W5-L7: Muscular fatigue Flashcards
What is Muscular Fatigue?
Loss in the capability to generate skeletal muscle force and/or velocity that is accompanied by recovery during rest
Is reversible with rest
What are the 2 types of fatigues?
Central Fatigue - before NMJ
Peripheral Fatigue - after NMJ
What is Central Fatigue?
Central fatigue is seen at the higher levels of motor input and transmission, activation of lower motor neurons, down to the propagation of the neural signal.
Central fatigue is a reduction in the ability of the central nervous system to activate muscles, leading to decreased muscle performance and endurance during prolonged physical or mental activity.
What is Peripheral Fatigue?
Peripheral fatigue is the reduction in muscle performance and endurance due to factors within the muscles themselves, such as depletion of energy stores, accumulation of metabolic byproducts, or impaired excitation-contraction coupling, occurring after the neuromuscular junction.
Fatigue occurs from a disruption in the chain of events between the CNS and muscle fibers.
How do Muscles Fatigue?
What are 3 factors they depend on?
Muscle fiber composition:
- Type I vs Type II
Type and intensity of contraction:
- Central fatigue is more present in endurance exercise
- Peripheral fatigue is more present high intensity exercise
Available oxygen (Enoka & Stuart, 1992)
- Lowering oxygen availability will increase the development of fatigue
- The degree of fatigue seems to be similar, but time exercising/intervening is shorter
- Increasing the FiO2 will decrease fatigability
Central Fatigue - Neurotransmitters
How do NT’s play a role in central fatigue?
Increased Dopamine decreases fatigue
Increased Serotonin increases fatigue
During prolonged exercise increased brain serotonergic activity may augment lethargy and loss of drive, resulting in a reduction in motor unit recruitment
slide 72
What mechanisms contribute to central fatigue during exercise?
Central fatigue involves a decrease in the excitatory drive to lower motor neurons, where afferent signals from mechanoreceptors and metaboreceptors in exercising muscles send signals to higher brain centers. This feedback depresses the motor unit firing rate of efferent signals to the muscles, reducing muscle activation and performance.
Central and peripheral fatigue are related
Hypothesis: peripheral fatigue only develops up to a threshold unique for each individual
- Constant load endurance exercise terminates voluntary once you reach a ‘critical threshold’
- Time trial exercise reduces central motor drive because of III/IV afferents in the exercising muscle
slide 73 reviweq
Hypothesis Testing Results:
Study Design:
- Two Trials: Time to exhaustion (TTE) tests at 83% and 67% of peak power output.
- Peripheral Fatigue Assessment: Using magnetic femoral nerve stimulation.
- Severe Fatigue: Observed at 83% peak power output.
- Moderate Fatigue: Observed at 67% peak power output.
- Three 5-km TTE Trials: Conducted on separate days with different pre-exercise conditions.
Pre-Exercise Conditions:
1. TT-Ctrl: No preexisting fatigue prior to the 5km test.
2. TT-severe: Performed 83% peak power TTE (severe fatigue) prior to the 5km test.
3. TT-moderate: Performed 67% peak power TTE (moderate fatigue) prior to the 5km test.
Results Interpretation:
1. TT-Ctrl (No Preexisting Fatigue):
- Likely resulted in the best performance on the 5km TTE test as there was no prior fatigue impacting the effort.
-
TT-severe (Severe Preexisting Fatigue):
- Performance on the 5km TTE test likely decreased significantly due to the severe fatigue induced by the 83% TTE effort. This high level of fatigue would impair muscle function and endurance.
-
TT-moderate (Moderate Preexisting Fatigue):
- Performance on the 5km TTE test likely decreased, but to a lesser extent compared to the severe fatigue condition. The moderate fatigue from the 67% TTE effort would still impair performance, but not as drastically as the severe fatigue.
Summary:
The results indicate that preexisting fatigue levels significantly impact performance in subsequent exercise tests. Severe preexisting fatigue (83% TTE) leads to a greater decline in performance compared to moderate preexisting fatigue (67% TTE), and no preexisting fatigue yields the best performance. This highlights the importance of managing fatigue for optimal athletic performance.
TTE - time to exhastion
Testing the hypothesis Slide 75
Block III/IV afferents by injecting fentanyl intrathecally into L3- L4
The graph demonstrates that blocking type III/IV afferents with fentanyl reduces muscle fatigue during exercise, highlighting the role of these afferents in the fatigue process. In contrast, the control and placebo trials show no significant difference, indicating that the reduction in fatigue is specific to the blockade of these sensory pathways.
What 4 things cause peripherial fatigue?
Peripheral Fatigue – Muscle Milieu
How do metabolic byproducts contribute to fatigue
- High concentrations of [H+] and [Pi], directly impact the power stroke in the contractile mechanism in the fibers AND can decrease the sarcolemma sensitivity for [Ca2+]
- Inorganic phosphate [Pi] impair sarcolemma Ca2+ release
How does lactate contribute to fatugue?
Lactate is correlative, but not causative of fatigue
Lactic Acid – dissipates to [H+], increasing
the acidity of the muscle
How does Peripheral Fatigue affect Sarcolemma excitability?
Peripheral fatigue reduces sarcolemma excitability by causing ion imbalances, accumulation of metabolic byproducts, and decreased ATP availability, all of which impair the sarcolemma’s ability to maintain proper membrane potential and propagate action potentials.
Effect of Pi and H+ together!
How does inorganic phospahte disrupt muscle contaction?
Pi can compete with Ca2+ for troponin binding, decrease cross bridge formation
Muscle Fiber Contributions to fatigue
What are Ways to Confirm fatigue?
- Decline in performance of a maximal effort
- Sub max effort with brief maximal tests
- Sub max effort until task failure
