Myofibre Types

Question 1

What are the 3 Myofibre Types?
What metabolic pathways do you think each fibre relys on?

Answer 1

1. Type I – slow oxidative (SO) - Aerobic
2. Type IIa–fast oxidative glycolytic (FOG)-Anaerobic + Glycolosis
3. Type IIx – fast glycolytic (FG)

Question 2

What are characteristics of Type I Fibres – Slow Oxidative?

6

Answer 2

• Smallest diameter
• High capacity for ATP production via OXPHOS
• High capillarization – for more oxygen transport
• High mitochondrial density – where OXPHOS takes place
• Least powerful but greatest fatigue resistance (endurance fibres)
• Speed of contraction (slow-twitch)

High myoglobin content gives more red colour (darker)

Question 3

What are characteristics of Type IIa Fibres – Fast Oxidative Glycolytic?

Answer 3

• Intermediate diameter
• Moderately powerful and moderate oxidative capacity
• Moderate capillarization
• Moderate-high mitochondrial density
• Moderately susceptible to fatigue
• Fast speed of contraction (fast-twitch

Question 4

What are characteristics of Type IIx Fibres – Fast Glycolytic?

Answer 4

• Largest diameter
• Most powerful but low oxidative capacity
• Low capillarization
• Low mitochondrial density
• Low fatigue resistance
• Fast speed of contraction (fast-twitch)

Question 5

Why does the size of a muscle fiber influence its force generating capacity?

Answer 5

Bigger muscle fibers have more sacromeres which allow for a larger contraction

Question 6

How do we determine someone’s fibre type distribution?

Answer 6

Step 1: Muscle Biopsy (Vastus Lateralis)
- Freeze skin and fascia, cut and sunction small piece of muscle

Step 2: Tissue Sectioning
- use a tiny deli slicer? to cut off thin slice of muscle

Step 3: Tissue Staining
- choose proteins that only exist in a certain muscle fiber

Step 4: Analysis

Question 7

How many fibre types does one Motor Unit have?

Answer 7

Motor units contain only one fibre type

Question 8

What is the order of motor unit recruitment?

Answer 8

Motor unit recruitment goes from smallest to largest (low to high recruitment threshold)

Question 9

Motor Unit function depends on size.
How do smaller and larger MU’s differ in function?

Answer 9

• Small MU = muscles for fine movements
• Large MU = muscle for gross/powerful movements

Question 10

How does Fibre Type Distribution differ?

Answer 10

• No muscle has a single fibre type
• Fibre type distribution (% of each fibre type found in that muscle) dictates muscle function
• Largely determined by genetics

Question 11

Thinking about the function of each of these muscles/muscle groups and estimate their fibre type distribution:
1. Postural Muscles of the back
2. Diaphragm
3. Muscles of the Arm
4. Muscles of the Leg

Answer 11

1. Slow-Oxidative - always active, fatigue resistent
2. Slow-Oxidative - also always active
3. Mixture
4. FOG

Question 12

What is the relationship between Fibre Type and Athletic Performance?

Answer 12

Question 13

What is Fatigue?

Answer 13

The decline in tension-generating capacity of the muscle.

Question 14

What does fatigue lead to?

Answer 14

• Reduced shortening velocities and rate of relaxation
• Onset and rate of development depends on
• Active myofibre type
• Intensity/duration of contractile activity
• Fitness status of the individual

Question 15

Tension Fatigue Graph

Answer 15

Question 16

How would an individual fatigue in this situation?

Answer 16

• Rapid fatigue but quick recovery
• Blood flow may cease to muscles due to blood vessel contraction (heavy weights)

Question 17

How would an individual fatigue in this situation?

Answer 17

• Slower fatigue development and slower recovery
• Cyclical periods of contraction and relaxation)

Question 18

What is Peripheral Fatigue?

Answer 18

• Due to factors distal of the NMJ
• Results from accumulation of metabolic by-products
• Greater in myofibres with greater glycolytic capacity

Question 19

What is Central Fatigue (Central Command Fatigue)?

Answer 19

• Due to factors proximal to the NMJ
• Results from changes within the nervous system
• Decreased motor neuron output

Question 20

What causes peripheral fatigue?

Answer 20

Metabolic byproducts of anaerobic glycolysis:

Question 21

What metabolic changes occure with Peripheral Fatigue?

Answer 21

Effects of Metabolic Changes:
1. Reduced Ca 2+ release, reuptake and storage by SR
2. Reduced sensitivity of thin filaments proteins to Ca 2+
3. Directly inhibit the binding/power-stroke motion of cross-bridges

Question 22

How does Peripheral Fatigue hinder us during Long-Duration Exercise?

Answer 22

1. Leaky SR Ca 2+ channels -> persistently high cytosolic [Ca 2+] -> protease activity -> contractile protein degradation
2. Depletion of fuel substrates:

• ATP
• Muscle glycogen
• Blood glucose
• Dehydration

Question 23

Weighlifting relies on anaerobic metabolism. Why then, does restricting blood flow lead to faster fatigue development?

Answer 23

need to get oxygen but also get rid of byproducts which cause fatigue.

Question 24

What about Central Command Fatigue?

Answer 24

• The muscles are not fatigued by the individual stops exercising
• Performance also depends on the mental ability to initiate central commands during periods of increasingly distressful sensations

We need more research!!

Question 25

Relative Role of Neural and Muscle Tissue Adaptation to Training

Answer 25