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NEUROMUSCULAR FATIGUE. In Exercise Physiology, neuromuscular fatigue can be defined as a transient decrease in muscular performance usually seen as a failure to maintain or develop a certain expected force or power. Importance of Neuromuscular Fatigue.

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neuromuscular fatigue
NEUROMUSCULAR FATIGUE
  • In Exercise Physiology, neuromuscular fatigue can be defined as a transient decrease in muscular performance usually seen as a failure to maintain or develop a certain expected force or power.
importance of neuromuscular fatigue
Importance of Neuromuscular Fatigue
  • Does O2 delivery alone limit exercise performance?
    • Is it just O2 transport and O2 fuel utilization?
    • Have we adequately explored other areas relating to muscle contractile function?
  • TD Noakes – South Africa
    • Only 50% of VO2 max trials result in a plateau – is there really a plateau?
    • Is fatigue biochemical or CNS controlled anticipatory response?
loss of strength with fatigue
Loss of Strength with Fatigue
  • Any volitional loss of strength during a sustained exercise is the basis of fatigue.
effect of fatigue on reflexes and coordination
Effect of Fatigue on Reflexes and Coordination
  • A reflex arc is fatigable.
    • If a reflex arc is stimulated repeatedly – it will eventually fail to elicit any type of expected reflex response.
      • The more interneurons and synapses involved, the more quickly it may become fatigued.
  • Coordination can be viewed the same way
    • Irradiation of motor impulses to neighboring motor nerve centers – coordination is lost.
effect of fatigue on industrial workers
Effect of Fatigue on Industrial Workers
  • How much work can be done in an 8-hour time period without fatigue?
    • Static work is more fatiguing than dynamic work
      • Blood flow
      • Rest periods
basic nature of fatigue
Basic Nature of Fatigue
  • Relationship between intensity of work and endurance appears to be a fundamental characteristic of performance…
    • Is there some equation that can be universally applied to calculate the highest sustainable workload?
      • Physical Working Capacity at Fatigue Threshold
      • PWCFT
central versus peripheral
Central versus Peripheral
  • Where does fatigue occur?
    • Central fatigue
      • Proximal to the motor unit
    • Peripheral fatigue
      • Residing within the motor unit
central fatigue
Central Fatigue
  • Brain and spinal cord; CNS fatigue
    • Studies that used voluntary exhaustion and then additional electrical stimulation
      • After voluntary exhaustion, electrical stimulation evoked sizable force production
      • Central location of fatigue
peripheral fatigue
Peripheral Fatigue
  • Fatigue occurring within the local motor unit; local fatigue
    • Studies that fatigued a muscle with electrical stimulation to the point of no muscle twitch
      • Muscle action potentials were relatively unaffected
      • Peripheral location of fatigue (but not at the NMJ)
so where does fatigue occur
So, where does fatigue occur?
  • In both central and peripheral locations.
    • The location of fatigue is intensity-dependent
      • Lower-intensity, longer duration fatigue will primarily occur centrally
      • Higher-intensity, short duration fatigue will primarily occur peripherally
  • Example  Why does pedaling rate decrease during the Wingate test?
  • Example  Why can’t we do another repetition after a 5RM lift?
  • Example  Why do we slow down during the course of a 1600 m race? Do we slow down?
what causes fatigue
What Causes Fatigue?
  • There are two hypotheses:
    • The Accumulation hypothesis
    • The Depletion hypothesis
  • The origin of fatigue is exercise-dependent and may be due to either accumulation, depletion, or both.
accumulation hypothesis
Accumulation Hypothesis
  • There is a buildup of metabolic by-products in the muscle fiber
    • Lactic acid (lactate)
    • Hydrogen ions (H+)
    • Ammonia
    • Inorganic phosphate
  • Lactate is the primary marker associated with the accumulation hypothesis
  • If you exercise at a high enough intensity, H+ accumulation interferes with force production
    • Applies to maximal exercise for 20 sec  3 minutes
four factors associated with the decrease in force production due to h accumulation
Four Factors Associated with the Decrease in Force Production Due to H+ Accumulation
  • H+ interferes with Ca++ release from the sarcoplasmic reticulum.
  • H+ interferes with actin-myosin binding affinity
  • H+ interferes with ATP hydrolysis
  • H+ interferes with ATP production
1 ca release from the sarcoplasmic reticulum
1. Ca++ release from the sarcoplasmic reticulum
  • Lactic acid (H+) accumulation disrupts the release of Ca++ from the sarcoplasmic reticulum, in part, by changing the membrane potential (ICF vs. ECF)
  • When Ca++ is not released as effectively, less is available to bind with troponin-C.
2 actin myosin binding affinity
2. Actin-myosin binding affinity
  • Actin and myosin do not bind as readily or as “tightly” in an increased acidic cellular environment (i.e., microenvironment).
3 atp hydrolysis
3. ATP hydrolysis
  • H+ accumulation decreases the effectiveness of mATPase.
  • Why?
4 atp production
4. ATP production
  • H+ accumulation interferes with enzymes that catalyze reactions that produce ATP.
    • What is the rate limiting step in glycolysis?
    • Allosteric inhibition:
acid removal
Acid Removal
  • What are the two primary ways to clear H+ accumulation?
    • Increased blood flow
    • Buffering
      • What is the body’s primary blood buffer?
depletion hypothesis
Depletion Hypothesis
  • 2 aspects to the depletion hypothesis:
    • Neural depletion
      • Depletion of acetylcholine
    • Depletion of energy substrates
      • Phosphagen depletion
      • Glycogen depletion
neural depletion
Neural Depletion
  • Neural fatigue that is caused by a depletion of the stimulatory neurotransmitter ACh.
    • You can induce neural depletion in an excised muscle, but can this happen in vivo?
    • Two possible instances where it might have occurred:
      • East German woman completing the final lap of a marathon
      • Ironman Triathalon competition in Hawaii (same occurance)
depletion of energy substrates
Depletion of Energy Substrates
  • 2 aspect of substrate depletion:
    • Phosphagen depletion
    • Glycogen depletion
phosphagen depletion
Phosphagen Depletion
  • 2 aspects to phosphagen depletion:
    • Reduction in ATP
      • Small ATP stores in skeletal muscle
      • Enough to provide 2 – 3 seconds of maximal muscular contraction
        • Used quickly
    • Depletion of phosphocreatine (PC)
      • Enough PC stored to provide up to 20 – 30 seconds of maximal muscular contraction
        • Nearly completely depleted during maximal exercise
glycogen depletion
Glycogen Depletion
  • Glycogen is a polymer of glucose that is created with glycogen synthase
    • Glycogen is stored in relatively large amounts in skeletal muscle.
      • About 2,000 kcals of energy stored in the form of glycogen (skeletal muscle)
        • Where are the two primary locations for glycogen storage in the body?
      • It takes approximately 100 kcals to run a mile, so we have enough glycogen stored for about 20 miles of running.
    • Glycogen depletion occurs during long-term activities that are done at a medium to moderate intensity
      • When this occurs, the body is forced to use alternative energy sources (that are not as powerful as glucose metabolism)
      • Example: “Hitting the runner’s wall”
      • What about glycogen supercompensation??
muscle temperature effect on fatigue
Muscle Temperature Effect on Fatigue
  • Optimal deep muscle temperature between 80 - 86 F
    • At 103, the endurance time decreased 65%
      • Due to metabolite accumulation or temperature effects of protein/enzyme function (titration).
    • At 68, the endurance time decreased 80%
      • Due to interference with neuromuscular transmission
electromyographic observations of fatigue
Electromyographic Observations of Fatigue
  • EMG Amplitude (submaximal workloads)
    • Increases linearly with exhaustion
    • PWCFT
  • EMG Amplitude (maximal workload)
    • Remains constant or decreases with exhaustion
      • “Muscle Wisdom” hypothesis
  • EMG Frequency (max and submax)
    • Decreases…
    • Why?
assignment for next week
Assignment for next week
  • Read handout
    • deVries & Housh
  • Read Enoka, 2003 pgs. 374-389.
  • Prepare for questions next week over this lecture.
course projects
Course Projects
  • Pick one of the five neuromuscular disorders:
    • Parkinsonism
    • Muscular/Myotonic Dystrophy
    • Cerebral Palsy
    • Low Back Pain
    • Peripheral neuropathy (generic)
course projects29
Course Projects
  • Give a 50-min lecture on the neuromuscular disorder that you chose
    • Etiology
    • Pathology
    • Common signs / symptoms
    • How does it affect motor unit function?
    • Describe how we could investigate this disorder with surface EMG and MMG:
      • Collect pilot data and report your results on 4 or 5 healthy subjects
      • Extrapolate your findings to the diseased subjects
course projects30
Course Projects
  • Lectures given on:
    • April 18
    • April 25
    • May 2
  • Choice must be made by next week.