Chapter 14

Neuromuscular Fatigue

Key Concepts

• accumulation hypothesis

• central fatigue• depletion hypothesis• electromyography (E

MG)• electronic integrator• glycogen depletion• integration• muscle action

potential (MAP)• neuromuscular

fatigue

• peripheral fatigue• phosphagen

depletion• planimetry• qualitative

electromyography• quantitative

electromyography• runner’s wall• Setchenov

phenomenon

Review Questions

Which is more fatiguing: continuous isometric muscle actions or intermittent isometric muscle actions? Why?

Continuous isometric muscle actions are more

fatiguing because they occlude circulation,

whereas intermittent isometric muscle actions

allow blood to flow to the muscle.

What conclusion did Bigland-Ritchie et al. reach regarding the source of fatigue?

Both central and peripheral fatigue areimportant factors in loss of muscle force,

andthere are many individual differences

involved.

• Mechanisms underlying central fatigue occur mainly where?– Brain

• Mechanisms underlying peripheral fatigue occur mainly within what?– Motor units

Which is more useful: a passive rest break or a pause with diverting activity?

A break with diverting activity. This is referred

to as the Setchenov phenomenon.

What are three factors that contribute to central fatigue?

• Sensory feedback from working muscles

• Accumulation of ammonia• Increase in serotonin in the brain

Explain the accumulation hypothesis.• It describes the fatigue-causing

effects of the buildup of metabolites within muscle fibers.

What metabolites are significant in

the accumulation hypothesis?• Lactate, inorganic phosphate, and

ammonia

What happens as a result of lactate buildup?

• It interferes with muscle contraction in the following ways:– calcium release from the sarcoplasmic

reticulum– actin–myosin binding– ATP breakdown– ATP production

How does the depletion hypothesis explain peripheral fatigue?

This hypothesis states that fatigue results from

the depletion of the phosphagens or glycogen,

which are used to produce ATP.

• When does phosphagen depletion occur?– During very high intensity activity

• When does glycogen depletion occur?– During moderate-intensity activity over

a long period of time

What is the most likely explanation for runner’s wall?

It is likely caused by glycogen depletion in themost active fibers of the quadriceps muscles.

What role does temperature play in muscle fatigue?• Muscle fatigue occurs

at both low and high muscle temperatures.

• At high temperatures, metabolites accumulate rapidly in the muscle.

• Low muscle temperatures may affect neuromuscular electrical transmission or the contractile properties of the muscle.

What has EMG shown regarding the quantification of electrical activity within the skeletal muscles during submaximal fatiguing tasks?• Electrical activity increases over time

Why does this occur?• The increase results from the

recruitment of additional motor units and increases in motor unit firing rates.

Useful Websites

Neuromuscular Research Centerhttp://nmrc.bu.edu

Electromyography Fundamentalswww.gcmas.org/EMGfundamentals.pdf

Neurohaven—EMGwww.neurohaven.com/emg1.htm

EMG

http://moon.ouhsc.edu/dthompso/pk/emg/emg.htm