Fatigue (Year 2) Flashcards
How can we test fatigue at the performance level?
We could set up a maximal voluntary contraction & identify fatigue as the point at which the subject fails to “maintain the required or expected force”
Or as a “failure to continue working at a given exercise intensity”
Some complications?
A modest contraction (20% MVC) can be sustained for some time without apparent failure or fatigue. However, if MVC is performed at intervals, a progressive reduction will be seen.
best definition of fatigue
“any exercise-induced reduction in force generating capacity, reversible by rest.”
series of events leading from the brain to cross bridges generating force?
Signal sent from Motor cortex–>
down the axon to the spinal cord–>
lower motor neurons effect muscle fibers–>
action potential arrives at neuromuscular junction–>
ACh is released, binds to receptors and opens sodium ion channels leading to action potential in sarcolemma–>
action potential travels along the T tubules–>
calcium is released and binds to the troponin, thick and thin filament interaction leads to muscle contraction–>
muscle shortens and produced tension
What is Central fatigue?
a form of fatigue that occurs upstream of the neuromuscular junction. It is associated with changes in the synaptic concentration of the neurotransmitters within the CNS and an inability to maximally drive muscle contraction.
What is Peripheral fatigue?
occurs downstream of the neuromuscular junction and involves changes in the muscle that cannot be overcome despite maximal voluntary contraction
Central & peripheral fatigue
Central fatigue:–Muscles are capable of greater output than the CNS is willing or able to request.
Peripheral fatigue: –Muscles are incapable of responding in the same manner as they were prior to the exercise that elicited the fatigue.
Assessing fatigue-Peripheral nerve stimulation
Electrode is placed over the anatomical nerve (such as the femoral nerve supplying the quadriceps).
Supra-maximal stimulation of the anatomical nerve activates the whole muscle
Allows comparison of peak force from an involuntary stimulated contraction to assess levels of peripheral fatigue pre & post exercise.
What is the M wave?
Electromyographic(EMG) response. “The M-wave represents the summated electrical activity of the motor units, whose motor axons were depolarized by the evoked stimulus”
Examples of M-max changes
- 10x10-sprints(30-rest)=50% reduction in M-max
* Cycling 80% VO2max for 30-min=No change
Twitch Interpolation-Central fatigue
- Deliver stimulus to nerve during voluntary contraction(interpolated twitch) was able to evoke additional force.
- In a fatigued state, a larger twitch is evoked during an MVC and voluntary activation is incomplete!
- Twitch interpolation provides a measure of central fatigue (lack of drive from CNS).
Central fatigue
Role of afferent feedback–Group III & IV afferents
- ↓firing frequency of the motor neuron
- An inhibition or facilitation of the motor neuron
- An inhibition of the motor cortex neuron
- Example:–Elevations in H+ or core temp can reduce muscle recruitment level
