Kinesiology Chapter 6: Muscular System

Normal Resting Length: The length of a muscle when it is neither shortened nor lengthened by active contraction or external force.
Four Properties of Muscle:
- Irritability:
- Definition: The ability of a muscle to respond to a stimulus, such as an impulse from a nerve or external stretch, resulting in contraction.
- Contractility:
- Definition: The ability of muscle to contract and generate force when adequately stimulated.
- Types of contractions:
  - Concentric: Muscle shortens as it contracts.
  - Eccentric: Muscle lengthens while under tension.
  - Isometric: Muscle length remains unchanged during contraction.
- Extensibility:
- Definition: The ability of muscle to lengthen when a force is applied.
- Elasticity:
- Definition: The ability of muscle to return to its normal resting length when the force is removed, allowing return from both lengthened and shortened positions.

A muscle is composed of muscle fibers organized into bundles called fascicles.
Each muscle fiber is made up of smaller bundles known as myofibrils.
Myofibrils are divided into units called sarcomeres, where length change occurs.
Sarcomeres include:
- Thin filaments (Actin)
- Thick filaments (Myosin)
Filaments are anchored to structures called z-lines.
- Two actin filaments are positioned on either side of a myosin filament.
- Actin filaments do not extend into the central region of a sarcomere.
Myosin filaments feature projecting myosin heads that contract and temporarily bind to actin filaments when the muscle is activated.

The process whereby myosin heads bind to actin is known as cross-bridges.
During contraction:
- Shortening Contraction: Actin filaments slide toward the sarcomere's middle.
- Lengthening Contraction: Actin filaments move apart from the center.
- Isometric Contraction: Minimal sliding occurs with no change in overall muscle length.
The force generated by a muscle is determined by the number of cross-bridges formed:
- Maximum cross-bridges occur at resting length.
- Length deviations from resting length lead to diminished cross-bridge formation.

Type I Fibers (Slow Twitch or Slow Oxidative):
- Characteristics:
  - Smaller diameter
  - Slower response to stimuli
  - Capable of sustaining longer contractions
  - Rich blood supply indicating a red color
  - Utilizes oxygen as its energy source.
- Significance: Better for endurance activities and lower force production; commonly found in postural muscles.
Type II Fibers (Fast Twitch or Fast Glycolytic):
- Characteristics:
  - Larger diameter
  - Quick to respond to stimulation
  - Relax quickly post-stimulation, reducing fatigue
  - Less blood supply giving them a pale (white) color
  - Utilizes stored glycogen as energy.
- Significance: Suitable for short bursts of high-force contraction, effective in fast, brief movements.

Definition: A motor unit consists of muscle fibers that are innervated by the same motor neuron.
Characteristics:
- Muscle fibers are not bundled together but are dispersed among several fascicles.
- Number of muscle fibers varies among motor units:
- Precision movements utilize motor units with fewer fibers.
- Power movements utilize motor units with many fibers.
Gradation of Motor Units: The recruitment of motor units depends on the force required.
Key Junctions:
- Musculotendinous Junction: Location where muscle attaches to tendon.
- Tenoperiosteal Junction: Location where tendon attaches to bone.
- Origin: The proximal attachment point of a muscle.
- Insertion: The distal attachment point of a muscle.

The force a muscle can exert is proportional to its cross-sectional area.
Cross-sectional area relates to fiber orientation:
- Parallel Muscle Fibers: Extend the entire length of the muscle.
- Oblique Muscle Fibers: Allow for more cross-sectional area with shorter fibers, attaching obliquely to the tendon.
Angle of Insertion: Angle at which a muscle attaches to a bone, influencing the range of motion (ROM).
Line of Pull: Direction from the origin to insertion, accounting for twists and turns.
Angle of Pennation: The angle at which oblique fibers attach to tendons in pennate or multipennate muscles.
Regardless of the angles associated with attachment, all generated muscle force contributes to osteokinematic motion at a joint axis and exerts traction or compression forces through joints.

Agonist: The muscle responsible for the desired motion defined by its size, location, line of pull, and contractile capacity.
Antagonist: The muscle that performs the opposing action to the agonist.
Co-contraction: The simultaneous activation of agonist and antagonist muscles.
Synergist: Collaborating muscles that work together to produce a motion that neither can achieve alone.

Isometric Contraction: Force is generated without a change in muscle length.
Concentric Contraction: Muscle attachments draw closer to each other during contraction; shortening occurs, typically overcoming the force of gravity:
- Open Kinetic Chain: The insertion moves toward the origin.
- Closed Kinetic Chain: The origin moves toward the insertion.
Eccentric Contraction: Muscle attachments move apart during contraction; lengthening occurs:
- Open Kinetic Chain: The insertion moves away from the origin.
- Typically works against gravity to slow down movement.

Multi-joint Muscles:
- Active Insufficiency: When a muscle cannot actively shorten to its fullest ROM at all joints it spans.
- Passive Insufficiency: When a muscle cannot lengthen through all joints it spans.
Tenodesis: The ability to close a fist via passive insufficiency, allowing those unable to actively grasp to perform some release and grasp functions.

Adaptive Lengthening: Occurs when muscles are maintained in a lengthened position for extended periods.
Adaptive Shortening: Occurs when muscles are kept in a shortened position for prolonged durations.
Not necessarily related to strength changes but affects the point in the ROM where maximum force production happens:
- Loss of Sarcomeres leads to adaptive shortening.
- Addition of Sarcomeres leads to adaptive lengthening.

Strain: Results from overstretching of muscles.
Rupture: A complete tear of a tendon.
Trigger Points: Hyperirritable and painful spots within a muscle.
Tendonitis: Inflammation of a tendon due to various causes, often repetitive strain or excessive use.