Mechanics of Muscle Contraction

Question 1

What is the stimulation to twitch ratio?

Answer 1

1 stimulation → 1 twitch

Question 2

What are the 3 muscle twitch phases?

Answer 2

• Latent phase
• Contraction phase
• Relaxation phase

Question 3

What is the latent phase?

Answer 3

The period of time between the stimulation of the muscle and the onset of contraction

Question 4

What is the contraction phase?

Answer 4

The period of time during which the muscle is actively contracting, from beginning to end of muscle tension

Question 5

What is the relaxation phase?

Answer 5

The period of time during which the muscle is returning to its resting state (peak tension to no tension)

Question 6

What happens during the latent phase of a muscle twitch?

Answer 6

The stimulus triggers an action potential (AP) which causes myosin to bind to the actin active site, beginning muscle contraction.

Question 7

What happens during the contraction phase of a muscle twitch?

Answer 7

Myosin heads slide along the actin filaments, generating tension in the muscle which lasts from the beginning to the end of the phase.

Question 8

What happens during the relaxation phase of a muscle twitch?

Answer 8

• Ca2+ ions are absorbed back into the cisternae
• tropomyosin moves back over actin binding site
• myosin head released from actin
• the filaments move back into resting position

Resulting in a decrease in muscle tension from peak to no tension.

Question 9

What determines the speed at which different muscles contract?

Answer 9

Various muscles contract at different speeds due to differences in their composition of muscle fibers.

Question 10

What are the implications of muscles having different types of muscle fibers?

Answer 10

The composition of muscle fibers in different muscles can affect their:
- strength
- endurance
- speed of contraction

Question 11

What factors determine the tension generated in a skeletal muscle fiber?

Answer 11

The tension generated in a skeletal muscle fiber is dependent upon its sarcomere length.

Question 12

What is sarcomere length?

Answer 12

Sarcomere length is the distance between two Z-lines in a muscle fiber

Question 13

How does sarcomere length affect muscle tension?

Answer 13

• muscle tension is maximal when sarcomeres are at an optimal length
• too much or too little stretching will result in decreased tension

Question 14

What happens to active tension in skeletal muscles when sarcomeres are too short or long?

Answer 14

When sarcomeres are too short
- active tension decreases
- because the myosin heads cannot reach the actin filaments effectively

Question 15

What other factor affects the tension generated in a skeletal muscle fiber?

Answer 15

• the frequency of APs across the skeletal muscle fiber
• higher frequencies of APs result in greater tension

Question 16

What factors determine the tension generated in a skeletal muscle as a whole?

Answer 16

• the frequency of action potentials transmitted along a somatic motor nerve
• recruitment

Question 17

What is the frequency of action potentials transmitted along a somatic motor nerve?

Answer 17

the number of action potentials that occur per unit of time
- higher frequencies of APs result in greater muscle tension

Question 18

What is recruitment in the context of muscle tension?

Answer 18

the activation of additional motor units in a muscle, resulting in an increase in muscle tension.
- used by the NS to regulate the amount of tension generated by a muscle

Question 19

What is the effect of frequency of action potentials on wave summation in skeletal muscles?

Answer 19

Increasing the frequency of action potentials can lead to wave summation in skeletal muscles.

Question 20

What is wave summation?

Answer 20

Refers to the
- increasing tension in a skeletal muscle
- due to summation of twitches
- caused by repeated stimulations (APs fired)
- before the end of the relaxation phase

Question 21

When does wave summation occur?

Answer 21

Wave summation occurs when successive stimuli arrive before the relaxation phase has been completed.

Question 22

What is the minimum stimulus frequency required to produce wave summation in skeletal muscles?

Answer 22

over 50/second

Question 23

What is Treppe?

Answer 23

Treppe is an increase in peak tension with each successive stimulus delivered shortly after the completion of the relaxation phase of the preceding twitch.

Question 24

When is Treppe observed in skeletal muscles?

Answer 24

Treppe is observed in skeletal muscles when successive stimuli are delivered shortly after the completion of the relaxation phase of the previous twitch.

Question 25

When is a skeletal muscle fiber’s maximum potential tension reached?

Answer 25

A skeletal muscle fiber’s maximum potential tension is not reached until tetanus.

Question 26

What is tetanus?

Answer 26

Tetanus is the maximum tension that a muscle can produce

Question 27

What is incomplete tetanus?

Answer 27

Sustained contraction of the muscle in response to repeated stimuli (increased stimulation frequency), where muscle relaxes briefly between each stimuli

Question 28

What is complete tetanus?

Answer 28

When a higher stimulation frequency (more, repeated stimuli) eliminates the relaxation phase, and the muscle is in continuous contraction
- does not relax between stimuli

Question 29

What happens when there are greater fibers per neuron?

Answer 29

The greater the number of fibers per neuron, the finer the movement, requiring more brain power.

Question 30

How is an increase in force made possible?

Answer 30

An increase in force is made possible by recruiting more motor units.

Question 31

What happens if the motor stimulus is strong enough?

Answer 31

If the motor stimulus is strong enough, it will cause multiple motor units to respond.

Question 32

What is the effect of the combined contractile force produced by motor units?

Answer 32

The combined contractile force produced by motor unit 1 and 2, and their combined 5 muscle fibers, is much greater than the force produced by either motor unit alone.

Question 33

How are muscle fibres of different motor units in a skeletal muscle arranged?

Answer 33

Muscle fibers of different motor units are intermingled

Question 34

What is the benefit of muscle fibers of different motor units being intermingled?

Answer 34

The benefit is that the forces applied to the tendon remain balanced regardless of which motor units are stimulated.

Question 35

Does the tension applied to the tendon remain constant?

Answer 35

Yes, the tension applied to the tendon remains fairly constant, even though individual motor units cycle between contraction and relaxation.

Question 36

What is an isotonic contraction?

Answer 36

A muscle contraction with movement
- concentric - muscle shortens
- eccentric - muscle elongates

Question 37

What is an isometric contraction?

Answer 37

A muscle contraction without movement

Question 38

Does a muscle shorten during an isometric contraction?

Answer 38

No, there is no muscle shortening during an isometric contraction.

Question 39

Can you provide an example of an isometric contraction?

Answer 39

Pushing against a wall