Exercise Physiology Flashcards: Skeletal Muscle

A comprehensive set of flashcards covering key concepts in exercise physiology related to skeletal muscle.

What is the role of satellite cells in muscle?

They facilitate muscle growth and repair by increasing the number of nuclei in a muscle fiber, enabling greater protein synthesis.

What is a myonuclear domain?

The region of cytoplasm surrounding each nucleus. Each nucleus can support only a limited myonuclear domain.

Why does adding nuclei to a muscle fiber matter for strength training?

More nuclei expand the myonuclear domain, allowing greater protein synthesis and therefore greater adaptation to resistance training.

What is a motor unit?

An alpha motor neuron and all the muscle fibers it innervates. When activated, all fibers in the unit contract together (all-or-none).

What is the innervation ratio?

The number of muscle fibers per motor neuron. Low ratios (e.g., 23:1 in eye muscles) allow fine motor control; high ratios (1,000:1+) are found in large powerful muscles.

Are motor units homogeneous in fiber type?

Yes. One alpha motor neuron innervates only Type I or Type II fibers, not both — and may actually determine the fiber type.

What triggers the release of acetylcholine (ACh) at the neuromuscular junction?

An action potential arriving at the axon terminal triggers ACh release into the synaptic cleft.

What must happen for an action potential to be generated in the muscle fiber?

Enough ACh must bind to receptors on the plasmalemma to threshold the fiber and generate an action potential.

What role do T-tubules play in EC coupling?

They conduct the action potential deep into the muscle fiber, triggering Ca2+ release from the sarcoplasmic reticulum (SR).

How does Ca2+ initiate cross-bridge formation?

Ca2+ binds to troponin, which pulls tropomyosin off the active sites on actin, allowing myosin heads to attach.

What is the state of myosin cross-bridges at rest?

Weak binding state — myosin is energized (ADP+Pi bound) but cannot attach to actin because tropomyosin covers the active sites.

What triggers the power stroke during cross-bridge cycling?

Release of phosphate (Pi) from myosin after strong binding to actin initiates the power stroke.

What does ATP do during cross-bridge cycling?

ATP binding to myosin breaks the cross-bridge (returns to weak binding). ATP is then hydrolyzed to ADP+Pi, re-energizing myosin for the next cycle.

What happens during the power stroke?

The myosin head tilts from 90° back toward 45°, dragging the actin filament toward the center of the sarcomere, shortening it.

How long does cross-bridge cycling continue?

As long as Ca2+ and ATP are both present.

What is the sliding filament theory?

Muscle shortening occurs because actin filaments slide over myosin filaments via cross-bridge formation and power strokes, reducing the distance between Z-lines.

What physically shortens during a muscle contraction?

The sarcomere — the Z-lines are pulled closer together as actin slides over myosin.

How does muscle relaxation occur?

When the AP ends, the SR stops releasing Ca2+ and pumps it back in (requiring ATP). Without Ca2+, tropomyosin re-covers the active sites, halting cross-bridge cycling.

Why does muscle relaxation require ATP?

The Ca2+-ATPase pump uses ATP to actively transport Ca2+ back into the sarcoplasmic reticulum.

What are the three sources of ATP for muscle contraction?

Phosphocreatine (PC) — immediate; Glycolysis — fast, anaerobic; Oxidative phosphorylation — slow, sustained, aerobic.

What enzyme breaks down ATP during contraction, and what does it produce?

Myosin ATPase breaks ATP into ADP + Pi, releasing the energy that powers the power stroke.

What is the difference between concentric and eccentric muscle action?

Concentric: muscle shortens while producing force. Eccentric: muscle lengthens while producing force.

What is an isometric muscle action?

The muscle produces force without changing length — e.g., pushing against an immovable object or maintaining posture.

What four factors determine how much force a muscle produces?

(1) Number of motor units recruited, (2) type of motor units (fast vs. slow), (3) initial muscle length, and (4) frequency of neural stimulation.

What is a muscle twitch?

A brief contraction in response to a single action potential. It has three phases: latent period (~5 ms), contraction (~40 ms), and relaxation (~50 ms).

What is summation?

When repeated stimuli arrive before full relaxation, twitches add together, producing greater force than a single twitch.

What is tetanus?

A sustained, fused contraction produced by very high-frequency stimulation where individual twitches can no longer be distinguished.

What is the length-tension relationship?

There is an optimal sarcomere length at which maximum cross-bridge overlap — and therefore maximum force — can be generated.

How does velocity relate to force in the force-velocity relationship?

Maximum shortening velocity is greatest at the lowest force load. As force increases, velocity decreases.

How does eccentric action compare to concentric in force production?

Eccentric (lengthening) actions generate more force than concentric (shortening) actions at any given velocity.

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What are the three skeletal muscle fiber types and their alternate names?

Type I (Slow, S) — fatigue-resistant, low force; Type IIa (Fast Fatigue Resistant, FR) — intermediate; Type IIx (Fast Fatigable, FF) — highest force and power.

Why do fast fibers produce more force than slow fibers?

Fast fibers have more myosin cross-bridges per cross-sectional area, allowing more simultaneous cross-bridge formation.

How does power compare across fiber types?

Peak power: Type IIx > Type IIa > Type I. All fiber types reach peak power at approximately the same percentage of peak force.

What is the size principle of motor unit recruitment?

Motor units are always recruited from smallest to largest as force demand increases: Type I then Type IIa then Type IIx.

Why are small motor units recruited first?

Small alpha motor neurons produce larger EPSPs relative to their threshold, so they reach action potential faster

Is the order of motor unit recruitment random or fixed?

Fixed — the same recruitment order (Type I, then IIa, then IIx) is used every time, regardless of the task.

What is the all-or-none principle of a motor unit?

When a motor unit is activated, all muscle fibers within it contract maximally — there is no partial activation.

What is the difference between transient and chronic hypertrophy?

Transient (the pump): fluid accumulation during/after exercise. Chronic: long-term structural increase in muscle size from resistance training.

What two mechanisms drive chronic hypertrophy?

Fiber hypertrophy (increase in fiber size — more actin/myosin filaments) and fiber hyperplasia (increase in number of fibers).

How does fiber hypertrophy occur at the molecular level?

Net increase in protein synthesis adds more actin and myosin filaments, creating more myofibrils and increasing cross-sectional area.

When does DOMS appear and what causes it?

24-48 hours after exercise. Caused by microscopic tears in muscle/connective tissue, leading to cellular degradation and inflammation — not lactic acid.

What type of muscle action is most associated with DOMS?

Eccentric (muscle-lengthening) actions, such as downhill running or the lowering phase of a lift.

What is the repeated bout effect?

After one bout of exercise that causes DOMS, the next identical bout causes significantly less damage and soreness.

What are the three theories explaining the repeated bout effect?

Neural (more fibers recruited, reducing load per fiber), connective tissue (increased costameric proteins), cellular (synthesis of protective proteins).

How does DOMS affect performance?

It temporarily reduces maximal force-generating capacity due to physical disruption, EC coupling failure, loss of contractile proteins, and insulin resistance.

What is rhabdomyolysis and why is it dangerous?

Acute skeletal muscle breakdown from extreme exercise. Dangerous because myoglobin released into the blood can clog the kidneys, causing renal failure.

What is the hallmark urine sign of rhabdomyolysis?

Dark or cola-colored urine, caused by myoglobin (Mb) released from destroyed muscle cells.

What actually causes exercise-associated muscle cramps?

Excessive motor neuron firing in the spinal cord — not dehydration or electrolyte imbalance. Altered muscle spindle and Golgi tendon organ function increases excitatory activity.

What is sarcopenia?

Age-related loss of muscle mass. Roughly 10% is lost between ages 25-50, and an additional 40% between ages 50-80.

Can resistance training reverse muscle atrophy from inactivity?

Yes — atrophy from bed rest, immobilization, or spaceflight is reversible with resistance training. However, age-related sarcopenia cannot be completely eliminated, only slowed.