Term Test 2: Muscle Mechanics

Question 1

Muscle fibres

Answer 1

• Single muscle cells
• Multi-nucleated, long thread-like cells
• Each muscle fibre is encased in a connective tissue
  sheath called the endomysium

Question 2

Fascicles

Answer 2

• Bundles of muscle fibres (each containing 10-100
  muscle fibres)
• Encased in a connective tissue sheath called the
  perimysium

Question 3

Whole muscles

Answer 3

• Are bundles of muscle fascicles (can vary widely).
• Encased in a connective tissue sheath called the
  epimysium

Question 4

Tendons

Answer 4

Cord-like and aponeuroses (sheet-like) are woven connective tissues that extend beyond the muscle and connect to bone

Question 5

Myofibrils

Answer 5

• Threadlike structures that lie parallel to each other
  and run the full length of the muscle fibre.
• Transverse light and dark bands appear across each
  myofibril and align with the same bands on adjacent
  myofibrils
• Bands of light and dark repeat every 2.5 um, giving
  skeletal muscle its striated appearance

Question 6

Sarcomere

Answer 6

Is the basic contractile unit of the muscle. It is the repeating unit of the myofibril between the stripes

Question 7

Myofilaments

Answer 7

Are protein filaments that overlap within the sarcomere

Question 8

Thin filaments contain

Answer 8

• Actin
• Troponin C
• Tropomyosin proteins

Question 9

Thick filaments contain

Answer 9

Myosin proteins

Question 10

I-band

Answer 10

Region that contains only actin and Z-line (no overlapping myosin), appears as light band

Question 11

A-band

Answer 11

• Region that contains myosin
• Overlapping actin, appears as dark band

Question 12

H-zone

Answer 12

• Region of the A band
• Contains only myosin and M-line (no overlapping
  actin)

Question 13

M-line

Answer 13

Transverse band that anchors myosin to each other

Question 14

Z-line

Answer 14

Transverse band that anchors actin to each other (Z-line to Z-line defines sarcomere)

Question 15

Sliding filament theory

Answer 15

Proposes that muscle force arises from cyclic binding between thin actin and thick myosin filaments of the sarcomere
- Absence of calcium, tropomyosin prevents
myosin from attaching to actin
- During AP, calcium is released from the
sarcoplasmic reticulum and binds to troponin
C
- This induces a conformal change in
tropomyosin, giving myosin head access to
actin
- Myosin head binds actin and a power stroke
causes muscle contraction
- ATP binds allowing the release of actin

Question 16

Electron micrographs

Answer 16

• Confirms the existence of thick myosin and thin
  actin filaments
• Stretching the muscle does not cause a change
  in the length of the thick and thin filaments,
  but only a change in the length of the I-band
  and H-zone

Question 17

Factors affecting muscle force development

Answer 17

• Length of muscle
• Velocity
• Physiological cross-sectional areas of the
  muscle
• Muscle geometry
• Level of activation

Question 18

If you have ever done a hamstring curl on a flat bench, you may have noticed that your hamstrings feel very weak when in a fully flexed position. Why do you think that is? Why might assuming some hip flexion may make the exercise easier?

Answer 18

• Flexing the hip makes the length of the
  hamstring longer
• Making exercise easier
• You are on the ascending limb
• You can generate more force

Question 19

Contractile element

Answer 19

Represents force development via cross-bridge attachments in the sarcomeres

Question 20

Series elastic element

Answer 20

Represents force-deflection properties of tendon

Question 21

Parallel elastic element

Answer 21

Represents force-deflection properties of the sarcolemma, epimysium, perimysium and endomysium

Question 22

Shortening does not equal short

Answer 22

• The force-length relationship tells us that the
  maximum force a muscle can generate
  depends on whether it is short or long
• The force-velocity relationship tells us that the
  maximum force a muscle can generate
  depends on whether it is shortening or
  lengthening

Question 23

Short and long

Answer 23

• Short does not equal shortening
• Long does not equal lengthening
• Short and long refer to the muscle length
• Short muscles can be shortening or lengthening

Question 24

Shortening and lengthening

Answer 24

• Short does not equal shortening
• Long does not equal lengthening
• Shortening and lengthening refers to the muscle velocity

Question 25

Muscle force-velocity relationship

Answer 25

- The greater the shortening velocity of the muscle, the smaller the force that can be produced (we cannot lift heavy objects quickly) - A muscle contracting eccentrically or isometrically is capable of producing more force than a muscle contracting concentrically

Question 26

Muscle force-length relationship

Answer 26

In a whole muscle, Total = active (muscle contraction) + passive (connective tissue) tension

Question 27

Activation

Answer 27

Number (or proportion of) muscle fibres that are stimulated to contract at any given time

Question 28

Motor unit

Answer 28

Composed of a single motor neuron and all of the muscle fibres with which it synapses - Electrical stimulation causes muscle to contract through the release of Ca2+

Question 29

How do we increase activation?

Answer 29

1. Increase the firing frequency of any given motor unit 2. Recruit more motor units to produce greater force (tension), you also recruit more motor units to increase the proportion of fibres running

Question 30

Heneman's size principal

Answer 30

Motor units are recruited from smallest (slow twitch) to largest (fast twitch)

Question 31

Physiological cross-sectional area I

Answer 31

- Increasing the number of sarcomeres lined up end to end = increasing number of cross- bridges = increasing the length of the myofibril - If adding the sarcomeres in parallel by increasing the number of myofibrils, the number of possible attachments of cross bridges have increased

Question 32

Physiological cross-sectional area II

Answer 32

Adding sarcomeres in parallel makes a muscle stronger, but not faster. Strength is thus proportional to muscle cross-sectional area because it is proportional to the number of parallel-acting sarcomeres