Muscle Structure and Adaptation

Question 1

Post-natally, how do muscles grow?

What are the isoforms of Myosin Heavy Chains/MHC?

What can influence muscle force?

What are the 3 types of muscle fibres? What isoform of MHC do they contain? What are their properties?

What factors affect muscle fibre type?

Answer 1

• Hypertrophy
• MHCIIa, MHCIIb, MHCIIX, MHCI, Perinatal Myosin
• Changes in expression patterns of MHC isoforms
• 1. Type 2B; contain MHCIIb
     o FASTEST contraction, Largest, Strongest fibres
     o Gets energy from glycolysis only (anaerobic)
     o WHITE = Doesn’t contain myoglobin

2. Type 2A; contain MHCIIa
   o INTERMEDIATE velocity contraction
   o Resistant to fatigue
   o Can use glycolysis and Krebs’s cycle for energy

3. Type 1; contain MHCI
o SLOWEST contraction, Weakest fibres
o More resistant to fatigue
o Uses aerobic respiration = rich in mitochondria
o RED = Contains lots of myoglobin

• Motor neurons (electrical activity, neurotrophic substances), Hormones, Load/Stretch

Question 2

How does muscle grow in response to intense exercise?

What controls muscle growth?

What hormones affect GH release from the anterior pituitary?

What does GH stimulate?

Effect of prolonged muscle contraction?

Answer 2

• Muscle suffers injury, which activates the Satellite cells; become myoblasts and migrate to injured site, proliferate and fuse to the damaged fibres
• Growth Hormone Axis
• GHRH, IGF-1, Somatostatin
• Stimulates IGF-1 release from liver, which goes on to promote muscle growth
• Causes the release of lots of IGF-1 to act as an Autocrine factor = ↑hypertrophy

Question 3

What is a Motor unit?

What can these units do when there’s more muscle contraction?

What is the Size Principle? Give an example

Answer 3

• Group of muscle cells innervated by the same motor neuron
• Can recruit more muscle fibres
• ↑Activity occurs as there’s increased selection of more, larger motor units as demand for force increases e.g. more motor units will be recruited for jumping vs walking

Question 4

How do you increase strength?

How is the muscle adapted for strength?

How do you increase endurance?

How is the muscle adapted for endurance?

Answer 4

• High impact resistance training with max. weights
• ↑MLC2, ↑IGF-1 release (for hypertrophy), ↑glycolytic pathway capability
• Low impact cardiovascular training
• ↑MLC1, induces mitochondrial replication + ↑size, ↑myoglobin content = ↑oxidative metabolic capability = fatigue resistance

Question 5

What are the muscles like of one who does Endurance exercises? e.g. Marathon runner

What are the muscles like of one who does Moderate activity? e.g. Sprinter

What are the muscles like of one who does Strength exercises? e.g. Powerlifter

Answer 5

• • Small, Good fatigue resistance
  • Dense and strong for their size
  • High oxidative capacity = can work over long periods of time
  • No explosive strength
• • Rapid, powerful contraction
  • Easily fatigued at max. effort
  • Low oxidative capacity
  • High force per cross-sectional area of muscle
• • Hypertrophied, Highly glycolytic
  • Fatigue easily
  • High muscle to total body mass ratio
  • Muscle size interferes with locomotion

Question 6

What occurs to muscles with age?

What can this due to?

Answer 6

• Atrophy, Weaker, Slower contraction
• • Adaptive reduction in fibre diameter
  • Pathological atrophy due to inactivity/disease