Energetics of muscle fatigue & training Flashcards
Muscle Fatigue
Defined as a reversible failure to maintain the required, or
expected, power output, leading to reduced muscle performance.
Protective strategy to prevent cellular damage.
Muscle Fatigue Aetiologies not yet clearly established:
• Many potential sites between ?
• Performance enhancing techniques used by athletes frequently
target pathways that are ______to cause muscle fatigue: eg?
the brain and the contracting
muscle.
assumed
eg. creatine supplementation, carbohydrate loading,high altitude training.
Where does Fatigue occur?
Central fatigue
Peripheral fatigue
Central fatigue
• CNS command – reduced excitatory input
• Motor neuron signal – altered input from
sensory fibres
Likely more relevant in untrained individuals
Peripheral fatigue
- Neuro-muscular transmission
- Muscle fibre action potential
- Excitation-contraction coupling
- Depletion of substrates for metabolism
- Accumulation of waste-product
How is Fatigue Studied?
Many experimental approaches:
- Trained athlete
- Exercising volunteer subject
(sedentary vs. active) - Experimental animals
- Isolated whole muscle
- Isolated single fibre (myocyte)
- Contractile proteins in a test-tube
Summation & tetanus
single, summation, unfuzed, fused
Exhaustion (fatigue) occurs at the intersection of what lines on force vs time graph.
max force
required output
Alterations in time taken to fatigue will occur with:
- Inc or dec in required force
- Inc or dec in maximum force muscle can produce
- Changes in the intrinsic fatiguability of muscles
How does a Fast-twitch fiber Type II, fatigue
(easily fatigued):
↓↓ Ca2+ and ↓↓ force
How does a Slow-twitch fibre Type I, Soleus fibre fatigue
fatigue-resistant):
Ca2+ and force relatively stable, even after
1000 pulses.
What kind of fiber?
predominantly anaerobic metabolism
- short bursts of fast contractions (e.g. sprinters)
Fast-twitch fiber (Type II, easily fatigued):
What kind of fiber?
predominantly aerobic metabolism
- rich in capillaries and mitochondria (dark)
- continuous extended contractions over time (e.g. marathon
runners)
Slow-twitch fibre (Type I, fatigue-resistant e.g. Soleus)
Fatigue at the Cellular level
• Changes in pH (due to accumulation of waste products) • Accumulation of phosphate (Pi) • Decreased Gibbs free energy of ATP • Excitation-contraction coupling impairment
Effects of decreased pHi
• At rest pHi ~7.05, following exhaustive exercise pHi ~6.5
• Competition of H+ with Ca2+ for binding sites on
Troponin-C – right-shift of the Force-[Ca2+]i relation
= ↓ Ca2+ sensitivity of the myofilaments
• Inhibition of Na-K-ATPase, myosin ATPase, cross-bridge
interaction
HOWEVER:
Following exhaustive exercise, ____recovers faster than ___
indicating that pH may contribute to fatigue but cannot be the sole cause.
force
pH
_________________
imaging is used to quantify Pi and PCr
Nuclear magnetic resonance (NMR)
Pi and PCr levels during fatigue?
- ↓ PCr with exercise
- ↑ Pi with exercise
- coincident with ↓ force production
Possible Actions of Pi during fatigue
• “Direct” inhibition of rotation of the actomyosin cross-bridge
(Pi is bound to myosin head)
• Effects (direct or indirect) on SR Ca2+
release and Ca2+-force dependence
- reduce Ca2+ release (inhibit RyR,
precipitation Ca2+-Pi )
- increase Ca2+ force activation threshold
• “Indirect” energetic effect:
- reduction of Free Energy of ATP hydrolysis (ΔGATP)
Role of Gibbs Free Energy (ΔGATP)
∆GATP is the energy released by ATP hydrolysis (kJ/mol)
• Depends on the concentrations of reactants and products
• ∆GATP is usually negative, because it is releasing energy
Gibbs Free Energy (ΔGATP)
The more negative GATP, the more ?
energy is transferred.
∆G required by ATPases is positive b/c they gain energy from
ATP
How can ΔGATP diminish while [ATP] remains constant?
as [CrP] falls, [Pi] rises but [ATP] and [ADP] stay constant.
High value of K means the reactions are driven far to the right and [\_\_\_] maintained at the expense of [\_\_\_].
ATP
PCr
Exhaustive Exercise
Increased [Pi
] will cause a decrease in
ΔGATP
Exhaustive Exercise
• If ΔGATP falls sufficiently then work of ATPases will be
compromised.
Exhaustive Exercise
• Metabolic end-products may inhibit
muscle activation
& force development.
Fatigue: excitation-contraction coupling 1. Na+ and K+ ionic gradients not fully restored = ? 2. Signal to open Ca2+ channels is ? 3. Inhibition of SERCA pump = 4. ↓ Ca transient =
impaired membrane excitability
impaired
↓SR Ca stores,
↓ force
Effects of aerobic training occur WHERE and WHAT?
Heart ↑ cardiac output
Increases O2 delivery to the muscles
Circulation ↑ plasma volume
Increases stroke volume and thus cardiac output.
Increases skin blood flow to optimise thermoregulation
Blood
vessels
↑ capillary proliferation
Increases O2 delivery and diffusion into the muscles
Myocytes ↑ mitochondria
Increases O2 extraction
Myocytes Enzyme adaptation
Optimizes metabolism
Increased reliance on fat thus reduced lactate production
Many potential sites of fatigue between the
• Can be either “_____” or “_________”.
brain and the contractile protein interactions.
central
peripheral
• Peripheral (cellular) fatigue may be a result of:
- Decreased energy supply and GATP
- Increased end-product accumulation
- Excitation-contraction coupling impairment
• Training induces changes in
muscle structure and function to optimize O2 delivery and extraction
