Fatigue Flashcards
Gandevia’s 2018 definition of fatigue?
The maximal force-generating capacity (MVC) of muscles starts to decline once exercise commences so that fatigue really begins almost at the onset of exercise and develops progressively before the muscle fails to perform the required task
Booth and Thomasson’s definition of fatigue?
A failure to continue working at a given exercise intensity
The need to distinguish between fatigue and exhaustion (Barry and Enoka 2007)
Although the impairment that contributes to fatigue will eventually limit the capacity of the individual to continue that task, fatigue and task failure is distinguishable
Typical power profile during a 3-min all-out test?
Peak power output achieved after 4-sec
The power output decline, however, exercise is maintained at the critical power within the first 30-sec to 1-min (fatigue threshold)
Define power output?
The amount of force that muscle can produce
The relationship between fatigue and high-intensity exercise?
Fatigue occurs very quickly following the onset of high-intensity exercise; however it doesn’t mean you failed at the task or became entirely exhausted, you are still able to perform physical exercise
The sensation of fatigue (Enoka & Stuart 1992)
Another significant, but less appreciated, interaction that influences motor performance involves the psychophysical phenomenon of the sense of effort
Define the sensation of fatigue?
Becoming aware of their exertion of the task, and eventually, you will disengage in that task (task disengagement) when RPE is 20
Outline the complexity of fatigue?
Fatigue is largely in the brain and in the conscious control of the individual
Fatigue is a multifaceted phenomenon
Fatigue is multi-factorial
A working definition of fatigue?
Any exercise-induced reduction in maximal voluntary force or power produced by a muscle or muscle group, regardless of whether or not the task can be sustained
NHLBI Workshop 1990 definition of fatigue?
A condition in which there is a loss in the capacity for developing force and/or velocity of a muscle, resulting from muscle activity under load and which is reversible by rest
What causes fatigue?
A loss in the muscle’s ability to produce force
How might fatigue arise? (Gandevia 2001)
Peripheral changes at the level of the muscle
The CNS fails to drive the motor neurons adequately
Define peripheral fatigue?
Fatigue resulting from the muscle itself that impairs the muscle contractility
Contributors to peripheral fatigue?
Excitation-contraction coupling inability Impaired calcium release and uptake Metabolite accumulation Substrate depletion Membrane excitability
How does excitation-contraction coupling inability contribute to peripheral fatigue?
Inability to converge electrical impulses that excite the muscle membrane resulting in the muscle not fully firing properly
How does impaired calcium release and uptake contribute to peripheral fatigue?
Impairs the interaction between actin and myosin to bring about a contraction which is caused by calcium release
How does metabolite accumulation contribute to peripheral fatigue?
Interferes with calcium release and sensitivity within the muslce which impairs the interaction between actin and myosin to bring about contraction
Limit membrane excitability which impairs the release of calcium
What metabolites interact with calcium to bring about peripheral fatigue?
Lactase acidosis
Inorganic phosphate
Adenosine diphosphate
Potassium
Define central fatigue?
When the CNS becomes impaired effecting electrical activity and propagation to a muscle
Subdivisions of central fatigue?
Supraspinal fatigue
Spinal fatigue
Define spinal fatigue?
Fatigue that originates in the spinal cord, resulting in the inability to appropriately conduct electrical impulses in the brain and the brain limits the amount of electrical motor output it is willing to generate
How does substrate depletion contribute to peripheral fatigue?
depletions in ATP and PCr during high intensity and muscle glycogen during endurance exercise limits fuel availability
Contributors to central fatigue?
Impaired central activation Reduced neural drive Ineffective motor unit recruitment Ineffective firing frequency Impaired muscle afferent feedback
How does impaired central activation contribute to central fatigue?
Impaired the excitability of the motor cortex
How does reduced neural drive contribute to central fatigue?
Less electrical impulses reaching the muscle
How do ineffective motor unit recruitment and firing frequency contribute to central fatigue?
Ineffective contraction of skeletal muscle
How does impaired muscle afferent feedback contribute to central fatigue?
Afferent feedback informs the CNS about the accumulation of metabolites and changes in oxygen levels, of which the CNS acts upon. It acts as a self-protection mechanism. If ineffective it will impact the ability to contract the muscle and could lead to lasting damage
Define multi-factorial?
Not limited by one thing
Consideration when assessing the mechanisms of fatigue development?
Neuromuscular Global fatigue Central fatigue Peripheral fatigue Voluntary activation percentage
How to assess the mechanism of fatigue through neuromuscular methods?
Measures the development of central and peripheral fatigue
Method -> electrical implise sent down the femoral nerve -> EMG assessing electrical activity -> measuring fource production
Define global fatigue?
Accounting for peripheral and central fatigue
How to assess central fatigue?
Perform an MVC
Assess the ability to maximally activate the muscle
How to assess peripheral fatigue?
Remove the contribution of the CNS
Potentiated twitch -> send an electrical impulse to the muscle and measure the amount of force production
How to assess supraspinal fatigue?
Transcranial magnetic stimulation
Magnetic stimulation of the motor cortex during an MVC (motor evoked potentials)
Provides a non-invasive measure of supraspinal fatigue
The mechanisms of fatigue are task-dependent? (Black et al. 2017)
Protocol
- exercise task to exhaustion
- take a muscle biopsy to assess pH, PCr, and muscle glycogen
Results
- muscle glycogen limits long-duration exercise performance more
- pH, lactate and PCr limits short duration exercise more (development of acidosis and depletion of PCr)
The mechanisms of fatigue are task-dependent? (Thomas et al. 2015)
Protocol - 4km, 20km, and 40km time trial
Results
- central and peripheral fatigue occurs in al exercise, however, the % contribution changes depending on the type of exercise task and the environmental conditions
- peripheral fatigue limits short-duration exercise performance more
- central fatigue limits long-duration exercise performance more
