Biological Basis of Muscle Hypertrophy Flashcards
Principle of Myoplasticity
What does Myoplasticity refer to
- When gene expression is altered in the cell, it results in an increase or decrease in the amount of specific proteins.
- This capacity for adaptive change we define as plasticity
- Myoplasticity therefore refers to the specific ability of the muscle cell to undergo adaptive change
Principle of Myoplasticity
- Approximately 20% of skeletal muscle is composed of proteins (balance being water, salts, other organic molecules)
- All types of protein can be regulated by alteration in gene expression
- Within connective tissue
- Within plasma membranes
- Within sarcoplasmic reticulum
- Within myofibrils
- Myoplasticity is therefore predicated on the ability of genetic machine to change either the quantity (Amount) or variant of protein it expresses for a desired outcome
Principle of Myoplasticity
- Old vs Young
- RT
Old vs Young
- In the younger individuals, we can see a larger distribution of the fast twitch muscle fibres than in the old individuals
- When we are older we see a preferential loss in type II muscle fibres
RT
- RT elicits hypertrophy of all fibre types
- Type IIx proportion decreases, IIa increases
- This may be because type IIx requires a lot of energy for less work which may be inefficient
Only size and quality/variant change?
- hyperplasia
- Hyperplasia = increase in number of muscle cells
- Some evidence of hyperplasia in animal models (cats and birds) , however, little evidence of hyperplasia in humans from training.
- Principle mechanism for muscular hypertrophy in adult mammals is cellular hypertrophy and not hyperplasia
Principle of Myoplasticity
- Synthesis vs degradation
Relates to protein turnover
- Every protein in a cell is transcribed (DNA to mRNA) Translated (mRNA to protein) And degraded (protein to amino acids)
Every protein has a half-life for its existence
The level of any protein in a muscle is proportional to the rate of synthesis relative to the rate of degradation
- Synthesis/degradation ratio
The goal for hypertrophy
- For synthesis to exceed degradation and lead to protein accretion within the muscle over time
Principle of Myoplasticity
HINT: skeletal muscle adaptations are…
That skeletal muscle adaptations are characterized by modifications of morphological, biochemical, and molecular variables that alter the attributes of fibres in specific motor units (functionally similar groupings of muscle fibres)
Protein synthesis pathway - important
Possible pathways for increase in protein synthesis
HINT: phosphorylation $ protein kinase
- Phosphorylation, allows coupling and addition of a phosphoryl group (e.g. to a lipid, protein, carbohydrate)
- Protein Kinase, an enzyme that modifies other proteins via phosphorylation
Possible pathways for increase in protein synthesis
- Resistance training can stimulate the increase in mTOR which leads to increase in protein synthesis
- Dietary Intake also stimulates mTOR which leads to protein synthesis
Protein synthesis pathway - is it valid?
HINT: Data shows a relationship…
Data shows a relationship between p70 and resistance training with
With an increase in p70, there is an increase in the % change in 1RM squat strength, % change in whole body FFM, % change I FFM(kg), % change in type IIA fCSA
Acute stimuli facilitating protein synthesis
HINT: myostatin
- Genetic factors
- Myostatin = is a negative regulator of myoblast proliferation and differentiation e.g. regulates hypertrophy.
- Myostatin deficiency or knockout can lead to increase muscle mass via not negatively regulating protein synthesis
What stimuli facilitates protein synthesis
Muscular tension
- Muscle damage
- Metabolic factors
- Hormones
Muscle Damage
- Exercise induced muscle damage (EIMD)
- Z discs
- EIMD - potential mechanisms of action
Very severe z disc streaming related to high levels of muscle damage. Shows broken points within the stream
Potential mechanisms of action
- Signalling via inflammatory cells
* InterLeukin-15
* fibroblast growth factor (FGF)
- Satellite Cell activity
* SC proliferate and adhere to damaged fibres
* Theorised that neurons innervating damaged fibres may also contribute to SC activity as there is a high pop of SC under the myoneural junction
- Insulin-Like Growth Factor - 1 Signalling
- Cell swelling
Muscle Damage
- EIMD
- Sarcoplasmic hypertrophy vs Myofibrillar hypertrophy
Sarcoplasmic hypertrophy: during first 4 weeks of training, no actual actin and myosin filaments laid down, it’s just swelling and contains enzymes
Myofibrillar hypertrophy: start to see a perceivable amount of actual myofibrillar hypertrophy at around 8 weeks of training. The edema-induced muscle swelling decreases within the cell.
Muscle Damage
- EIMD
- RT
HINT: the first couple of …
- The first couple of sessions of resistance training, we see the most amount of z disc streaming or muscle damage.
- We only see a high amount of myofibrillar hypertrophy when the muscle damage is low
- In the first couple of sessions at the gym, the activity in the muscle including from the satellite cells is actually just repairing the muscle, not trying to make it bigger. Around 2-3 weeks, repair of the muscle decreases and muscle growth increases
In the initial training protocol, the z disc streaming is high while the increase in muscle fibre cross-sectional area is low. It is only when there is no z disc streaming that the muscle cross-sectional area is at its highest.
Metabolic Factors
- Metabolic influence?
- Hypoxic training inclusion…
Support:
- Hypoxic training inclusion reduces muscle mass loss post surgery 7 vs 15 % 14 days post
- Improvement in hypotrophy outcomes relative heavy load with intermittent hypoxic
- Therefore, potential metabolites may induce / contribute to hypertrophy pathway
Arguments against:
- Reduce energy clearance or intake capacity invokes fatigue in muscles which consequently requires additional fibers to sustain force output. So the mechanism is still load/tension related
* Can be beneficial for individuals who are injured or are recovering from an injury and want to increase muscle mass quickly without lifting heavy loads
However, recent work found that the addition of blood flow restriction training to a traditional resistance training program preferentially enhanced type 1 fiber cross sectional area in a cohort of elite powerlifters.
Metabolic influence - possible mechanisms
Metabolic stress is a vague concept given that 2,700 metabolic enzymes catalyze 900 metabolic reactions (129) and that 4,000 metabolites can be detected in human serum alone (123)
However possible candidates
- Lactate
- α ketoglutarate
- phosphatidic acid
- skeletal muscle protein breakdown
Effects may be acute in nature as chronic energy stress regulators (AMPK) exist in the cell that would likely counteract some of the proposed mechanisms of above named metabolites.
AMPK can block mTOR from phosphorylating. mTor is important for p70
