muscle (Pt. 1) Flashcards
what are three type of muscular tissue?
skeletal, cardiac and smooth muscle
Q: What are the main characteristics of skeletal muscle?
A: Moves bones, has striations, consciously controlled with some subconscious functions, connected to somatic nervous system
Q: Where is cardiac muscle found and what controls it?
A: Found only in heart, controlled by hormones and neurotransmitters, part of autonomic nervous system
What is unique about cardiac muscle’s function?
A: Has natural pacemaker (autorhythmicity) and cannot be consciously controlled
Q: Where is smooth muscle located?
A: Found in hollow internal structures, blood vessels, airways, organs, and skin (hair follicles)
Q: What are the visual characteristics of smooth muscle?
A: No striations, appears smooth
Q: How is smooth muscle controlled?
A: Controlled by autonomic nervous system and hormones, some have autorhythmicity
Q: What type of muscle has striations?
A: Both skeletal and cardiac muscle have striations
Q: Which muscles are involuntary?
A: Cardiac and smooth muscle are involuntary
Which muscle type is voluntary?
A: Skeletal muscle is voluntary (with some subconscious control)
Functions of Muscular Tissue
Producing body movements
Stabilizing body positions
Storing and mobilizing substances within the body
Generating heat
Q: What is the role of muscles in producing body movements?
A: Controls whole body movements (like walking and running) and localized movements (like grasping and typing), working with bones and joints.
Q: How do muscles stabilize body positions?
A: Maintains posture and joint stability, enabling standing and sitting (e.g., neck muscles keeping the head upright).
Q: How do muscles aid in moving substances in the body?
A: The heart pumps blood, blood vessels regulate flow, and muscles in the digestive, reproductive, and urinary systems move substances.
Q: What role do skeletal muscles play in fluid movement?
A: They aid in lymph and venous flow in the body.
Q: How do muscles contribute to generating heat?
A: Produce heat during contraction, helping to maintain body temperature.
Q: What is shivering related to muscle function?
A: Involuntary contractions of muscles that increase heat production.
Q: What are the basic states of skeletal muscle activity?
A: Two primary states: contraction and relaxation, working via antagonistic pairs
Q: What percentage of total body mass is skeletal muscle in average adults?
A: 40-50% of total body mass
Q: What factors affect skeletal muscle percentage in the body?
A: Age, sex, fitness level, and overall health status
Q: What are the main effects of exercise on skeletal muscle?
A: Increased muscle mass (hypertrophy), enhanced strength, improved endurance, better neural recruitment, and increased metabolic efficiency
Q: How is skeletal muscle’s functional capacity measured?
A: Through muscle strength, power output, endurance, and recovery rate
Q: What is the primary energy conversion function of skeletal muscle?
A: Converting chemical energy to mechanical energy
Q: What factors influence skeletal muscle efficiency?
A: Training status, muscle fiber type, metabolic health, neuromuscular coordination, and energy system utilization
Q: What is the clinical significance of skeletal muscle?
A: Important for physical performance, metabolic health, daily function, quality of life, and disease prevention
Q: What is a skeletal muscle composed of?
A: Hundreds to thousands of elongated cells called muscle fibers (myocytes), along with connective tissues, blood vessels, and nerves
Q: What is the role of myoblasts in muscle development?
A: They are early muscle cells that combine/merge to form muscle fibers
Q: What are satellite cells and their function?
A: Special cells that act as muscle stem cells, helping with muscle growth, repair, regeneration, and maintenance
Q: What is a myonuclear domain?
A: A specific area of the muscle fiber controlled by each nucleus, critical for cell function and maintenance
Q: What characterizes mature muscle fiber structure?
A: They are multinucleated cells with multiple myonuclear domains that control specific areas
Q: How do satellite cells change with age?
A: They are abundant during embryonic development but gradually decrease with age, leading to reduced regenerative and repair capacity
Q: What are the consequences of aging on muscle tissue?
A: Progressive decrease in muscle mass (sarcopenia), reduced repair capacity, increased fibrosis, decreased strength and power output
Q: What clinical implications result from age-related muscle changes?
A: Impacts physical independence, fall risk, overall mobility, quality of life, and metabolic health
Q: What are the main strategies for preventing age-related muscle decline?
A: Exercise, proper nutrition, hormone balance, and lifestyle modifications
Q: What is the outermost protective layer of a whole muscle?
A: The epimysium
Q: What are fascicles and what surrounds them?
A: Bundles within the muscle, surrounded by perimysium
Q: What two layers protect individual muscle fibers?
A: Endomysium and sarcolemma (cell membrane)
Q: What are myofibrils?
A: Thread-like structures inside muscle fibers responsible for contraction
Q: What is the basic functional unit of muscle cells?
A: The sarcomere
Q: What are the two main types of myofilaments in muscle tissue?
A: Thick filaments (myosin) and thin filaments (actin)
Q: What is the role of troponin?
A: A regulatory protein that binds to calcium and helps control muscle contraction
Q: How are muscles connected to bones?
A: Through tendons
Q: What is the hierarchical organization of muscle tissue (from largest to smallest)?
A: Whole muscle → Fascicles → Muscle Fibers → Myofibrils → Myofilaments
Q: What are the main components at the skeletal muscle level?
A: Muscle fascicles, muscle fibers, blood vessels, and nerves, all enclosed by epimysium
Q: What is a muscle fascicle and what surrounds it?
A: A bundle of muscle fibers surrounded by perimysium
Q: What are the key components of a muscle fiber?
A: Sarcoplasmic reticulum, sarcolemma, myofibrils, sarcoplasm, multiple nuclei, T tubules, terminal cisterns, and mitochondria
Q: What is the role of the sarcoplasmic reticulum?
A: Stores calcium, which is crucial for muscle contraction
Q: What are T tubules and terminal cisterns responsible for?
A: T tubules transmit electrical signals into the muscle fiber, and terminal cisterns work with T tubules to release calcium
Q: What is a myofibril and what are its main components?
A: Contractile elements within sarcoplasm containing sarcomeres, Z discs, thick filaments (myosin), and thin filaments (actin)
Q: What is a sarcomere?
A: The basic functional unit of a myofibril, defined by Z discs
Q: Where are the nuclei located in a muscle fiber?
A: Multiple nuclei are located on the periphery of the muscle fiber
Sarcomere
Basic functional unit of a myofibril.
Z Disc
Defines the boundaries of each sarcomere.
Q: What are the two types of myofilaments?
A: Thick filaments (composed of myosin) and thin filaments (composed of actin, tropomyosin, and troponin)
Q: What defines the boundaries of each sarcomere?
A: Z discs
Q: How are thick and thin filaments arranged in a sarcomere?
A: Thick and thin filaments are arranged between the Z discs
Q: What mechanism allows for muscle shortening during contraction?
A: Thin filaments slide past thick filaments, known as the sliding filament mechanism
Q: What is the structure of thick filaments?
A: Composed of myosin with two intertwined tails known as the myosin heavy chain (types one and two)
Q: What is the role of thin filaments in muscle contraction?
A: Muscle contraction occurs through the sliding of thin filaments (actin) over thick filaments (myosin)
What are the two main components involved in muscle contraction at the filament level?
A: Myosin (thick filaments) and actin (thin filaments)
Q: How does muscle contraction occur at the sarcomere level?
A: Myosin and actin filaments slide past each other, causing the sarcomere to shorten
Q: What is the hierarchical structure of muscle from largest to smallest component?
A: Muscle → Muscle Fascicles → Muscle Fibers → Myofibrils → Sarcomeres → Filaments
Q: What surrounds muscle fascicles to support their function?
A: Arteries, veins, and nerves for blood supply and control
Q: What defines the boundaries of a sarcomere?
A: Z lines mark the boundaries, with M lines in the center
Q: What is the relationship between myofibrils and sarcomeres?
A: Myofibrils are composed of repeating units called sarcomeres, which are the functional units of contraction
Q: How does muscle movement ultimately occur?
A: Myofibrils shorten during contraction through the sliding of filaments, leading to muscle movement
Q: What is a sarcomere and what defines its boundaries?
A: The basic contractile unit of muscle, extending from one Z disc to the next
Q: What is the function of Z discs?
A: Mark the boundaries of sarcomeres and anchor thin filaments (actin)
Q: What is the I band and where is it located?
A: Contains only thin filaments and is located adjacent to the Z disc
Q: What is the A band?
A: Spans the length of thick filaments and includes areas where thin and thick filaments overlap
Q: What is the H band and where is it found?
A: The central region within the A band that contains only thick filaments (myosin) when the muscle is at rest
Q: What is the M line and its function?
A: Located at the center of the H band, serves as an anchor point for thick filaments
Q: How does the sliding mechanism work in muscle contraction?
A: Myosin pulls on actin, drawing the Z discs closer together, causing contraction
Q: What are zones of overlap?
A: Areas where thick and thin filaments overlap, critical for muscle contraction
Q: What occurs during muscle contraction in the sliding filament mechanism?
A: Myosin heads attach to thin filaments and “walk” toward the M line, pulling thin filaments inward.
Q: How do the sarcomeres change during contraction?
A: Thin filaments slide toward the center, causing the I band and H zone to narrow or disappear.
Q: What happens to the A band during muscle contraction?
A: The width of the A band remains the same, while the lengths of thick and thin filaments do not change.
Q: How do Z discs move during contraction?
A: As thin filaments slide inward, the Z discs move closer together, shortening the sarcomere.
Q: What is the overall effect of the sliding filament mechanism on muscle fibers?
A: The shortening of sarcomeres leads to the shortening of the muscle fiber and the entire muscle.
Q: What is the correct order of muscle components from largest to smallest?
A: 1. Whole muscle 2. Muscle fascicles 3. Muscle fibers 4. Myofibrils 5. Sarcomeres 6. Actin and myosin 7. Myosin heavy chains
Q: What type of tissue surrounds muscle fascicles?
A: Perimysium
Q: What is the smallest functional unit of muscle tissue?
A: The sarcomere
Q: What are the two protective layers of muscle fibers?
A: Endomysium and sarcolemma
Q: What wraps the whole muscle?
A: Epimysium
Q: What are myofibrils composed of?
A: Repeated units called sarcomeres
Q: What is the relationship between actin and myosin?
A: Actin (thin filament) interacts with myosin (thick filament) to facilitate contraction
Q: Which proteins are considered “contractile proteins”?
A: Actin and myosin are the contractile proteins
Q: Which proteins are considered “regulatory proteins”?
A: Troponin and tropomyosin are the regulatory proteins
Q: Which protein binds to calcium?
A: Troponin binds to calcium
Q: What structure is bordered by Z-discs?
A: The sarcomere is bordered by Z-discs
Q: Which protein blocks binding sites on actin?
A: Tropomyosin blocks binding sites on actin
Q: Which protein controls the speed of contraction?
A: Myosin heavy chains control the speed of contraction
Q: What encloses a muscle fiber?
A: The sarcolemma (cell membrane)
Q: What is the role of calcium ions in muscle contraction?
A: Released from sarcoplasmic reticulum, binds to troponin, causing tropomyosin to move and expose binding sites on actin
Q: What are T tubules and their function?
A: Indentations of the sarcolemma that facilitate rapid transmission of contraction signals throughout the muscle fiber
Q: What is the function of myoglobin?
A: Binds to oxygen, providing an oxygen reserve for muscle metabolism
Q: What is the purpose of glycogen granules in muscle fibers?
A: Serve as an energy reserve, utilized during muscle contraction
Q: What are the main energy-producing structures in muscle fibers?
A: Mitochondria, which provide ATP for muscle contractions
Q: What are the main structural components of muscle fibers?
A: Myofibrils containing sarcomeres (made of thick and thin filaments) and Z discs
Q: What initiates muscle contraction?
A: Calcium ions are released into the muscle cell, bind to troponin, which moves tropomyosin to expose binding sites on actin
Q: What happens in the first step of the contraction cycle?
A: Myosin head breaks down ATP, stores energy and gets into a “cocked” position
Q: What occurs during the connection step?
A: The energized myosin head attaches to actin, forming a “cross-bridge”
Q: What happens during the power stroke?
A: Myosin head pivots from 90° to 45°, pulls the thin filament creating muscle force, and releases ADP
Q: How does the contraction cycle end?
A: A new ATP molecule binds, causing the myosin head to release from actin
Q: What is a cross-bridge?
A: The connection formed when an energized myosin head attaches to actin
Q: Is the contraction cycle a one-time event?
A: No, this process repeats many times during muscle contraction
Q: What are the characteristics of slow oxidative fibers?
A: They are darker due to higher myoglobin and mitochondria content, used for endurance, and can work for long periods without fatigue.
Q: How do fast glycolytic fibers differ from slow oxidative fibers?
A: Fast glycolytic fibers are lighter and larger, contain less myoglobin, and produce quick bursts of energy but tire quickly.
Q: What defines fast oxidative-glycolytic fibers?
A: They have properties between slow oxidative and fast glycolytic fibers, suitable for activities that require both speed and endurance.
Q: What is the role of myoglobin in muscle fibers?
A: Myoglobin stores and carries oxygen within muscle cells, facilitating energy production during exercise.
Q: What is the general percentage distribution of muscle fibers in humans?
A: Approximately 50% slow oxidative (SO) and 50% fast fibers.
Q: What are the three types of muscle fibers?
A: Slow oxidative (SO), fast oxidative-glycolytic (FOG), and fast glycolytic (FG) fibers.
Q: What is a characteristic of slow oxidative (SO) fibers?
A: They are good for endurance and use oxygen efficiently; they appear dark red due to high myoglobin.
Q: What is the main energy source for slow oxidative fibers?
A: Aerobic (using oxygen).
Q: What activities are slow oxidative fibers ideal for?
A: Marathon running and maintaining posture; they are highly resistant to fatigue.
Q: What defines fast oxidative-glycolytic (FOG) fibers?
A: They are a mix of speed and endurance, large, dark red, and contract quickly with moderate fatigue resistance.
Q: What is the main energy source for fast oxidative-glycolytic fibers?
A: They can use both aerobic and anaerobic systems.
Q: What activities are fast oxidative-glycolytic fibers good for?
A: Activities like walking and sprinting.
Q: What are the characteristics of fast glycolytic (FG) fibers?
A: Focused on quick bursts of energy, appear white, and tire quickly.
Q: What is the main energy source for fast glycolytic fibers?
A: Anaerobic (without oxygen).
Q: Where are slow oxidative fibers primarily found?
A: In muscles used constantly, like those in the neck, back, and legs.
Q: Which muscles primarily contain fast glycolytic (FG) fibers?
A: Muscles in the shoulders and arms, used for quick, strong actions like lifting and throwing.
Q: Where are fast oxidative-glycolytic (FOG) fibers found?
A: In leg muscles, which need both endurance and strength.
Q: How are motor units activated based on task demands?
A: SO fibers for light tasks, FOG fibers for more force, and FG fibers for maximum effort.
Q: Who controls the activation of muscle fibers during tasks?
A: The brain and spinal cord.
Q: What is a key characteristic of slow oxidative (SO) fibers?
A: High myoglobin content, appears dark red, with many capillaries and mitochondria, and has a smaller diameter.
Q: What defines fast oxidative-glycolytic (FOG) fibers?
A: High myoglobin content (dark red), many capillaries and mitochondria, and an intermediate diameter.
Q: What are the characteristics of fast glycolytic (FG) fibers?
A: Low myoglobin content (white), fewer capillaries, few mitochondria, and a larger diameter.
Q: What determines the mix of muscle fiber types in an individual?
A: It is mostly determined by genetics.
Q: How does the diameter of slow oxidative (SO) fibers compare to fast glycolytic (FG) fibers?
A: SO fibers have a smaller diameter compared to larger FG fibers.
Q: What benefit do the high capillarity and mitochondrial content in SO and FOG fibers provide?
A: They enhance endurance by improving oxygen delivery and energy production.
Q: What are the three main energy systems in muscle fibers?
A: 1. Immediate system (ATP-phosphocreatine) 2. Short-term anaerobic system (glycolysis) 3. Long-term aerobic system (oxygen-dependent)
Q: How long does the immediate energy system last?
A: Seconds (uses stored ATP and phosphocreatine)
Q: How long can the short-term anaerobic system sustain activity?
A: Up to a few minutes
Q: Which energy system is used for extended activities?
A: The long-term aerobic system, which uses oxygen to break down glucose
Q: Why do we continue breathing heavily after exercise?
A: To provide extra oxygen needed to convert lactic acid back to energy and restore ATP levels
Q: What is Creatine Kinase (CK) and its primary role?
A: An enzyme that converts creatine and ATP into phosphocreatine and ADP, helping store and release energy quickly for intense activities.
Q: Which muscle fiber type has the highest amount of CK?
A: Fast Glycolytic (FG) Fibers
Q: Which fiber type has the medium amount of CK?
A: Fast Oxidative-Glycolytic (FOG) Fibers
Q: Which fiber type has the lowest amount of CK?
A: Slow Oxidative (SO) Fibers
Q: How does glycogen storage compare between fiber types?
A: FG fibers have large stores for quick power (like a sports car), SO fibers have less but use it more efficiently (like a hybrid car)
Q: What happens to excess glucose in the body?
A: It’s converted into glycogen and stored in muscle cells and the liver
Q: What is the order of muscle fiber recruitment during activity?
A: 1. SO fibers (low intensity) 2. FOG fibers (moderate intensity) 3. FG fibers (maximum effort)
Q: What was the traditional view of muscle fiber types?
A: Muscle fibers were thought to be strictly one type (either Type I/SO, Type IIa/FOG, or Type IIb/FG)
Q: What are hybrid fibers?
A: Single muscle fibers that can contain characteristics of multiple fiber types
Q: What unique characteristic can hybrid fibers display?
A: Different segments of the same fiber can show properties of both Type I and Type II fibers
Q: What factors can cause muscle fibers to change their properties?
A: Training type, activity level, and environmental demands
Q: How has our understanding of muscle fiber adaptability changed?
A: We now know muscles are more adaptable than previously thought, with fibers able to change their characteristics
Q: Why is the discovery of hybrid fibers significant for athletes?
A: It helps explain why athletes can excel at both power and endurance activities
Q: How many muscle fiber types do healthy, active humans typically have?
A: Only two main types: Type I (Slow Oxidative) and Type IIa (Fast Oxidative-Glycolytic)
Q: Under what conditions do Type IIx/IIb fibers appear?
A: During prolonged periods of inactivity, spaceflight, and spinal cord injury
Q: What is the relationship between Type IIx and Type IIa fibers?
A: Type IIx are essentially a “downgraded” version of Type IIa fibers
Q: What muscle fiber types do cheetahs possess?
A: All types: Type I, IIa, IIx, and IIb
Q: Why do cheetahs need all muscle fiber types?
A: For a combination of speed, power, and endurance necessary for survival and hunting
Q: Do active humans have the Type IIx gene?
A: Yes, but it isn’t actively expressed
Q: What are the two main proteins found in a sarcomere?
A: Actin and Myosin
Q: Which proteins are primarily involved in muscle contraction?
A: Actin and Myosin
Q: What proteins can prevent muscle contraction?
A: Tropomyosin and Troponin
Q: What is the slowest contracting fiber type?
A: Type I fiber
Q: What is the energy currency of a cell?
A: ATP
Q: Which enzyme is involved in muscle contraction?
A: ATPase
Q: What is muscular hypertrophy?
A: Enlargement of existing muscle fibers due to increased production of myofibrils, mitochondria, sarcoplasmic reticulum, and other organelles
Q: What causes muscular hypertrophy?
A: Forceful repetitive muscle activity, strength training, growth hormone during childhood, and testosterone
Q: What are myofibrils?
A: Tiny contractile threads (2 µm in diameter) in muscle fiber sarcoplasm that give muscles their striped appearance
Q: What is the sarcoplasmic reticulum’s role?
A: Stores calcium ions in relaxed muscles and releases them to initiate muscle contraction
Q: What is muscular atrophy?
A: Decrease in size of muscle fibers due to loss of myofibrils
Q: What are the two main types of muscular atrophy?
A: Disuse atrophy and denervation atrophy
Q: What is disuse atrophy?
A: Occurs when muscles aren’t used (e.g., in bedridden patients); is reversible with exercise
Q: What is denervation atrophy?
A: Occurs when nerve supply is cut off; muscles shrink to 1/4 original size; is irreversible and muscle is replaced by connective tissue
Q: What is a triad in muscle structure?
A: A structure formed by terminal cisterns of the sarcoplasmic reticulum connecting with T tubules
Q: What muscle changes occur between ages 30-50?
A: 10% of muscle tissue is replaced by fat and fibrous connective tissue
Q: What muscle changes occur between ages 50-80?
A: An additional 40% of muscle tissue is replaced (total loss up to 50% of original muscle mass)
Q: What is fibrosis?
A: The process where fibroblasts synthesize collagen fibers and extracellular matrix materials to form scar tissue
Q: What are three main consequences of age-related muscle changes?
A: 1. Decreased muscle strength and flexibility 2. Slower reflexes 3. Increase in slow oxidative fibers
Q: How does aging affect muscle fiber composition?
A: More Type I (slow-twitch) fibers and fewer fast-twitch fibers develop
Q: What is sarcopenia?
A: Age-related muscle loss
Q: How do skeletal muscle fibers switch between activity levels?
A: They switch between low activity (relaxed, using little ATP) and high activity (contracting, using ATP quickly).
Q: How long does the ATP available in muscles last?
A: Only a few seconds.
Q: What are the three methods muscles use to generate more ATP?
A:
1. Creatine phosphate (unique to muscles)
2. Anaerobic glycolysis (no oxygen needed)
3. Aerobic respiration (uses oxygen)
Q: What is the primary purpose of ATP in muscles?
A: It is needed for muscle contraction, calcium pumping, and other processes.
Q: What happens to extra ATP in relaxed muscles?
A: It is used to create creatine phosphate, an energy-rich molecule stored in muscles.
Q: What enzyme transfers a phosphate group from ATP to creatine to form creatine phosphate?
A: Creatine Kinase (CK).
Q: How does creatine phosphate provide energy during muscle contraction?
A: Creatine kinase transfers the phosphate from creatine phosphate to ADP, rapidly creating ATP.
Q: How long can ATP and creatine phosphate energy stores sustain maximum effort?
A: About 15 seconds.
Q: What is the top reaction in biologic work?
A: ATP is broken down into ADP and phosphate (P) by ATPase, releasing energy for biological work like muscle contraction.
Q: What is the bottom reaction in biologic work?
A: Creatine phosphate donates its phosphate to ADP to produce ATP, catalyzed by creatine kinase.
Q: What happens if creatine phosphate levels drop during high-intensity exercise?
A: ATP production decreases, limiting muscle performance.
Q: How can creatine supplementation help during intense exercise?
A: It can increase creatine phosphate levels by 20%, enhancing ATP production and improving performance.
Q: What happens when creatine phosphate stores are depleted?
A: Glucose is broken down into pyruvic acid via glycolysis to generate ATP.
Q: Where does glucose come from for muscle energy?
A: From the blood or stored glycogen in muscles.
Q: What happens during glycolysis?
A: Glucose is broken down into two pyruvic acid molecules, producing 2 ATPs, without requiring oxygen (anaerobic process).
Q: What happens to pyruvic acid when oxygen is low?
A: It is converted into lactic acid, which enters the blood.
Q: How does the liver handle lactic acid?
A: The liver converts some lactic acid back into glucose, reducing blood acidity.
Q: What can lactic acid buildup cause?
A: Muscle soreness.
Q: How long can anaerobic glycolysis provide energy?
A: About 2 minutes of intense activity.
Q: How does anaerobic glycolysis compare to aerobic respiration?
A: It is faster but produces less ATP.
Q: What happens to pyruvic acid when oxygen is available?
A: It enters the mitochondria for aerobic respiration, producing ATP, carbon dioxide, water, and heat.
Q: What are the two stages of aerobic respiration?
A: The Krebs cycle and the electron transport chain.
Q: How much ATP does aerobic respiration generate?
A: About 30–32 ATP molecules per glucose molecule.
Q: Where do muscles get oxygen for aerobic respiration?
A: From hemoglobin in the blood and myoglobin in muscle fibers.
Q: How does aerobic respiration compare to anaerobic glycolysis?
A: It is slower but produces much more ATP.
Q: What fuel sources does aerobic respiration use?
A: Pyruvic acid, fatty acids, and amino acids.
Q: What are the byproducts of aerobic respiration?
A: Carbon dioxide (CO₂), water (H₂O), and heat.
Q: How long can aerobic respiration provide energy?
A: For several minutes to hours, depending on the activity.
Q: What is rigor mortis?
A: The stiffening of muscles after death where muscles cannot contract or stretch, due to locked cross-bridges between myosin and actin.
Q: What causes rigor mortis to begin?
A: Cellular membranes become leaky, causing calcium ions to leak from the sarcoplasmic reticulum into the sarcoplasm.
Q: Why can’t muscles relax during rigor mortis?
A: ATP synthesis stops after death, so cross-bridges cannot detach from actin (muscle relaxation requires ATP).
Q: How long do fibers remain contracted in rigor mortis?
A: Until myofilaments decay.
