뒤로Skeletal Muscle: Structure, Function, and Mechanics
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Skeletal Muscle
Classification of Skeletal Muscle Fibers
Skeletal muscle fibers are classified based on their speed of contraction and resistance to fatigue. These properties are determined by their metabolic pathways, myoglobin content, and structural characteristics.
Slow-twitch fibers (ST or Type I): Rely primarily on oxidative phosphorylation for ATP production. They are adapted for endurance and continuous activity.
Fast-twitch fibers: Develop tension more rapidly, split ATP more quickly, and pump Ca2+ into the sarcoplasmic reticulum more rapidly.
Fast-twitch oxidative-glycolytic fibers (FOG or Type IIA): Use both oxidative and glycolytic metabolism. They are moderately resistant to fatigue.
Fast-twitch glycolytic fibers (FG or Type IIB/X): Rely primarily on anaerobic glycolysis. They fatigue quickly but generate powerful contractions.
Myoglobin: A protein that binds oxygen in muscle, making it readily available for aerobic processes. Oxidative fibers have higher myoglobin content, giving them a darker (red) appearance.
Example: Postural muscles contain more slow-twitch fibers, while muscles used for rapid, intense movements (e.g., eye muscles) have more fast-twitch fibers.
Characteristics of Muscle Fiber Types
Muscle fibers differ in their contraction speed, fatigue resistance, metabolic pathways, and structural features. The following table summarizes these differences:
Characteristic | Slow-Twitch Oxidative; Red Muscle (Type I) | Fast-Twitch Oxidative-Glycolytic; Red Muscle (Type IIA) | Fast-Twitch Glycolytic; White Muscle (Type IIB/X) |
|---|---|---|---|
Speed of Development of Maximum Tension | Slowest | Intermediate | Fastest |
Myosin ATPase Activity | Slow | Fast | Fast |
Diameter | Small | Medium | Large |
Contraction Duration | Longest | Short | Short |
Ca2+ ATPase Activity in SR | Moderate | High | High |
Endurance | Fatigue resistant | Fatigue resistant | Easily fatigued |
Use | Most used: posture | Standing, walking | Least used: jumping, quick fine movements |
Metabolism | Oxidative; aerobic | Glycolytic but becomes more oxidative with endurance training | Glycolytic; more anaerobic than fast-twitch oxidative-glycolytic type |
Capillary Density | High | Medium | Low |
Mitochondria | Numerous | Moderate | Few |
Color | Dark red (myoglobin) | Red | Pale |
Mechanics of Body Movement
Length-Tension Relationship
The force a muscle can generate depends on its length at the start of contraction. This is known as the length-tension relationship.
Optimal length (L0): Sarcomeres contract with maximum force when at an optimal length—neither too long nor too short—before contraction begins.
Tension generated: Directly proportional to the number of crossbridges formed between actin and myosin filaments.
Example: Muscles that are overly stretched or overly contracted before activation produce less force than those at optimal length.
Summation and Tetanus
Muscle contraction strength can be increased by summation, which occurs when a muscle does not relax completely between action potentials.
Summation: Stronger contraction due to incomplete relaxation between stimuli.
Tetanus: Maximal, sustained contraction. Can be incomplete (unfused) or complete (fused) depending on the frequency of stimulation.
Motor Units and Recruitment
A motor unit consists of one motor neuron and all the muscle fibers it innervates. The force of muscle contraction depends on the number and type of motor units activated.
Recruitment: Activation of additional motor units increases contraction force.
Asynchronous recruitment: Alternating activation of motor units helps prevent fatigue during sustained contractions.
Example: Fine motor control (e.g., in the fingers) requires small motor units, while large, powerful muscles (e.g., quadriceps) have large motor units.
Additional info:
Equation for Force Generation: The force generated by a muscle is proportional to the number of active crossbridges:
Length-Tension Curve: The relationship between sarcomere length and tension is typically represented as a bell-shaped curve, with maximum tension at optimal sarcomere length.
