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Neuromuscular Fatigue


... for continuous isometric, intermittent isometric, and dynamic muscle ... to exhaustion and muscle temperature during submaximal isometric muscle actions ... – PowerPoint PPT presentation

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Title: Neuromuscular Fatigue

Chapter 14
  • Neuromuscular Fatigue

What is neuromuscular fatigue?
  • A temporary decrease in muscular performance seen
    as a failure to maintain or develop an expected
    force or power output.
  • Central fatigue occurs mainly in the brain.
  • Peripheral fatigue occurs in the motor neurons,
    the peripheral nerves, the motor endplates, and
    the muscle fibers.

Effects of fatigue
  • Causes loss of muscle strength
  • Causes reflexes to fade with repeated stimulation
  • Degrades coordination of complex movements

The relationships between force production and
time to exhaustion for continuous isometric,
intermittent isometric, and dynamic muscle actions
Central versus peripheral causes of fatigue
  • Central factors are not involved to any great
    extent in fatigue during high-intensity exercise.
  • Fatigue in low-intensity effort over one or two
    hours is likely a failure in nervous system drive.

Factors contributing to central fatigue
  • Sensory feedback from working muscles
  • Accumulation of ammonia
  • Increase in serotonin in the brain

Setchenov phenomena
  • More work can be produced after a pause with
    diverting activity than after a passive rest
  • Sensory feedback of nerve impulses from fatigued
    muscles impinges on the reticular formation in
    the brain and inhibits voluntary effort.
  • Diverting activities, such as exercise with
    non-fatigued muscle group, produce an increased
    inflow of impulse from non-fatigued body parts to
    the reticular formation, shifting balance between
    inhibition toward facilitation.

Accumulation hypothesis
  • Build up of fatigue-causing metabolites within
    muscle fibers
  • Build-up of lactate interferes with muscle
    contraction by affecting
  • Calcium release from the saracoplasmic reticulum
  • Actin-myosin binding
  • ATP breakdown
  • ATP production

Depletion hypothesis
  • Depletion of fuels used to produce ATP
  • Phosphagen depletionduring very high intensity
  • Glycogen depletionover a long period of time
    during moderate-intensity exercise

Temperature and muscle fatigue
  • Muscle fatigue occurs at both low and high muscle
  • At high temperatures, metabolites accumulate in
    the muscle.
  • Low muscle temperatures may affect neuromuscular
    electrical transmission of the contractile
    properties of the muscle.

The relationship between time to exhaustion and
muscle temperature during submaximal isometric
muscle actions
  • Records muscle action potentials (MAPs)
  • Qualitative electromyography--Developed for use
    of physicians in diagnosing abnormal
    neuromuscular function
  • Quantitative electromyographyStudy of amount of
    electrical activity present in a given muscle
    under varying conditions

Differences between recordings of single
motor-unit potentials with needle electrodes
(left) and summated potentials from many motor
units with surface electrodes (right)
Integration of muscle action potentials to get
true mean value of their amplitude.
Electromyography and fatigue
  • Electrical activity in muscle tissue increases
    over time during submaximal fatiguing tasks.
  • Increase results from recruitment of additional
    motor units and increases in the firing rate of
    already activated motor units to make up for the
    force that is lost as motor units fatigue.

The relationship between EMG voltage and time to
exhaustion during fatiguing tasks with increasing
force outputs
The relationship between EMG voltage and time at
non-fatiguing and fatiguing power outputs during
cycle ergometry