Types of skeletal muscle fibers and Exercise physiology + pathologies

What is the speed characteristic of myosin ATPase in Slow Oxidative (SO) fibers?

Slow myosin ATPase

How would you describe the blood supply to Slow Oxidative (SO) fibers?

Plentiful blood supply

What are the levels of myoglobin found in Slow Oxidative (SO) fibers?

High levels of myoglobin

How does the diameter of a Slow Oxidative (SO) fiber compare to other fiber types?

It has a smaller diameter

How many mitochondria are present in Slow Oxidative (SO) fibers?

Many mitochondria

What is the primary metabolic pathway used by Slow Oxidative (SO) fibers for energy?

Aerobic respiration

What specific fuel source do Slow Oxidative (SO) fibers primarily use?

Fatty acids

How resistant to fatigue are Slow Oxidative (SO) fibers?

They are resistant to fatigue

What is the speed characteristic of myosin ATPase in Fast Glycolytic (FG) fibers?

Fast myosin ATPase

What kind of internal fuel stores are found inside Fast Glycolytic (FG) fibers?

Glycogen stores

How much myoglobin is present in Fast Glycolytic (FG) fibers?

Little myoglobin

How does the diameter of a Fast Glycolytic (FG) fiber compare to other fiber types?

It has a larger diameter

How many mitochondria are found in Fast Glycolytic (FG) fibers?

Fewer mitochondria

What system do Fast Glycolytic (FG) fibers use initially for energy production?

Initially aerobic respiration

After the initial phase, what system do Fast Glycolytic (FG) fibers quickly move to for energy?

Anaerobic respiration

What specific fuel source do Fast Glycolytic (FG) fibers primarily use?

Glycogen stores

What type of contraction is produced by Fast Glycolytic (FG) fibers?

Rapid, forceful contraction

How quickly do Fast Glycolytic (FG) fibers fatigue?

They fatigue more quickly

Structurally, how do the characteristics of Fast Oxidative (FO) fibers compare to SO and FG fibers?

They are similar to SO and similar to FG (intermediate between SO and FG)

What type of contractions do Fast Oxidative (FO) fibers produce?

Strong, repetitive contractions

How fatigue-resistant are Fast Oxidative (FO) fibers?

They are moderately fatigue resistant

Does a gross skeletal muscle contain only one twitch type or both?

Gross muscle contains both fast-twitch and slow-twitch fibers

What factor determines the specific ratio of fiber types within a given muscle?

muscle use

What type of muscle fiber is predominantly found in the biceps brachii?

Mostly fast twitch

What type of muscle fiber is predominantly found in the soleus (antigravity) muscle?

Mostly slow twitch

Besides muscle use, what biological factor predetermines the ratio of fiber types?

genetics

Can the ratio of muscle fiber types change over time?

Yes, the ratio can change with conditioning

How many different muscle fiber types are found within a single motor unit?

Each motor unit contains a single muscle fiber type

3 types of skeletal muscle fibers

slow oxidative, fast glycolytic, fast oxidative

what meat is there a high concentration of slow oxidative fibers

dark meat

what is myoglobin

Protein that stores oxygen in muscle cells.

what meat is there a high concentration of fast glycolytic fibers

white meat

the types of skeletal muscle fibers in order, from least resistant to fatigue, to most resistant to fatigue?

fast glycolytic, fast oxidative, slow oxidative

Which type of muscle fiber has the greatest level of myoglobin?

slow oxidative

Each gross muscle contains multiple __________, but each motor unit contains ____________

fiber types, only one type of fiber

fast twitch

type of muscle that contracts quickly

slow twitch

type of muscle that contracts slowly and is fatigue resistant

What happens to your creatine phosphate (CP) and glycogen reserves when you're working out?

They get used up super fast / rapidly depleted.

Does "oxygen debt" mean you are actually running out of oxygen in your blood?

Nope. Your blood and body \(O_{2}\) levels stay normal; the "debt" just means your muscles want more than your heart and lungs can physically deliver.

Since your heart and lungs can't keep up with the intense oxygen demand during heavy exercise, what pathway does your body start leaning on?

Glycolysis.

What does EPOC stand for?

Excess Postexercise Oxygen Consumption (basically, why you keep heavy breathing after a workout).

Why does EPOC even happen after you stop moving?

Because during the workout, your muscles' demand for oxygen totally out-stripped what your body could supply right then.

How do well-trained athletes handle oxygen debt compared to normal people?

They are much more efficient, so they rack up less of an oxygen debt.

Why does your metabolism stay cranked up high even after you finish exercising?

Your body is working hard to restore homeostasis. It has to cool you down (heat dissipation), balance out your cell ions, and fix your blood pH.

What three specific energy stores inside your skeletal muscles need to be refilled during recovery?

Creatine Phosphate (CP)

Glycogen

Oxygen (stored in myoglobin)

Where does all that leftover lactic acid go after a workout, and what happens to it?

It hitches a ride to the liver, which has the special enzymes needed to turn it back into glucose and dump it back into your blood.

What's the main vibe of endurance training when it comes to motor units and force?

You're firing your motor units a ton (high frequency), but only producing a moderate amount of force.

What 4 long-term upgrades happen to your muscle fibers from doing endurance training?

You don't tire out as fast (fatigue resistance)

More capillaries (better blood flow)

More mitochondria (more energy factories)

More myoglobin (more oxygen storage

Endurance training changes all your muscle fibers, but which specific type gets the biggest upgrade?

SO (Slow-Oxidative) fibers

What's the main vibe of resistance training when it comes to motor units and force?

You're firing motor units at a moderate frequency, but you are cranking out a massive amount of force.

What 3 structural changes actually make your muscles stronger from resistance training?

Bigger muscle fibers (hypertrophy)

More mitochondria

Tougher, thicker connective tissue

If your heart/lungs can't keep up with your muscles during a hard workout, why don't your blood oxygen levels drop?

Your cardiovascular system pumps blood like crazy to keep systemic arterial blood fully saturated. The oxygen shortage is purely local—inside the working muscle tissue itself—which is why the muscle has to switch to anaerobic glycolysis.

How does shipping lactic acid to the liver protect your body during recovery?

Your liver spends energy to turn that lactate back into glucose. This prevents your blood from getting too acidic (metabolic acidosis) and helps rebuild your fuel stores.

Why does endurance training build up things like capillaries and myoglobin, while resistance training builds up fiber size and connective tissue?

Endurance is all about aerobic delivery—getting oxygen into the cell to fight fatigue.

Resistance is all about handling heavy loads—making fibers physically wider to generate tension and beefing up connective tissue so your tendons don't snap under the stress.

Why does "cooling down" after a workout keep your metabolic rate high?

Muscle contractions are inefficient, throwing off a ton of waste heat. Getting rid of that heat (shunting blood to the skin, sweating) requires active cellular work and ATP, which keeps your metabolism elevated during recovery.

Why does endurance training have a much bigger effect on Slow-Oxidative (SO) fibers compared to other fiber types?

Because endurance training uses a high frequency of motor unit activation with low-to-moderate force. SO fibers have a low activation threshold, meaning they are recruited first and stay active the entire time, forcing them to do almost all the work and make the biggest aerobic adaptations.

What is muscle atrophy?

The loss of muscle mass, usually because you stop using the muscle (disuse).

What are the four major consequences of muscle atrophy?

Loss of strength

Decreased endurance

Reduced joint support

Loss of muscle tone

What is the general physiological definition of fatigue?

A decreased capacity to generate force or handle contractions.

What is psychological fatigue and what part of the body causes it?

It's your brain (Central Nervous System) perceiving that you are tired, even if your muscles could technically keep going.

What is physically happening during muscular fatigue?

The muscle is running out of glucose inside its fibers.

True or False: During muscular fatigue, your actual ATP levels drop down to zero.

False. ATP levels are always maintained relatively constant.

How common is true muscular fatigue, and who does it typically happen to?

It's rare! It usually only happens in highly trained athletes who have pushed themselves to absolute maximal effort.

What causes synaptic fatigue?

The nerve physically runs out of the neurotransmitter acetylcholine (ACh)

How common is synaptic fatigue?

It is very, very rare.

Why is "reduced joint support" a consequence of muscle atrophy?

Skeletal muscles act as dynamic stabilizers for joints by maintaining baseline tone. When muscle mass and tone shrink, the passive structures (like ligaments and joint capsules) take on all the stress, increasing the risk of injury.

If ATP is the literal fuel for muscle contraction, why do ATP levels stay relatively constant even when a muscle is completely fatigued?

The body has strict protective mechanisms (like the phosphocreatine system and rapid adenylate kinase tracking) to keep ATP stable. If ATP actually plummeted to zero, the myosin heads would lock onto actin permanently, causing rigor-mortis-like cramping rather than just fatigue.

Why is synaptic fatigue so incredibly rare compared to psychological fatigue?

Your motor neurons are highly efficient at recycling and restocking acetylcholine (ACh) vesicles in the axon terminal. Your brain's Central Nervous System (psychological fatigue) will almost always shut down the movement to protect the body long before the peripheral nerves physically run out of chemical signals.