Lecture 4: Contraction Types, Energy Systems, Fiber Types & Fatigue

What does isometric mean?

Same length.

Isometric contraction

A muscle produces internal tension without changing its overall length.

Why does an isometric contraction produce tension without movement?

External resistance prevents the muscle from shortening.

What are the three purposes of isometric contractions listed in the lecture?

Maintaining posture, stabilizing joints, and acting as a prelude to movement.

What type of contraction holds a dumbbell stationary?

Isometric contraction: tension develops, but muscle length and joint position remain the same.

What does isotonic mean?

Same tension.

Isotonic contraction

A muscle changes length while maintaining tension; the lecture describes this as no change in tension.

What are the two types of isotonic contraction?

Concentric and eccentric contractions.

Concentric contraction

The muscle shortens as it maintains tension.

Eccentric contraction

The muscle lengthens as it maintains tension.

What type of contraction occurs when the biceps lifts a dumbbell?

Concentric contraction: the biceps shortens while maintaining tension.

What type of contraction occurs when the biceps slowly lowers a dumbbell?

Eccentric contraction: the biceps lengthens while maintaining tension.

Does a contracting muscle always shorten?

No. It can maintain the same length during an isometric contraction or lengthen during an eccentric contraction.

What inputs does muscle ATP production require, depending on the pathway?

Organic energy sources alone, or oxygen plus organic energy sources.

What are the two main ATP synthesis pathways in muscle?

Anaerobic fermentation and aerobic respiration.

Does anaerobic fermentation require oxygen?

No oxygen is required.

How much ATP does anaerobic fermentation produce compared with aerobic respiration?

Anaerobic fermentation produces little ATP; aerobic respiration produces much more ATP.

What product of anaerobic fermentation is disposed of by the liver?

Lactate, called lactic acid in the lecture.

What does aerobic respiration need continuously?

Oxygen.

Does aerobic respiration produce lactate as its end product?

No. The lecture contrasts it with lactate-producing anaerobic fermentation.

What is the lecture's sequence of ATP supply and recovery stages during exercise?

Immediate aerobic respiration → phosphagen system → glycogen–lactate system → long-term aerobic respiration → EPOC.

What supplies oxygen during immediate aerobic respiration?

Myoglobin in the muscle fiber.

What happens to the oxygen supplied by myoglobin at the start of exercise?

It is quickly depleted.

Besides immediate aerobic respiration, what ATP source is used at the start of exercise?

ATP already stored in the muscle fiber.

About how long does the immediate energy supply described in the lecture last?

About 5 seconds.

About how long does the phosphagen system last, according to the lecture?

About 10 seconds.

How does the phosphagen system make most of its ATP?

By transferring phosphates from other molecules to ADP.

What two enzymes are used by the phosphagen system?

Myokinase and creatine kinase.

Myokinase

An enzyme that transfers phosphate from one ADP molecule to another ADP molecule.

Creatine kinase

An enzyme that takes phosphate from creatine phosphate and gives it to ADP.

How do the phosphate donors for myokinase and creatine kinase differ?

Myokinase uses ADP as the donor; creatine kinase uses creatine phosphate.

What ATP pathway does the glycogen–lactate system use?

Anaerobic fermentation.

Where does the glycogen–lactate system obtain glucose?

From glycogen stored in the muscles.

About how long does the glycogen–lactate system last in the lecture's timeline?

About 40 seconds; it supplies enough ATP for 30–40 seconds of maximum activity.

What sequence converts glucose to lactate in the glycogen–lactate system?

Glucose undergoes glycolysis to form pyruvic acid and a net 2 ATP; pyruvic acid is then converted to lactate.

Why is the net ATP yield of glycolysis 2 ATP per glucose?

Four ATP are produced, but two ATP were invested: 4 − 2 = 2 net ATP.

What allows long-term aerobic respiration to support muscle activity?

The respiratory and cardiovascular systems catch up and deliver enough oxygen.

According to the lecture's bullet points, about how much ATP is produced per glucose during aerobic respiration?

About 30 ATP per glucose.

According to the lecture's simplified timeline, when do muscle glycogen stores become exhausted?

After about 30 minutes. This is the timing stated in the lecture.

According to the lecture, what becomes the fuel source after muscle glycogen stores are exhausted?

Fatty acids.

What product leaves the electron (hydrogen) transport system besides ATP?

Water (H2O).

What does EPOC stand for?

Excess Postexercise Oxygen Consumption.

What does EPOC meet, according to the lecture?

Oxygen debt: the metabolic demand for oxygen after exercise.

What five needs does EPOC serve in the lecture?

Aerobically replenish ATP; restore oxygen in myoglobin; restore creatine phosphate levels; supply oxygen to the liver eliminating lactate; supply oxygen to other cells with higher metabolic rates.

How does EPOC restore the muscle fiber's immediate ATP supply?

It supplies oxygen to aerobically replenish ATP.

What oxygen-binding protein has its oxygen restored during EPOC?

Myoglobin.

Which phosphagen reserve is restored during EPOC?

Creatine phosphate.

Why does the liver need oxygen during EPOC, according to the lecture?

To eliminate lactate.

Why do cells other than muscle and liver need extra oxygen after exercise?

They have higher metabolic rates.

What are the key exercise time values given in this lecture?

Immediate supply: about 5 seconds; phosphagen system: about 10 seconds; glycogen–lactate system: about 40 seconds (30–40 seconds of maximum activity); glycogen-to-fatty-acid fuel shift: after about 30 minutes in the lecture's simplified timeline.

What are the two main physiological classes of muscle fibers in the lecture?

Type I: slow-twitch fibers; Type II: fast-twitch fibers.

What intermediate muscle fiber type is identified in the lecture?

Type IIA.

Do most muscles contain only one muscle fiber type?

No. Most have both Type I and Type II fibers, but one type usually predominates in a single muscle.

What kind of energy supply is associated with slow-twitch fibers in the comparison figure?

Aerobic energy production using oxygen, providing a continuous energy supply.

How does the comparison figure describe slow-twitch contraction speed and duration?

Slow firing and extended contraction.

Which athlete in the comparison figure illustrates the use of slow-twitch fibers?

A marathon runner, illustrating endurance activity.

How does the comparison figure describe fast-twitch activity?

Rapid firing for short bursts of speed; the fibers fatigue quickly and use anaerobic energy production.

Which athlete in the comparison figure illustrates the use of fast-twitch fibers?

A sprinter, illustrating short bursts of speed and power.

What other names are given for Type I muscle fibers?

Red fibers and slow oxidative fibers.

Why are Type I fibers called red fibers?

They contain more myoglobin, which gives them a deep red color.

What are the endurance and fatigue characteristics of Type I fibers?

High endurance and fatigue resistance.

What ATP pathway are Type I fibers specialized for?

Aerobic respiration.

What function is associated with muscles rich in Type I fibers?

Maintaining posture.

Which two postural muscles are listed as examples for Type I fibers?

Erector spinae and soleus.

What is the size of Type I muscle fibers?

Thin.

How does the lecture describe Type I motor units?

Small and easily excitable, making contraction easier to maintain.

What three physiological features support aerobic activity in Type I fibers?

Many mitochondria, many capillaries, and abundant myoglobin for oxygen.

Which fiber type has many mitochondria and capillaries?

Type I slow oxidative fibers.

Which names does the lecture group under its Type II fast-twitch category?

White fibers, Type IIB, Type IIx, and fast glycolytic fibers. These are the labels grouped together on this lecture's comparison slide.

What are the endurance and response characteristics of the lecture's fast glycolytic fibers?

Low endurance, with quick and powerful responses.

What ATP pathway are the lecture's fast glycolytic fibers specialized for?

Anaerobic fermentation.

What kinds of muscles are listed as examples for fast glycolytic fibers?

Muscles used for quick, powerful movements: eye muscles, hand muscles, gastrocnemius, and biceps brachii.

What is the size of the lecture's fast glycolytic muscle fibers?

Thick.

How does the lecture describe fast glycolytic motor units and their neurons?

Large motor units that are less excitable, with larger neurons for powerful movements.

What two energy reserves are especially high in fast glycolytic fibers?

Glycogen and creatine phosphate.

How much myoglobin do fast glycolytic fibers have?

Little myoglobin, compared with slow oxidative fibers.

How do Type I and fast glycolytic fibers differ in fiber thickness and motor unit excitability?

Type I: thin fibers with small, easily excitable motor units. Fast glycolytic: thick fibers with large, less excitable motor units.

How do Type I and fast glycolytic fibers differ in their main energy-related features?

Type I: many mitochondria, many capillaries, and abundant myoglobin. Fast glycolytic: high glycogen, high creatine phosphate, and little myoglobin.

What other name is given to Type IIA intermediate fibers?

Fast oxidative fibers.

What combination of twitch speed and fatigue resistance characterizes Type IIA fibers?

They are fast-twitch fibers but fatigue resistant.

How does the lecture describe the prevalence of Type IIA fibers in humans?

The slide says “very rare in humans.” This is the lecture's wording.

Which fiber classes are paired with slow oxidative, fast oxidative, and fast glycolytic in the comparison figure?

Type I: slow oxidative; Type IIA: fast oxidative; Type IIB: fast glycolytic.

What seven factors influencing muscle organ strength are listed in this lecture?

Length–tension relationship, muscle fatigue, muscle temperature, muscle hydration, stimulus frequency, muscle fiber stimulus voltage, and nerve stimulus voltage.

How does repeated use affect contraction strength as muscle fatigue develops?

More use produces less strength.

How does the lecture relate muscle temperature to contraction strength?

Warmer muscles produce stronger contractions because their enzymes work more quickly.

How do dehydration and overhydration affect contraction strength in the lecture?

Both can make the muscle weak.

What two aspects of contraction does muscle hydration affect?

Myofilament overlap and cross-bridge formation.

Which contraction-strength factor changes how quickly muscle enzymes work?

Muscle temperature.

Which contraction-strength factor can alter myofilament overlap and cross-bridge formation?

Muscle hydration.

What happens when a stimulus to a muscle fiber is below threshold?

There is no contraction.

Does increasing stimulus voltage above threshold make a single muscle fiber's twitch stronger under the same conditions?

No. Increasing stimulus strength above threshold does not increase that fiber's contraction strength under the same conditions.

Multiple motor unit summation

An increase in whole-muscle contraction strength as more motor units are activated; also called motor unit recruitment.

What structures are bundled together in a motor nerve?

Axons (nerve fibers) from many motor neurons.

How does activation of more nerve fibers increase whole-muscle contraction strength?

More motor neuron axons fire action potentials, more motor units contract, and the muscle produces greater tension.

What happens as the voltage used to stimulate a motor nerve increases?

More nerve fibers reach threshold and fire action potentials, recruiting more motor units.

Why can increasing nerve stimulus voltage increase muscle force even though increasing voltage to one muscle fiber does not?

A nerve contains many axons. Higher voltage can activate additional axons and motor units; one muscle fiber does not produce a stronger twitch merely because a stimulus is further above threshold.

How does multiple motor unit summation differ from temporal summation?

Multiple motor unit summation activates more motor units; temporal summation increases stimulus frequency so successive twitches add together.

What do the increasing vertical lines labeled stimuli 1–9 represent in the recruitment graph?

Progressively higher stimulus voltages applied to a nerve.

What does the dashed horizontal line in the recruitment graph indicate?

Threshold stimulus voltage.

Which numbered stimuli in the recruitment graph are below threshold, and which first reaches it?

Stimuli 1 and 2 are below threshold; stimulus 3 first reaches the dashed threshold line.