Muscles

Question 1

List Functions of Skeletal Muscles (6)

Answer 1

1. Produce skeletal movement
2. Maintain posture
3. Support soft tissue (think abdominal muscles)
4. Guard openings (sphincter muscles / mouth)
5. Maintain body temperature (waste heat)
6. Store nutrient reserves (proteins -> ketone bodies)

Question 2

List Properties of Skeletal Muscles (4)

Answer 2

1. Electrical excitable - respond to electrical signals
2. Contractile
3. Extensibility: can be stretched without being damaged
4. Elasticity: return to original shape.

Question 3

List tissue types of skeletal muscles

Answer 3

1. Connective Tissues
   - Epimysium: Dense irregular connective tissue that surrounds the entire muscle.
   - Perimysium: Dense irregular connective tissue that surrounds bundles of muscle fibers (fascicles).
   - Endomysium: Loose areolar connective tissue that surrounds individual muscle fibers.
   - Deep fascia: Dense irregular connective tissue located external to the epimysium, separating muscles and groups of muscles from one another.
2. nerves - skeletal muscles are voluntary, controlled by central nervous system.
3. Blood vessels - supply oxygen, nutrients, and carry away waste
4. muscle tissue

Question 4

Describe skeletal muscle formation and shape

Answer 4

Skeletal muscle is:
- striated
- multi-nucleated (allows for efficient control through length of muscle)
- very long and cylindrical

Skeletal muscle fibers are formed:
- through the fusion of mesodermal stem cells, called myoblasts

Question 5

Describe Muscle Organization

Answer 5

• Muscles are surround by a dense irregular connective tissue layer called epimysium
• Within muscles there are many muscle fascicles, bundles of muscle fibers (muscle cells) surrounded by perimysium.
• Each muscle cell is surrounded by endomysium and contains many myofibrils, nucleuses, and mitochondria.

The endomysium, perimysium, and epimysium come together at ends of muscles to form tendons or aponeurosis, connecting muscle to bone.

aponeurosis -> CT sheet

Question 6

Describe Components of Muscle Fiber / Muscle Cell

Answer 6

• sarcolemma: Muscle cell plasma membrane
• sarcoplasm: Muscle cell cytoplasm
• sarcoplasmic reticulum: specialized smooth endoplasmic reticulum. Stores calcium and regulates calcium release. Sarcoplasmic reticulum surrounds each myofibril.
• terminal cisterna: Enlarged areas of the SR with large amounts of calcium. Are on either side of t-tubules. Release of calcium from cisternae triggers muscle contractions.
• T-tubules: invaginations of the sarcolemma which penetrate into the interior of the muscle fiber surrounding each sarcoplasmic reticulum. Transmit action potential through the cell allowing simultaneous contraction of entire cell.
• Triad: Two terminal cisterna surrounding a t-tubule.
• myofibril: Contractile component, composed of bundles of myofilaments. There are thin and thick filaments.
• mitochondria: energy producers.
• multiple nuclei

Question 7

Draw / Describe structure of Sarcomeres

Answer 7

• contractile unit of muscle, structural unit of myofibrils.

Thick filaments: Composed primarily of myosin protein, these filaments are responsible for muscle contraction through their interaction with actin.

Thin filaments: Made mainly of actin, along with troponin and tropomyosin, these filaments slide past thick filaments during contraction.

A band: The region of the sarcomere that contains thick filaments, including areas where they overlap with thin filaments.

I band: The lighter region that contains only thin filaments and is divided by the Z disc

Zone of overlap: The area within the A band where thick and thin filaments overlap, crucial for muscle contraction.

H band: The central part of the A band where only thick filaments are present, without any overlapping thin filaments.

M line: The middle line of the sarcomere that anchors and aligns the central part of the thick filaments.

Z disc / z line: The boundary structure of the sarcomere that anchors the thin filaments and connects adjacent sarcomeres.

dArk = A bands
lIght = I bands

Question 8

Describe structure and function of thin filaments

Answer 8

• F-actin: A polymerized string of G-actin. Active sites on G-actin bind to myosin
• Tropomyosin: A regulatory protein that wraps around actin filaments, blocking myosin binding sites on G-actin when the muscle is relaxed.
• Troponin: A complex of three proteins that binds to tropomyosin and associates with actin. When calcium binds to troponin, it causes a conformational change, shifting tropomyosin, exposing myosin-binding sites on G-actin and enabling muscle contraction

Question 9

Describe the structure of thick filaments

Answer 9

• primarily composed of myosin (roughly 300) which has a long tail and two globular heads. The tails intertwine to form the filament’s backbone.
• Each myosin head has binding sites for actin (on thin filaments) and ATP. The heads use ATP hydrolysis to pivot and pull thin filaments during contraction.
• Titin, an elastic protein, anchors the thick filaments to the Z-disc, contributing to muscle elasticity and alignment

Question 10

Describe changes that occur in sarcomere during muscle contraction

Answer 10

• Calcium Ions Released: Calcium ions are released from the sarcoplasmic reticulum into the sarcoplasm.
• Troponin Binds Calcium: Calcium ions bind to troponin, causing a conformational change.
• Tropomyosin Moves: The conformational change in troponin shifts tropomyosin away from the myosin-binding sites on actin filaments.
• Myosin Binding Sites Exposed: With tropomyosin moved, the myosin-binding sites on actin are exposed.
• Cross-Bridge Formation: Myosin heads bind to the exposed sites on actin, forming cross-bridges.
• Power Stroke: Using energy from ATP hydrolysis, the myosin heads pivot, pulling the actin filaments toward the center of the sarcomere.
• Z Discs Move Closer: The Z discs at each end of the sarcomere move closer together.
• I Bands Shorten: The I bands, which contain only thin filaments, shorten.
• H Zone Narrows: The H zone, where only thick filaments are present, becomes narrower.
• A Band Stays the Same: The A band, the length of the thick filaments, remains unchanged.
• Zone of Overlap Increases: The overlap between thick and thin filaments

Question 11

Define action potential. Describe action potential in muscles

Answer 11

A rapid depolarization and repolarization of the membrane potential that propagates along the axon of a neuron and along a muscle fiber.

• resting membrane potential is between -70 to -90 volts. The sodium potassium ATPase maintains this potential by moving two potassium (K+) ions into the cell and and three sodium (Na+) ions out. It is more negative directly bellow cell membrane surface. There is a higher concentration of Na+ out of the cell (wants to come in) and a higher concentration of K+ in the cell (wants to leave)

1. AP reaches synaptic terminal, triggering release of ACh.
2. ACh binds nicotinic acetylcholine receptors, a ligand gated sodium channel.
3. sodium enters the cell, causing initial depolarization.
4. initial depolarization opens voltage gated sodium channels (around -55mV). More sodium enters the cell, membrane, membrane potential increases to around +30 V.
5. AP propagates along sarcolemma (diffusion), and deep into the muscles via T-tubules .
6. Depolarization along T-tubules opens voltage gated calcium ion channels in the cisternae of the SR, triggering the release of calcium into the sarcoplasm.
7. Ca+ release causes contraction. Allows myosin to bind to action and the muscle to contract.
8. Maximum depolarization (+30mV) causes the opening of voltage gated potassium channels, potassium exits the cell, depolarizing the membrane (often overshoots a bit). Around the same time time-dependent inactivation gates close the sodium channels (gates or “reset” when membrane reaches resting potential).

excitation-contraction coupling: Link between electrical signals and muscle contraction

With rapid or continuous stimulation (such as during tetanus), the action potentials occur in quick succession, but the basic sequence of Na⁺ channel activation, inactivation, and K⁺ channel opening still occurs. The channels cycle through their states more frequently, but they do not bypass the inactivation and opening processes. This is why there’s a limit to how rapidly action potentials can occur, defined by the refractory period.

Question 12

Define synaptic terminal and cleft and motor end plate

Answer 12

synaptic terminal: The end of a neuron where neurotransmitters are stored and released into the synaptic cleft.

synaptic cleft: The space between the synaptic terminal of a neuron and the target cell where neurotransmitters are released.

motor end plate: The region of the muscle fiber membrane directly opposite the synaptic terminal of a motor neuron, contains acetylcholine receptors.

Question 13

Define Acetylcholine, nicotinic acetylcholine receptors, and acetylcholine esterase

Answer 13

acetylcholine: A neurotransmitter. In muscle contractions it is released at the the synaptic terminal and binds nicotinic ACh receptors in the motor end plate.

nicotinic acetylcholine receptors: ligand gated sodium channels in motor end plate. Open when bound to ACh causing initial depolarization of membrane.

acetylcholine esterase: An enzyme located in the synaptic cleft that breaks down acetylcholine into acetate and choline, terminating the signal at the neuromuscular junction and allowing the muscle to relax.

Question 14

Describe the contraction cycle

Answer 14

1. At resting, myosin heads are in their cocked position and bound to ADP and P. The tropomyosin / troponin complex is blocking the myosin binding sites on G-actin.
2. Calcium binds troponin. Causing a conformational change which moves the tropomyosin, exposing G-actin binding sites.
3. The myosin head binds the G-actin binding site, creating a cross bridge.
4. Myosin releases the bound ADP + P. This causes the myosin head to pivot, pulling the actin filament towards the center of the sarcomere (power stroke)
5. Myosin then binds ATP causing detachment from the G-actin myosin binding site.
6. Myosin cleaves ATP to ADP + P and returns to the resting cocked position.

Question 15

What factors influence contraction duration

Answer 15

1. Duration of the neural stimulus
2. Number of free calcium ions in the sarcoplasm
3. ATP availability

Question 16

What is the all-or-none principle of muscle fiber contraction

Answer 16

When a muscle fiber is stimulated to the threshold level, it will contract fully (engage in contraction). If that threshold in not reached, no contraction will occur.

Of note the muscle tension depends on how many cross-bridge cycles occur.

Question 17

What factors influence the amount of tension in a single muscle fiber?

Answer 17

The number of piviting cross-bridges formed.

1. The fibers’ resting length at time of stimulation
2. The frequency of the stimulus

With a single contraction (twitch) ~20-40% of myosin heads form cross-bridges. At tetanus 70-80%.

Question 18

What is the length-tension relationship?

Answer 18

• Maximum tension is can be generated at optimal sarcomere length. This allows for maximal overlap between actin and myosin -> the maximum number of active cross-bridges at a given instant can be formed. If the sarcomere is too short, overlap is excessive, reducing cross-bridge formation and tension potential. If the sarcomere is too stretched, there is insufficient overlap for cross-bridge formation.
• Normal sarcomere length is between 75-130% optimal.
• Tension directly proportional to the number of active cross-bridges at a given time.

For example the optimal position for the bicep is around 90 degrees, when the fore-arm is closer to the shoulder there is excessive overlap and when the forearm is extended passed 90, there is insufficient overlap. In theory we can produce the maximum tension in our biceps at 90 degrees

Question 19

What is a twitch, what are the phases?

Answer 19

twitch: a single muscle contraction produced by a single neural stimulation (only happens in a lab)

Phases:
1. Latent period: Action potential moves through sarcolemma, triggers Ca2+ release
2. Contraction phase: Calcium ions bind, muscle tension builds
3. relaxation phase: Ca2+ is taken back up by SR, contraction ends, tension falls to resting levels.

Question 20

Describe four types of muscle tension

Answer 20

1. Treppe: occurs when a muscle is stimulated repeatedly with identical stimuli, and each successive contraction produces slightly more tension than the previous one, even when the muscle is allowed to fully relax between contractions. This increase in tension continues until it reaches a plateau. The mechanisms behind treppe include a gradual accumulation of calcium ions in the muscle fibers, which allows more cross-bridges to form in subsequent contractions
2. Wave summation: occurs when successive action potentials arrive before the muscle has fully relaxed from the previous contraction. Each subsequent action potential causes more calcium to be released into the cytoplasm, increasing the number of cross-bridges that can form and thereby increasing tension. As a result, the tension produced by each successive contraction is greater than the tension produced by the previous one, leading to a progressively stronger contraction if the stimuli continue.
3. Incomplete tetanus: muscle stimulated by a series of action potentials at a frequency high enough that individual twitches begin to sum together, increasing tension, but not so high that the muscle has no time to relax between stimuli. In this state, the muscle fibers partially relax between stimuli, leading to a wavering or oscillating tension that is not maximal but higher than what is produced by individual twitches
4. Complete tetanus: occurs when the frequency of stimulation is so high that the muscle does not have time to relax at all between stimuli. As a result, the twitches fully merge, producing a smooth, sustained contraction with maximal tension. In this state, the muscle remains in a constant state of contraction, with no observable relaxation phase, leading to the highest possible tension that the muscle fiber can produce

Question 21

How do you increase whole skeletal muscle tension

Answer 21

1. The more motor units recruited the greater the tension (motor unit = all muscle fibers innervated by a neuron).
2. The greater the frequency of stimulation the more cross-bridges formed. As the frequency increases, individual twitches begin to summate, leading to incomplete tetanus. If the frequency is high enough, the muscle reaches complete tetanus, where cross-bridge formation is maximized, and the muscle generates peak tension with no relaxation phases in between.

Question 22

Describe Maximum Tension vs Sustained Tension and Muscle Tone

Answer 22

Maximum tension Achieved when all motor units are recruited, are at complete tetanus, and optimal length. This level of contraction can only be maintained for a very short period of time.

Sustained Tension: A whole skeletal muscle achieves sustained tension by cycling the activation of different motor units in an asynchronous manner. While some motor units contract, others rest and recover. This rotation allows the muscle to maintain a steady level of tension over time without leading to rapid fatigue, as not all motor units are active simultaneously

Muscle Tone: Muscle tone is the continuous and passive partial contraction of muscles,which maintains the normal tension and firmness of a muscle at rest. It is maintained by the nervous system’s constant, low-level activation of a small number of motor units, which keeps muscles firm and ready for action while also stabilizing joints and maintaining posture.

Question 23

What are the three primary sources of ATP for skeletal muscles

Answer 23

1. Aerobic respiration: Complete oxidation of glucose in the presence of oxygen in the ETC of the mitochondria. Primary source of ATP at rest (oxidation of fatty acids) and during moderate exercise (oxidation of glycogen). Produces 34 ATP per glucose
2. Anaerobic glycolysis: Break down of glucose (normally from glycogen) into pyruvate without oxygen. Primary energy source for peak muscular activity. Only produces 2 ATP per glucose, lactic acid produced as byproduct.
3. Creatine phosphate: Provides a rapid source of ATP. Directly donates a phosphate to ADP. Only lasts for about the first 15 seconds of activity.

Question 24

What are the four different types of muscle contraction

Answer 24

1. Isotonic: When skeletal muscle changes in length, it results in motion.
2. Concentric: When the muscle shortens.
3. Eccentric: When the muscle lengthens.
4. Isometric: When skeletal muscle develops tension but length does not change.

Question 25

Describe muscle metabolism at rest, during light activy and during maximum activity.

Answer 25

1. At rest, the demand for ATP is low and oxygen is plentiful. Fatty acids are metabolized. Surplus ATP is used to rebuild glycogen stores from glucose, and convert creatine to creatine phosphate.
2. During moderate activity ATP can still be produced aerobically but the primary source is glucose stored as glycogen (or other substrates such as fatty acids). All ATP generated goes to power contraction.
3. During max activity, the demand for ATP outpaces aerobic respiration, there is not sufficient oxygen. Only ⅓ of energy needs can be met by mitochondria. The rest of the ATP comes from glycolysis. This causes a build up of lactic acid and hydrogen ions. The hydrogen buildup increases cytosol acidity which will eventually inhibit muscle contraction.

Question 26

What is the Cori Cycle

Answer 26

The Cori cycle is the removal and recycling of lactic acid by the liver, converting lactic acid back to glucose. This glucose is released in the blood and can be converted back to glycogen in the muscle.

Question 27

What is oxygen debt

Answer 27

The amount of oxygen that the body needs to recover after intense exercise. Returns the body to homeostasis. Results in heavy breathing.

Question 28

What are the three types of muscle fibers.

Answer 28

1. Fast fibers: Contract strongly and quickly, but fatigue rapidly. They are large in diameter, have large glycogen reserves and fewer mitochondria.
2. Slow fibers: Are slower to contract but also slower to fatigue. They are smaller in diameter, have more mitochondria, and more oxygen (stored in red myoglobin).
3. Intermediate fibers: Between slow and fast fibers.

Pale muscle is mostly fast fibers, red muscle, mainly slow (more mitochondria and blood).

Question 29

What is muscle hypertrophy and muscle atrophy

Answer 29

Muscle hypertrophy is the increase in muscle size due to the growth of muscle fibers, typically resulting from resistance training or other forms of exercise that stress the muscles. It involves an increase in the cross-sectional area of muscle fibers due to enhanced protein synthesis and the addition of myofibrils. There will also be an increase in mitochondria and glycogen reserves.

Muscle atrophy is the decrease in muscle size due to the reduction in muscle fiber size, often caused by disuse, aging, or certain medical conditions. It involves a loss of muscle mass and strength as a result of the breakdown of muscle proteins.

inactivity -> muscle atrophy -> decrease in glycogen -> loss of muscle tone -> loss of water (less glycogen and creatine)

Question 30

What is an agonist, antagonist, and synergist

Answer 30

• agonist/prime mover: produces the target movement
• Antagonist: opposes the target movement
• Synergist: Assists the target movement.

when an agonist contracts the antagonist relaxes -> flexors vs extensor, abductors vs adductors

Question 31

Features of cardiac muscle

Answer 31

• Cardiac muscles are smaller, short and branched.
• have one nucleus
• have shorter wider t-tubules
• lack triads (SR do not have terminal cisternae). Instead SR tubules contact T tubules at the z lines
• are aerobic (high in myoglobin and mitochondria)
• have intercalated discs: specialized structures found between adjacent cardiac muscle cells consist of gap junctions (allow direct transmission of electrical signals), and desmosomes.

intercalated discs link heart cells mechanically, chemically, and electrically allowing the heart to function like a single, fused mass of cells

Question 32

What is muscle origin and insertion

Answer 32

origin: fixed point of attachment.
insertion: moving point of attachment.

origin is usually proximal to insertion

Question 33

Five functions of cardiac muscle

Answer 33

1. Automaticity: Contraction without neural stimulation. Controlled by pacemaker cells. Neural control limited to moderating pace and tension.
2. Variable contraction tension: regulated by the nervous system and circulating hormones. Influences the strength of contraction by adjusting calcium ion availability within the cells. This modulation allows the heart to increase or decrease its force of contraction based on physiological demands, such as during exercise or rest.
3. Extended contraction time: Contractions are about ten times as long as those of skeletal muscle.
4. Prevention of wave summation: After each contraction there is a long refractory period which ensures that each contraction is followed by full relaxation. Cardiac muscle never reaches full tetanus.
5. aerobic metabolism, usually lipid or carbohydrate substrates.

Question 34

Where is smooth muscle found

Answer 34

Smooth muscle is found around other tissues.

• Around blood vessels it regulates blood pressure and blood flow.
• In the digestive and urinary system it forms sphincters and produces coordinated contractions to facilitate the movement of substances.
• In the integumentary system it forms the arrector pili muscles.

Question 35

Features of smooth muscle

Answer 35

• Long and slender
• Has a single nucleus
• Does not have t tubules, myofibrils, nor sarcomeres.
• Is not associated with tendons nor aponeuroses.
• Has scattered myosin fibers with a higher concentration of heads.
• Thin filaments are attached to dense bodies.
• Dense bodies transmit contractions from cell to cell.
• primarily aerobic metabilism.
• slow onset to contractions, may be tetanized, are resistant to fatigue.

Smooth muscle can function at a wide variety of lengths. Are controlled by neurons directly or indirectly (gap junctions), hormones, chemicals, or pacesetter cells

Question 36

Compare cardiac to skeletal to smooth.

Answer 36

Question 37

List muscles of head and functions (9)

Answer 37

1. occipitofrontalis (frontal belly and occipital belly): The frontal belly is located on the forehead, while the occipital belly is at the back of the head. The frontal belly raises the eyebrows and wrinkles the forehead, while the occipital belly retracts the scalp. Connected by the epicranial aponeurosis
2. orbicularis oculi: Surrounds the eye socket. Closes the eyelids; involved in blinking, squinting, and winking.
3. temporalis: Covers the temporal bone on the side of the skull. Elevates and retracts the mandible, aiding in chewing.
4. masseter: Located at the angle of the jaw, covering the lateral aspect of the mandible. Elevates the mandible, playing a key role in chewing.
5. Buccinator: Located in the cheek, between the maxilla and mandible. Compresses the cheek, as in sucking or blowing; helps in holding food between the teeth during chewing.
6. Orbicularis Oris: Surrounds the mouth. Closes and puckers the lips, as in kissing or speaking.
7. Zygomaticus major: Extends from the zygomatic bone (cheekbone) to the corner of the mouth. Raises the corners of the mouth, contributing to smiling.
8. zygomaticus minor: Extends from the zygomatic bone (cheekbone) to the upper lip, positioned just above the zygomaticus major. Elevates the upper lip, contributing to facial expressions like smiling or showing the upper teeth.
9. Risorius: Located at the sides of the mouth, extending horizontally across the cheek. Draws the corner of the mouth laterally, as in expressions of grinning or smirking.

Question 38

List Muscles of the neck / back and functions (10)

Answer 38

1. sternocleidomastoid: Extends from the sternum and clavicle to the mastoid process of the temporal bone, on the side of the neck. Unilateral action laterally flexes (brings head to shoulders) or rotates the neck, bilateral action flexes the neck forward (nodding).
2. scalenes anterior / middle / posterior: Located on the lateral aspect of the neck, extending from the cervical vertebrae to the first and second ribs. Elevate the first and second ribs during inspiration, lateral flexion of the neck (head to shoulder).
3. Levator Scapulae: Extends from the transverse processes of the upper cervical vertebrae to the superior angle of the scapula. Elevates the scapula aids in lateral neck flexion (head to shoulder)
4. Trapezius: Extends from the occipital bone and spinous processes of the thoracic vertebrae to the clavicle, acromion, and spine of the scapula. Extends / hyperextends neck and laterally flexes neck. Depresses / elevates shoulder, retracts / adducts scapula, rotates scapula (allows abduction beyond 90)
5. Rhomboid Major: Extends from the spinous processes of T2-T5 to the medial border of the scapula. Retracts (adducts scapula) and performs downward rotation.
6. Rhomboid Minor: : Extends from the spinous processes of C7-T1 to the medial border of the scapula, near the spine. Adducts scapula, retracting shoulder and performs downward rotation.
7. Serratus Anterior: Extends from the lateral surface of the first to eighth ribs to the anterior surface of the medial border of the scapula. Protracts the scapula (brings shoulder forward), aids in upward rotation of the scapula.
8. Erector Spinae longissimus: Runs along the length of the spine, from the sacrum to the base of the skull. Longissimus is the intermediate column of the erector spinae. Spine extension and hyperextension, lateral flexion of spine.
9. erector spinae iliocostalis: Runs from the iliac crest and sacrum to the ribs and cervical vertebrae. Iliocostalis is the most lateral column of the erector spinae. Primarily in involved in lateral flexion. Also involved in spinal extension and hyperextension.
10. erector spinae spinalis: The most medial group, runs adjacent to the vertebral column from the upper lumbar region to the skull. Spinalis is the most medial column, running closest to the vertebral column. Spinal extension, thoracic lateral flexion.
11. Latissimus Dorsi: Large muscle of mid back. Shoulder retraction (scaupla adduction), Shoulder depression, arm medial rotation (rolling shoulder forward), shoulder adduction (adducting arm when over head), shoulder extension, hyperextension (bringing arm back (tricep pulls)).

Question 39

List Muscles of the Anterior Trunk

Answer 39

1. Diaphragm: Separates the thoracic cavity from the abdominal cavity, located below the lungs. Primary muscle of respiration; contracts to increase thoracic volume for inhalation.
2. External intercostals: Between the ribs, running diagonally from the rib above to the rib below. Elevate the ribs during inhalation, expanding the thoracic cavity.
3. Internal intercostals: Between the ribs, deep to the external intercostals, running perpendicular to them. Depress the ribs during forced exhalation, reducing thoracic volume.
4. Pectoralis major: Covers the upper chest, spanning from the clavicle and sternum to the humerus. Transvers abduction of shoulder (bringing arm forward (peck fly)), medial rotation of humerus, shoulder flexion (raising the arm), shoulder depression, shoulder abduction (bringing arm back down)
5. Pectoralis Minor: Deep to the pectoralis major, extending from the ribs to the coracoid process of the scapula. Protracts and depresses the shoulder.
6. Rectus Abdominis: Runs vertically along the front of the abdomen, from the pubic bone to the sternum. Flexes the vertebral column and compresses abdominal contents.
7. External Oblique: Lateral sides of the abdomen, running diagonally from the lower ribs to the iliac crest. Aids in trunk flexion, trunk rotation, trunk lateral flexion compresses abdominal contents.
8. transverse abdominis: Deepest abdominal muscle, running horizontally around the abdomen.Trunk lateral flexion, trunk flexion, trunk rotation

Question 40

Sarcomeres durring isometric contraction

Answer 40

Cross-Bridges Form and Cycle: Myosin heads bind to actin, pivot (power stroke), and generate force, but instead of causing the sarcomere to shorten significantly, the force they generate is balanced by the load you’re holding (like the weight in your hand).

No Net Sarcomere Shortening: The sarcomeres in your bicep muscle do not shorten overall because the force produced by the cross-bridges is countered by the external load. The myosin heads continue to cycle, attaching, pulling, and releasing, but the muscle length stays constant because the external force is equal to the internal force.

Tension Without Movement: The tension generated by the cycling of cross-bridges maintains the position of your arm. The muscle fibers are in a dynamic equilibrium where cross-bridges are constantly forming and breaking, but there is no overall movement because the forces are balanced.