Muscular System: Fascicle Arrangement and Muscle Power

The Muscular System

• This chapter explores the muscular system's gross anatomy and functional relationships between muscles and bones
• Skeletal muscle fibers contract at similar rates and shorten to the same degree, but microscopic and macroscopic variations dramatically affect the power, range, and speed of movement

Fascicle Arrangement and Muscle Power

• Muscle fibers form bundles called fascicles
• Fascicle arrangement varies: the relationship between fascicles and tendons also varies
• Skeletal muscles are classified based on fascicle arrangement: parallel, convergent, pennate, and circular muscles

Parallel Muscles

• Fascicles are parallel to the muscle's long axis
• Some are flat bands with broad attachments (aponeuroses) at each end
• Others are plump and cylindrical, with tendons at one or both ends, and a central body (belly)
• Biceps brachii is a parallel muscle with a central body
• When a parallel muscle contracts, it shortens and gets larger in diameter
• Parallel muscles include the rectus abdominis and supinator
• A skeletal muscle fiber shortens by about 30 percent
• The tension developed depends on the total number of myofibrils

Convergent Muscles

• Fascicles extend over a broad area but converge on a common attachment site
• The muscle may pull on a tendon, aponeurosis, or raphe (slender collagen fiber band)
• Muscle fibers spread out, like a fan or broad triangle, with a tendon at the apex
• Examples include the pectoralis muscles of the chest
• Convergent muscles can adapt to different activities because stimulating different portions changes the direction of pull
• Convergent muscle fibers pull in different directions, so do not pull as hard on the attachment site as parallel muscles of the same size

Pennate Muscles

• Fascicles form a common angle with the tendon
• Contracting pennate muscles move their tendons less far than parallel muscles
• Pennate muscles contain more muscle fibers (and myofibrils), and produce more tension, than parallel muscles of the same size
• Pennate muscles include: unipennate, bipennate, and multipennate
  • In unipennate muscles, all fascicles are on the same side of the tendon
  • An example is the extensor digitorum
  • In bipennate muscles, fascicles are on both sides of a central tendon
  • An example is the rectus femoris
  • If the tendon branches within the muscle, it is multipennate
  • An example is the deltoid of the shoulder

Circular Muscles

• Fascicles are concentrically arranged around an opening
• When the muscle contracts, the diameter of the opening becomes smaller
• Circular muscles (sphincters) surround body openings or hollow organs and act as valves in the digestive and urinary tracts
• An example is the orbicularis oris of the mouth

Bone Levers and Muscle Efficiency

• Skeletal muscles don't work in isolation
• With muscles attached to the skeleton, their connections determine the force, speed, and range of movement produced
• These characteristics are interdependent and explain the organization of the muscular and skeletal systems
• Attaching muscle to a lever modifies the force, speed, or direction of movement produced by contraction
• A lever is a rigid structure (board, crowbar, or bone) that moves on a fixed point (fulcrum)
• A lever moves when pressure (applied force) overcomes a load
• In the body, each bone is a lever and each joint is a fulcrum
• Muscles provide applied force
• The load can vary from the weight of an object, a limb, or the entire body
• Levers change the following: direction of an applied force, distance, speed of movement produced by an applied force, and effective strength of an applied force
• Levers are classified by the relative positions of: applied force, fulcrum, and load
• Regardless of class of lever, all follow the same mechanical principles:
  • A mechanical advantage occurs when the applied force is farther from the fulcrum than the load
  • A mechanical disadvantage when the applied force is closer to the load than the fulcrum

First-Class Levers

• The fulcrum (F) lies between the applied force (AF) and the load (L), positions = L-F-AF
• The body has few first-class levers
• One is involved with extension of the neck and lifting of the head

Second-Class Levers

• The load lies between the applied force and the fulcrum, positions = F-L-AF
• A small force can move a larger weight b/c the force is always farther from the fulcrum than the load
• Effective force is increased at the expense of speed and distance
• The body has few second-class levers
• Ankle extension (plantar flexion) by the calf muscles involves a second-class lever

Third-Class Levers

• The applied force is between the load and the fulcrum, positions = F-AF-L
• Third-class levers are the most common levers in the body.
• The effect is the reserve: speed and distance traveled are increased at the expense of effective force
• Not every muscle is part of a lever system, but levers provide speed and versatility beyond just muscle physiology

Origins, Insertions, and Muscle Actions

• Skeletal muscle actions require an understanding of where they're connected to the bones that act as levers, and which joints they cross
• When both myofibril ends are free to move, the ends move toward the center during contraction
• In the body, the ends of a skeletal muscle are always attached to structures that limit their movement
• The fixed end is called the origin of the muscle, and the movable end is the insertion of the muscle
• The origin is typically proximal to the insertion
• Almost all skeletal muscles either originate or insert on the skeleton
• The decision as to which end is the origin is based on movement from the anatomical position
• When movement does not easily show origins/insertions from the anatomical position:
  • A muscle that extends between a broad aponeurosis and a narrow tendon, the aponeurosis is the origin
  • For multiple tendons at one end, and just one at the other, the muscle has multiple origins and a single insertion

Actions

• When a muscle contracts, it produces a specific action, or movement, which may involve flexion, extension, adduction, abduction, and so on
• Actions described in two ways, by focusing on the bone or the joint
• In complex movements, several muscles commonly work in groups
• Their cooperation improves the efficiency of a particular movement
• The range of motion is commonly more extended with groups
• Those muscles cannot produce powerful movements at full extension, so they are usually paired with smaller muscles
• The larger muscle produces minimum tension when smaller ones produce maximum tension.
• These types are: agonist, antagonist, synergist, and fixator

Agonist

• (Prime mover): is a muscle that is mostly responsible for producing a particular movement
• The biceps brachii is an agonist that produces flexion at the elbow

Antagonist

• A muscle whose action is opposite that of a particular agonist
• the triceps brachii extends the elbow and is an antagonist
• Agonists and antagonists are functional opposites (flexors-extensors, abductors-adductors) and are stretched

Synergist

• Helps a larger agonist work efficiently, may provide additional pull near the insertion or stabilize the point of origin
• Importance changes as the movement progresses.
• Eg latissimus dorsi and teres

Fixator

• Assist an agonist by preventing movement at another joint, thereby stabilizing the origin of the agonist

Skeletal Muscle Names

• Anatomists assign names including descriptive terms
• A muscle name may indicate its body region, position/direction/fascicle arrangement, structural characteristics, and/or action

Axial and Appendicular Muscles

• The separation of the skeletal system into axial and appendicular serves as a guideline to subdivide the muscular system
• Axial muscles constitute ~60 percent of skeletal muscles, position the head/vertebral column, move the rib cage, form the pelvic floor
• Appendicular muscles constitute ~40 percent of skeletal muscles, stabilize and move the pectoral/pelvic girdles and upper/lower limbs

Axial Muscles Key Functions

• The axial muscles includes muscles of head and neck, vertebral column, trunk and pelvic floor
• The general functions of axial muscles are:
  • They move the face, tongue, and larynx
  • They are responsible for speech and nonverbal communication
  • Assist in eating and looking for food
  • Include muscles involved with movements of the vertebral column
  • This group includes numerous flexors, extensors, and rotators of the vertebral column
  • Forms the muscular walls of the trunk between the first thoracic vertebra and the pelvis and supports muscles of the pelvic floor

Appendicular Muscles Key Functions

• The general function of appendicular muscles are to:
  • To stabilize or move structures of the appendicular skeleton
  • They Include those that move and support the pectoral (shoulder) and pelvic girdles and the upper and lower limbs
  • Support the arms and hands
  • Help move the lower back and legs