OTA125_Ch. 6: Muscular System [Lippert]
Characteristics of Muscle
Normal Resting Length:
The length of a muscle when it is not shortened or lengthened by active contraction or the application of external force.
Four Fundamental Properties of Muscle:
Irritability: Ability to respond to a stimulus, such as an impulse from a nerve or an external stretch, resulting in a contraction.
Contractility: Ability to contract, generating force when an adequate stimulus is applied.
Concentric: Shortening contraction.
Eccentric: Lengthening contraction.
Isometric: Contraction where no length change occurs.
Extensibility: Ability of a muscle to lengthen when a force is applied.
Elasticity: Ability of a muscle to return to normal resting length when force is removed, returning from either a lengthened or a shortened position.
Anatomy of a Muscle
Structural Hierarchical Organization:
A muscle is a collection of muscle fibers bound together into bundles termed fascicles.
Each muscle fiber is composed of smaller bundles termed myofibrils.
Myofibrils are divided into functional units termed sarcomeres.
The sarcomere is the specific anatomical structure where length change occurs.
Filament Arrangement:
Sarcomeres contain two types of filaments:
Thin actin filaments.
Thicker myosin filaments.
Filaments are attached to z-lines.
Two actin filaments are situated on each side of a myosin filament.
Actin filaments do not extend into the middle of a sarcomere.
Projecting from myosin filaments are myosin heads, which contract and temporarily bind with actin filaments when the muscle is activated.
Sliding Filament Theory
Mechanism of Contraction:
The binding of myosin heads to actin is termed cross-bridges.
When myosin heads bind with actin, the actin filament slides closer to or away from the actin filament on the other side of the gap:
Shortening Contraction: Actin filament moves toward the gap in the middle of the sarcomere.
Lengthening Contraction: Actin filaments move apart.
Isometric Contraction: Minimal sliding occurs, resulting in no change in overall muscle length.
Cross-Bridge and Force Relationships:
The number of cross-bridges formed directly determines the force generated.
At resting length, the maximum number of cross-bridges can be formed.
When muscle length is greater than or less than resting length, there is a decrease in the formation of cross-bridges.
Muscle Fiber Types
Type I Fibers:
Terminology: Termed slow twitch or slow oxidative.
Diameter: Smaller in diameter.
Stimulation Response: Slower to respond to stimulus.
Endurance: Able to sustain longer periods of contraction.
Blood Supply: Rich blood supply, giving them a red appearance.
Energy Source: Uses oxygen as an energy source.
Functional Capability: Characteristics make Type I fibers better suited for endurance and low levels of force production.
Primary Location: Postural muscles.
Type II Fibers:
Terminology: Termed fast twitch or fast glycolytic.
Diameter: Larger in diameter.
Stimulation Response: Respond quickly to stimulation.
Relaxation Rate: Relax quickly after stimulation, avoiding fatigue.
Blood Supply: Less blood supply, giving them a pale (white) appearance.
Energy Source: Uses existing stores of glycogen as an energy source.
Fatigue Profile: Prone to fatigue.
Functional Capability: Effective for short bursts of contraction involving fast, brief contractions of high force.
Motor Units and Attachments
Motor Unit Organization:
Definition: Muscle fibers innervated by the same motor neuron.
Fiber Distribution: Muscle fibers are not all bundled together; instead, they are distributed among several fascicles.
Size and Precision Variability:
Precision movements use motor units with a fewer number of muscle fibers.
Power movements use motor units with a greater number of muscle fibers.
Force Regulation: Gradation of the number of motor units recruited is based on the force required.
Anatomical Junctions and Attachments:
Musculotendinous Junction: The place where a muscle joins a tendon.
Tenoperiosteal Junction: The place where a tendon joins a bone.
Origin: The proximal muscle attachment.
Insertion: The distal muscle attachment.
Muscle Fiber Arrangement and Structural Mechanics
Cross-Sectional Area and Force Production:
The amount of force a muscle can generate is directly proportional to the cross-sectional area of the muscle.
Cross-sectional area is related to muscle fiber orientation.
Fiber Orientations:
Parallel Muscle Fibers: Fibers extend the entire length of the muscle.
Oblique Muscle Fibers: Shorter fibers that attach obliquely to a tendon, enabling more cross-sectional area.
Angles and Mechanical Lines:
Angle of Insertion: The angle at which a muscle attaches to a bone or other structure, which changes through range of motion (ROM).
Line of Pull: A line drawn from origin to insertion of a muscle, accounting for twists and turns along the path.
Angle of Pennation: The angle at which oblique fibers within a pennate or multipennate muscle attach to muscle tendons.
Mechanical Output Contribution: Regardless of the angle of insertion or angle of pennation, all muscle force generated contributes to osteokinematic motion about a joint axis and the traction or compression force through a joint.
Naming Conventions of Muscles
Muscles are named based on specific defining criteria:
Location: Example — anterior tibialis
Shape: Example — quadratus lumborum
Action: Example — flexor digitorum
Number of Heads/Divisions: Example — biceps brachii
Attachments: Example — sternocleidomastoid
Fiber Orientation: Example — external oblique
Size: Example — extensor radialis longus
Functional Roles of Muscles
Agonist: The muscle responsible for desired motion. Determined by size, location, line of pull, and contractile ability.
Antagonist: The muscle that performs the opposite motion of the agonist.
Co-contraction: Simultaneous contraction of the agonist and antagonist.
Synergist: Two or more muscles contracting together to produce a motion that neither can perform individually.
Types of Muscle Contraction
Isometric Contraction:
Force is produced without any change in muscle length.
Concentric Contraction:
Muscle attachments move toward each other during a shortening contraction.
Generally functions to overcome the effect of gravity.
Open Kinetic Chain Configuration: Insertion moves toward origin.
Closed Kinetic Chain Configuration: Origin moves toward insertion.
Eccentric Contraction:
Muscle attachments move away from each other during a lengthening contraction.
Generally functions to slow down the effect of gravity.
Open Kinetic Chain Configuration: Insertion moves away from origin.
Active Insufficiency, Passive Insufficiency, and Tenodesis
Multi-Joint Muscle Dynamics:
Both active insufficiency and passive insufficiency occur in multi-joint muscles.
Active Insufficiency:
Occurs when a multi-joint muscle is unable to actively shorten simultaneously through full ROM at all joints spanned.
Passive Insufficiency:
Occurs when a multi-joint muscle is unable to lengthen simultaneously through full ROM at all joints spanned.
Tenodesis:
Closing of the fist through passive insufficiency.
Functionally allows some grasp and release capabilities for individuals who cannot actively grasp and release.
Adaptive Muscle Length Changes
Prolonged Postural Adaptations:
Muscle length changes when maintained in a lengthened or shortened position for prolonged periods.
A length-changed muscle is not necessarily stronger or weaker.
The point within the ROM of greatest force production shifts.
Sarcomere Quantity Adaptations:
Adaptive Shortening: Results in a loss of sarcomeres.
Adaptive Lengthening: Results in an addition of sarcomeres.
Common Pathologies of Muscle and Tendon
Strain: Overstretching of muscle or tendon.
Rupture: Complete tearing of a tendon.
Trigger Points: Hyperirritable painful points located within a muscle.
Tendonitis: Inflammation of a tendon.