6. Muscle Metabolism, Fibre Types, and Fatigue

Explain how ATP is produced in muscle

Distinguish and discuss the characteristics of muscle fibre types

Explain how the characteristics of muscle fibre type determines different features of muscle contraction (tension, fatiguablility, speed of contraction)

Discuss the association between muscle fibre properties and athletic performance / training

Distinguish and describe the two sources of fatigue

Discuss two theories on muscle cramps and explain the mechanism behind hypocalcemia tetany

the fuel for the contraction cycle (aka crossbridge cycling)

ATP is the fuel

How do muscles gain the ATP that’s needed for the contraction cycles? (three ways)

• creatine phosphate (phosphocreatine)

• glycolysis

• aerobic cellular respiration (oxidation phosphorylation: aerobic glycolysis and beta oxidation)

Creatine Phosphate system (aka phosphocreatine system) (aka ATP-CP)

• immediate and rapid production of ATP

• occurs in cytosol

• no oxygen required

• very small amount of ATP

• 3-15 sec of energy

• about 90 sec to recover

• lasts until PCR runs out

• comes from food we eat

ATP-CP system fatigue results from

• depletion of PCr stores

• ATP levels don’t drop off immediately because it is being maintained by PCr in the cytosol

• PCr levels drop off immediately as those stores are used to produce ATP to power the muscle

• exhaustion occurs when PCr and ATP levels are both depleted

Phosphocreatine system: muscle at rest vs working muscle

Glycolysis (anaerobic)

Oxidative Phosphorylation (aerobic glycolysis)

oxidative phosphorylation (beta / fat oxidation)

diagram of products going in and coming our of oxidative phosphorylation

3 types of fuel and energy storage

• immediate

• short term

• long term

immediate fuel and energy storage

stored in muscle cytosol (ADP, PCr)

short term fuel and energy is stored in…

blood (blood glucose), muscle cytosol (muscle glycogen)*, liver (liver glycogen)

* = larger stores of fuel

long term fuel and energy is stored…

in adipose tissue (subcutaneous and intramuscular)

(huge stores of fuel)

Fuel and Energy Storage

how long does each ATP supply provide energy for at a time

• (Stored ATP in muscle: 0-2 seconds of exercise)

• Phosphocreatine System: 3-15 seconds of exercise

• Anaerobic Glycolysis: 30 seconds - 2 minutes

• Aerobic Glycolysis: up to 40 minutes

• Fat Oxidation: hours

3 types of muscle fibre

• Type I – Slow oxidative (SO) fibres

• Type IIa – fast oxidative-glycolytic (FOG) fibres

• Type IIx – fast glycolytic (FG) fibres (aka Type IIb)

Name the structures from top to bottom

• slow oxidative fibre

• fast glycolytic fibre

• fast oxidative-glycolytic fibre

Type I Fibres

slow oxidative (SO)

• small in diameter = least powerful

• greatest oxidative capacity

  • high capillary density

  • high mitochondrial density

  • high density myoglobin

• fatigue resistent - high endurance

• slow speed of contraction (slow twitch)

  • myosin head ATPase activity is sloooowwwww

Type IIa Fibres

Fast Oxidative-Glycolytic (FOG)

• intermediate diameter = moderately powerful

• moderate oxidative capacity

  • Moderate capillary density

  • Moderate mitochondrial density

  • Moderate myoglobin density

    • Relies on glycolysis AND phosphorylation for ATP

• Moderate rate of fatigue

• fast speed of contraction (fast twitch)

  • myosin head ATPase rate is moderate

Type IIx / IIb

Fast glycolytic (FG)

• Largest diameter = very powerful

• Low oxidative capacity

• Relies on anaerobic processes for ATP

  • Lower capillary density

  • Lower myoglobin density

  • Fewer mitochondria

• Very fatigable (low endurance)

• Fastest contraction speed (fast twitch)

  • Myosin head ATPase activity very fast

how is muscle fibre type distribution determined

genetically

can muscle type distribution be changed?

a little bit with training

FG and FOG can become each other, but nothing can become or stop being SO

all muscles have all fibre types

depending on their typical function, Dif muscles will have different fibre type profiles - like our posture muscles will be more SO because need endurance vs another muscle that is mostly strength

How do adaptations to muscle fibre / muscle fibre types occur?

through the FITT principles

Frequency
Intensity
Time
Type - of exercise

how does aerobic (cardio) training affect muscle fibre?

Slow twitch fibres become more efficient at oxidative processes

• increase in mitochondria

• increase in capillary density

• increase in myoglobin

  • improved O2 delivery and metabolite removal

FG become FOG

Increase type I motor units / connections

how power training affects the muscle fibres

• muscle cells undergo hypertrophy - increase in cross-sectional area of muscle fibre - leads to increased cross-section of entire muscle

• increase in muscle cell number (hyperplasia) - satellite cells activated to differentiate into muscle fibres

• more cross bridge connections = more tension

• FOG become FG

• Increase in type II motor units / connections

Henneman’s Size principle

relates the input and output properties of motoneurons and their muscle fibers to size and is the basis for size-ordered activation or recruitment of motor units during movement

fatigue

the decline in the tension capacity of a muscle

two types of fatigue

• peripheral fatigue (muscle)

• central fatigue (neural)

peripheral fatigue

• muscle fatigue

• slower shortening velocity

• slower relaxation rate

  • due to accumulation of metabolic by-products

muscle fibre types and their fatiguability

FG = fast fatigue

FOG = moderate fatigue

SO = slow fatigue

lactic acid is made of ____ and ______

an acid and a lactate molecule

effect of intensities / lengths of exercise on lactic acid levels

levels do change, but lactic acid disappears quickly from the blood

T / F: lactic acid helps make you sore

False. Levels do not seem to correlate with reported soreness

• fatigue is multi-faceted, not completely understood

factors to consider when looking at fatigue

• dehydration

• fuel / lack of fuel

• damage to muscle tissue

• other metabolites

• central command factors / neural factors

• electrolyte balances

• many others

Muscle cramps are…

involuntary tetanic contractions of a muscle

MVC

acronym for maximum voluntary contraction

T or F: during muscle cramps, AP (action potential) firing rates are higher than they are during MVC

True

Potential causes for muscle cramping

Electrolyte imbalance

• may impair ability of muscle or neuron to maintain concentration gradient

  • impair ability of membrane to depolarize / repolarize

• may result from overexercising and / or dehydration

• treatments : drink electrolyte solutions, heat therapy, massage, hydration, rest

Stimulation of sensory receptors in muscle

• activation of reflex arc with motor neuron as effector

• treatment : eat spicy foods - may activate reflex arc that decreases motor unit output

Hypocalcemic tetany

the involuntary tetanic contraction of skeletal muscle that occurs when extracellular Ca2+ concentration decreases to about 40% of normal values

Symptoms of hypocalcemic tetany