Skeletal muscles

Question 1

What type of control is skeletal muscle under?

Answer 1

Voluntary control

Question 2

What are the bundles of individual muscle fibers called?

Answer 2

Fascicles

Question 3

What are the three layers of connective tissue that wrap the components of a muscle?

Answer 3

Fibers: Endomysium
Fasciculi: Perimysium
Muscle: Epimysium

Question 4

What are the basic repeating units between two Z lines that make up a myofibril?

Answer 4

Sarcomeres. They are approximately 2.5 microns long and are the functional unit of a muscle fiber.

Question 5

Why does muscle have a striated appearance?

Answer 5

Due to the organization of actin and myosin within the sarcomere.

Question 6

What is the primary factor used to classify muscle fiber types?

Answer 6

The predominant myosin heavy chain (MHC) content.

Question 7

What complicates the classification of muscle fiber types?

Answer 7

Single fibers can contain different portions of MHC (transitional fibers), suggesting that fiber types exist on a continuum rather than as distinct types.

Question 8

What is the muscle biopsy technique?

Answer 8

The removal and analysis of a small portion of whole muscle, allowing for direct measurement of muscle fiber morphology and metabolism

Question 9

What method helps identify different muscle fiber types?

Answer 9

Immuno-histochemical staining. For example, in the source material, Type IIb fibers are stained dark in Panel A, Type IIa in Panel B, and Type I in Panel C. Panel D is a control with no stain.

Question 10

True or False: Fiber type distribution is consistent across all muscles in the body.

Answer 10

False. Fiber type varies within and between different muscles.

Question 11

Give an example of how fiber type distribution varies between different muscles.

Answer 11

The soleus muscle is predominantly Type I, while the gastrocnemius is a mix of Type I and II.

Question 12

Can fiber type distribution vary between individuals?

Answer 12

Yes, fiber type can vary between the same muscles of two different people. For example, a marathoner’s quadriceps might have ~70% Type I fibers, while a sprinter’s might have ~70% Type II fibers.

Question 13

How does muscle function influence fiber type distribution?

Answer 13

Postural muscles, which require fatigue resistance for hours of continuous tension, are predominantly Type I.

Question 14

What two factors influence muscle fiber type?

Answer 14

Genetics and training.

Question 15

What comprises a motor unit (MU)?

Answer 15

A motor unit includes the anterior alpha motor neuron and the muscle fibers it innervates. It is considered the basic functional unit of movement

Question 16

Describe the characteristics of the anterior (α) efferent motor neuron.

Answer 16

It is relatively large in diameter and myelinated, allowing for fast conduction velocity (30-60 m/s).

Question 17

What is the neuromuscular junction?

Answer 17

Also called the motor end plate, it is the interface between the nerve and muscle fibers where neurotransmitters, such as acetylcholine, are released.

Question 18

How many primary types of motor units are there in skeletal muscle?

Answer 18

Three, possibly four.

Question 19

What are the three (possibly four) primary types of motor units?

Answer 19

○ Fast Fatigable (FF)
○ Fast fatigue-resistant (FR)
○ Slow (S)
○ Fast fatigue Intermediate (FI)

Question 20

Relate the types of motor units to the types of muscle fibers.

Answer 20

• FF MU → FG or Type IIb fiber type
• FR MU → FOG or Type IIa fiber type
• S MU → SO or Type I fiber type
• FI MU → Type IIx fiber type (not a lot of research in this area

Question 21

What is important to remember about the relationship between motor units and muscle fibers?

Answer 21

Muscle fibers “belong to” or are considered to be “within” a motor unit.

Question 22

Describe the concept of muscle fiber number to nerve ratio within a MU.

Answer 22

○ A single motor neuron (MN) can stimulate a varying number of fibers.
○ Muscles used for gross motor movements have a high muscle fiber to nerve ratio (e.g., the gastrocnemius has ~1776 muscle fibers per MN).
○ Fine, more precise movements have a low muscle fiber to nerve ratio (e.g., finger muscles have ~342 muscle fibers per MN).

Question 23

Explain Henneman’s Size Principle.

Answer 23

It states that smaller motor neurons are recruited before larger ones. This occurs because smaller MUs have a lower recruitment threshold and depolarize more easily than larger MUs.

Question 24

What is the order of motor unit recruitment during muscle contractions?

Answer 24

S - FR - FF. This order is consistent for all voluntary muscle contractions, including both power and endurance movements. However, FR/FF MUs contribute more to force production in fast and maximal contractions.

Question 25

Describe the sliding filament theory of muscle contraction.

Answer 25

During a concentric contraction, the thick (myosin) and thin (actin) filaments slide past each other, pulling the Z lines closer together. ATP hydrolysis provides the energy for this process

Question 26

Briefly explain excitation-contraction coupling.

Answer 26

It is the process by which an electrical signal (action potential) in the muscle fiber leads to the release of calcium ions, which initiate the sliding filament mechanism and muscle contraction.

Question 27

What are the three main types of skeletal muscle contractions?

Answer 27

○ Isotonic: Same tension
○ Isokinetic: Same velocity
○ Isometric: Same length

Question 28

What are the three muscle actions?

Answer 28

○ Concentric: Shortening of muscle under tension
○ Eccentric: Lengthening of muscle under tension
○ Static: Tension within the muscle but no visible movement.

Question 29

What factors influence muscle tension production?

Answer 29

○ Muscle length
○ Joint angle
○ Velocity of movement

Question 30

At what muscle length is tension typically greatest?

Answer 30

Near the resting length of the muscle. This is due to the maximum overlap of myosin cross-bridges with actin active sites.

Question 31

How does muscle pennation impact force production?

Answer 31

Increased pennation leads to a greater physiological cross-sectional area and, therefore, increased force production.

Question 32

Why does muscle tension vary with joint angle?

Answer 32

Primarily due to lever arm mechanics and the angle of pull a muscle can exert at a specific joint.

Question 33

Compare muscle tension produced during different contraction types.

Answer 33

Isometric contractions generally produce the greatest tension. Fast concentric movements produce less tension than slow contractions because there is less time for optimal motor unit recruitment.

Question 34

What are eccentric contractions?

Answer 34

Eccentric contractions occur when a muscle lengthens under tension, often acting as a braking force or controlling movement.

Question 35

Why do eccentric contractions produce more force than concentric or isometric contractions?

Answer 35

○ Increased CNS drive leading to greater MU recruitment.
○ Passive tension contribution from connective tissue.
○ Myofibrillar cross-bridges remain bound longer, resulting in more cross-bridge attachments.

Question 36

True or False: Eccentric contractions are more energy-efficient than concentric contractions.

Answer 36

True. They utilize less ATP and exhibit lower EMG activity for the same power output.

Question 37

What is the stretch-shortening cycle?

Answer 37

Pre-stretching or eccentrically loading a muscle before the shortening phase can enhance force and power production.

Question 38

What is Post-activation potentiation (PAP)?

Answer 38

Short-term increases in maximum force/power (muscle performance) following heavy strength exercises, lasting 2-30 minutes

Question 39

List some proposed mechanisms for PAP.

Answer 39

○ Increased CNS drive and MU recruitment
○ Enhanced myosin binding responsiveness
○ Greater myofilament Ca++ sensitivity
○ Increased contribution of elastic energy from muscle-tendon properties
○ Increased muscle spindle activation contributing to concentric force

Question 40

What is hypertrophy?

Answer 40

An increase in the size of muscle fibers. There may be 2 types: sarcoplasmic and myofibrillar, but this is controversial.

Question 41

What is hyperplasia?

Answer 41

An increase in the number of muscle fibers

Question 42

Does muscle increase in size through hypertrophy, hyperplasia, or both?

Answer 42

○ Research on cats and rats has shown an increase in both the size and number of muscle fibers with extreme weight training.
○ Human muscle research has shown a definite increase in size, but there is little research to support hyperplasia in humans with training.

Question 43

What did MacDougall et al. find in their research on bodybuilders?

Answer 43

They found a similar number of muscle fibers in the bicep muscles of bodybuilders compared to untrained individuals, suggesting that muscle size increases are primarily due to hypertrophy. They arrived at this conclusion by taking biopsies and CT scans of the same biceps brachii muscle in bodybuilders and control subjects. They counted the number of fibers and measured the area they occupied in the muscle biopsy. They also determined the area of the whole biceps muscle from the CT scan and did calculations to determine the total number of fibers in the whole muscle.

Question 44

What did Sjostrom et al. discover about the tibialis anterior (TA) muscle?

Answer 44

They found that the dominant TA muscle had a greater number of muscle fibers than the non-dominant TA in the same subject. It is unclear whether this is due to hyperplasia or genetic differences.

Question 45

What are some possible mechanisms of hyperplasia?

Answer 45

○ Muscle fiber splitting?
○ Muscle satellite cell incorporation into a new fiber?

Question 46

Can training induce muscle fiber splitting?

Answer 46

Research by MacDougall et al. suggests this is possible.

Question 47

Can training induce myofibril splitting?

Answer 47

There isn’t much evidence supporting this.

Question 48

What happens to muscle during limb immobilization, limb suspension, space flight, or spinal cord injury?

Answer 48

○ The muscle atrophies (reduced cross-sectional area).
○ The muscle fiber type profile shifts from slow twitch (ST) to fast twitch (FT).
○ The muscle’s oxidative capacity is reduced.
○ General weakness occurs

Question 49

What happens to muscle as we age?

Answer 49

There is motor neuron apoptosis (pre-programmed cell death) and some muscle fiber necrosis (pre-mature death), especially in FT fibers. About 40% of individual muscle fibers are lost from birth to age 80.

Question 50

What are some specific changes in muscle due to aging?

Answer 50

○ Loss of overall muscle mass - sarcopenia (e.g., 30% by age 70; 50% by age 80).
○ Decrease in muscle fiber size, especially in FT fibers.
○ Increase in muscle connective tissue content.
○ Increased “grouping” of fiber types because there are fewer motor units due to motor neuron loss.
○ Decrease in overall metabolic activity.

Question 51

What is the “Repeated Bout Effect”?

Answer 51

This refers to the protective adaptations that occur with training, such as increases in connective tissue content, structural sarcomere proteins, muscle fiber size, and flexibility throughout a full range of motion, which reduce DOMS.

this effect does not eliminate DOMS completely.

Question 52

How does training affect the neuromuscular system (specifically Neural properties)

Answer 52

■ Increase in CNS drive.
■ Increase in the synchronization of motor units, which can lead to increased motor unit recruitment patterns and greater force production.
■ Enhanced coordination of motor units, increasing efficiency.
■ “Asynchronous” motor unit firing, allowing some motor units to fire while others relax, which may reduce fatigue.
■ Less inhibitory neural response due to a “resetting” of tension-limiting mechanisms in the central and peripheral nervous systems

Question 53

How does training affect the neuromuscular system (specifically muscle fibre properties)

Answer 53

■ Increase in the size of muscle fibers (hypertrophy), especially with strength training. This increase is likely due to both sarcoplasm and myofibrillar content.
■ An increase in muscle fiber number in humans remains difficult to prove.
■ Muscle fascicle length appears to be greater (longer) and the angle of pennation is less in the vastus lateralis and gastrocnemius muscles of sprint-trained athletes (100m) compared to endurance runners and control subjects. This difference is suggested to provide enhanced muscle shortening speed in sprinters. It is unclear whether this is due to genetics or training.
■ Increase in anaerobic potential or aerobic potential of fast twitch and slow twitch fibers. Type I muscle fibers become more like Type II, and Type II become more like Type I. This represents a shifting of the metabolic profile within the muscle itself, not the entire motor unit.