required and the muscles on the anterior side of the joint are shortening, then the anterior muscles are actively producing the movement.
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Muscle attachments
The human body has over 600 muscles, and the study of the muscular system involves memorising details about each muscle, such as their attachments and joint motions. Skeletal muscles are part of the musculoskeletal system and work with bones, tendons, and ligaments to support the body's weight and enable movement.
The points of attachment and their positioning determine the range and direction of movement, influencing flexibility and the ability to perform specific motions efficiently. Efficient force transmission depends on the proper alignment of these attachments, which ensures the force produced by muscle contractions is effectively translated into movement. For example, the flexor digitorum profundus originates on the ulna and inserts on the distal phalanges of the fingers, enabling intricate finger movements necessary for grasping and dexterity.
In muscle actions, the prime mover or agonist is the muscle that provides the primary force driving the action. An antagonist muscle works in opposition to the agonist, providing resistance or reversing a movement. Synergists assist the agonist, while stabilisers keep bones immobile when needed.
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Muscle contraction
The human body contains over 600 muscles, and these muscles are responsible for a wide range of movements and functions. Skeletal muscles, attached to bones, provide structure and enable movement. Cardiac muscles, found in the heart, facilitate blood pumping, while smooth muscles line our blood vessels, organs, and other structures.
The process of muscle contraction, known as excitation-contraction coupling, involves a sequence of events. It begins with an action potential that causes depolarization in the myocyte membrane, triggering a chain reaction. This leads to the release of calcium from the sarcoplasmic reticulum, which binds to troponin C, resulting in conformational changes. These changes allow the myosin heads to attach to the actin filaments, forming cross-bridges. The cycling of these cross-bridges, fuelled by ATP, generates the force necessary for muscle contraction.
Furthermore, isotonic contractions can be further categorized into concentric and eccentric contractions. Concentric contractions occur when muscle tension exceeds the opposing load, resulting in the muscle shortening, such as lifting a lightweight object. In contrast, eccentric contractions happen when the muscle lengthens as the tension is insufficient to overcome the load, like slowly lowering the lightweight object.
Understanding the specific muscles involved, their attachments, and the type of contraction they produce is crucial for determining muscle action. This knowledge enables us to comprehend the complex interplay of muscles that facilitates the wide range of movements and functions our bodies are capable of.
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