Lecture 4.3: Muscle Systems

Muscle Systems

Introduction to the Muscular System

The muscular system consists exclusively of skeletal muscles, which are responsible for voluntary movements of the body. The organization of these muscles significantly influences the power, range, and speed of movement.

• Skeletal muscles are attached to bones and facilitate movement by contracting and pulling on the skeleton.

• Muscle structure and arrangement determine their functional capabilities.

Fascicle Arrangement in Skeletal Muscles

Overview of Fascicle Arrangements

Skeletal muscle fibers are grouped into bundles called fascicles. The arrangement of these fascicles forms the basis for classifying muscles and directly impacts their function.

• Parallel muscles: Fascicles run parallel to the long axis of the muscle.

• Convergent muscles: Fascicles spread out over a broad area and converge at a common attachment site.

• Pennate muscles: Fascicles are arranged at an angle to the tendon.

• Circular muscles (sphincters): Fascicles are arranged concentrically around an opening.

Parallel Muscles

In parallel muscles, the fascicles are aligned with the long axis of the muscle, allowing for efficient contraction and movement. Most skeletal muscles are of this type.

• Contraction shortens and fattens the muscle.

• Examples include the biceps brachii and rectus abdominis.

Convergent Muscles

Convergent muscles have a broad origin and converge to a single tendon. This arrangement allows for versatile movement but less force than parallel muscles.

• Example: Pectoralis major of the chest.

Pennate Muscles

Pennate muscles have fascicles that attach obliquely to a central tendon, resembling a feather. This arrangement allows for greater force production but less range of motion compared to parallel muscles.

• Unipennate: Fascicles on one side of the tendon (e.g., extensor digitorum).

• Bipennate: Fascicles on both sides of a central tendon (e.g., rectus femoris).

• Multipennate: Tendon branches within the muscle (e.g., deltoid).

Circular Muscles (Sphincters)

Circular muscles, or sphincters, are arranged in concentric rings and function as valves to open and close body passages.

• Example: Orbicularis oris of the mouth.

Levers and Muscle Efficiency

Levers in the Musculoskeletal System

Most skeletal muscles act on bones as levers, which are rigid structures that move around a fixed point called a fulcrum. The arrangement of the applied force, fulcrum, and load determines the type of lever and its mechanical advantage.

• Levers can change the direction, distance, speed, and strength of movement.

• There are three classes of levers in the body:

1. First-class lever: Fulcrum between applied force and load (e.g., extension of the neck).

2. Second-class lever: Load between applied force and fulcrum (e.g., ankle extension by calf muscles).

3. Third-class lever: Applied force between load and fulcrum (most common; e.g., biceps brachii at the elbow).

Origins, Insertions, and Muscle Actions

Origins and Insertions

The origin of a muscle is its fixed attachment point, usually proximal, while the insertion is the movable attachment, usually distal. Muscle contraction pulls the insertion toward the origin, producing movement at a joint.

Muscle Actions and Interactions

Muscles work together to produce coordinated movements. The main roles include:

• Agonist (prime mover): Main muscle responsible for a movement.

• Antagonist: Opposes the action of the agonist.

• Synergist: Assists the agonist in performing its action.

• Fixator: Stabilizes the origin of the agonist to allow efficient movement.

Agonists and antagonists typically work in pairs, such as flexors and extensors.

Naming Muscles

Principles of Muscle Nomenclature

Muscle names often provide clues about their location, attachments, fascicle arrangement, position, structure, or action. Common naming conventions include:

• Location: e.g., abdominus (abdomen)

• Origin and insertion: e.g., genioglossus (chin to tongue)

• Fascicle organization: e.g., rectus (straight), oblique (angled)

• Position: e.g., externus (external), profundus (deep)

• Structural characteristics: e.g., trapezius, biceps (two heads)

• Action: e.g., flexor, extensor, abductor

Effects of Exercise and System Integration

Muscular System and Other Body Systems

The muscular system interacts closely with other body systems to maintain homeostasis and support movement:

• Cardiovascular system: Delivers oxygen and nutrients, removes waste.

• Respiratory system: Meets increased oxygen demand during activity.

• Integumentary system: Disperses heat generated by muscle activity.

• Nervous and endocrine systems: Regulate and coordinate muscle function.

Age-Related Changes to the Muscular System

With aging, muscles may lose mass and strength, and recovery from injury slows. Regular exercise can help mitigate these effects by maintaining muscle tone and function.

Summary Table: Muscle Types by Fascicle Arrangement

Muscle Type	Arrangement	Example
Parallel	Fascicles parallel to long axis	Biceps brachii
Parallel with tendinous bands	Parallel with transverse bands	Rectus abdominis
Wrapping	Parallel, wrapping around bone	Supinator
Convergent	Broad origin, converges to tendon	Pectoralis major
Unipennate	Fascicles on one side of tendon	Extensor digitorum
Bipennate	Fascicles on both sides of tendon	Rectus femoris
Multipennate	Tendon branches within muscle	Deltoid
Circular	Concentric rings	Orbicularis oris

Key Equations

• Mechanical Advantage (MA) of a Lever:

$MA = \frac{Length\ of\ effort\ arm}{Length\ of\ resistance\ arm}$

• Force Output:

$Force_{output} = Force_{input} \times MA$

Additional info: Equations above are general for lever mechanics and help in understanding how muscle attachment affects force and movement.