Neuromuscular Fatigue Chapter 14

Learning Objectives Be able to define neuromuscular fatigue. Understand basic concepts regarding the central and peripheral causes of fatigue. Be able to describe and discuss the task-dependency model, central governor model, accumulation hypothesis, and depletion hypothesis of fatigue. Understand the basic concepts of electromyography and the electromyographic observations of fatigue.

Neuromuscular Fatigue A temporary decrease in muscular performance A failure to maintain or develop an expected force or power output

Effects of Fatigue Causes loss of muscle strength Causes reflexes to fade with repeated stimulation Degrades coordination of complex movements seen in athletics

Force Production and Time to Exhaustion for Different Muscle Actions

Causes of Fatigue Central or peripheral mechanisms Central fatigue: Mainly in the brain, proximal to motor neurons Peripheral fatigue: In motor units (peripheral nerves, muscle fibers, etc.)

Task-Dependency Model Both central and peripheral mechanisms contribute to fatigue. Cause depends on characteristics of exercise being performed. Physiological changes in the neuromuscular system also depend on the task involved.

Sites of Fatigue in the Neuromuscular System

Possible Factors Contributing to Central Fatigue Sensory feedback from working muscles Accumulation of ammonia Increase in serotonin in the brain Or, some combination of all

Setchenov Phenomenon Definition:  More work can be produced after a pause with diverting activity than after a passive rest pause. Diverting activities produce an increased inflow of nerve impulses from non-fatigued parts of the body.

Your Perspective When you are tired, do you prefer to rest, or do you like to do something new and different? Will knowing about the Setchenov phenomenon help you to change your habits? Imagine you are the coach of a high school track team, and the members of the team complain about being tired. What do you advise them?

Central Governor Model The central nervous system regulates exercise performance to ensure that catastrophic physiological failure does not occur. Redefines fatigue; not a physical event but a sensation or emotion A form of central fatigue

Possible Mechanisms Underlying Peripheral Fatigue Accumulation of metabolites Depletion of energy substrates (fuels)

Accumulation and Depletion Hypotheses Accumulation hypothesis:  Postulates that fatigue- causing metabolites have accumulated in muscle fibers: Lactate Inorganic phosphate Ammonia Depletion hypothesis:  Describes fatigue in terms of depleted fuels used to produce ATP: Phosphagen Glycogen

Your Perspective Have you ever hit the wall (i.e., experienced “runner’s wall”) while running or bicycling long distances? How did you work through it?

Muscle Temperature and Fatigue Muscle fatigue occurs at both low and high muscle temperatures: High temps: Metabolites accumulate in activated muscle fibers and reduce endurance. Low temps: Neuromuscular electrical transmission of the contractile properties of the muscle may lead to fatigue.

Relationship Between Time to Exhaustion and Muscle Temperature

Myoelectric Manifestations of Fatigue Time domain:  Electrical activity in muscle tissue increases over time during submaximal fatiguing tasks. Frequency domain:  Neuromuscular fatigue results in a shift in the power density spectrum (PDS) for the surface EMG signal to lower frequencies.

Relationship Between EMG Voltage and Time to Exhaustion, with Increasing Force Outputs

Effect of Fatigue on the PDS of the Surface EMG Signal

Where to Learn More Fatigue in exercise:  www- rohan.sdsu.edu/dept/coachsci/csa/vol65/table.htm www- rohan.sdsu.edu/dept/coachsci/csa/vol65/table.htm Neuromuscular Research Center: 