NEUROMUSCULAR FATIGUE - PowerPoint PPT Presentation

View by Category
About This Presentation
Title:

NEUROMUSCULAR FATIGUE

Description:

NEUROMUSCULAR FATIGUE. In Exercise Physiology, neuromuscular ... Ironman Triathalon competition in Hawaii (same occurance) Depletion of Energy Substrates ... – PowerPoint PPT presentation

Number of Views:963
Avg rating:3.0/5.0
Slides: 31
Provided by: joeltc
Learn more at: http://www.uta.edu
Category:

less

Write a Comment
User Comments (0)
Transcript and Presenter's Notes

Title: NEUROMUSCULAR FATIGUE


1
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.

2
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?

3
Loss of Strength with Fatigue
  • Any volitional loss of strength during a
    sustained exercise is the basis of fatigue.

4
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.

5
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

6
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

7
Central versus Peripheral
  • Where does fatigue occur?
  • Central fatigue
  • Proximal to the motor unit
  • Peripheral fatigue
  • Residing within the motor unit

8
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

9
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)

10
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 cant 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?

11
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.

12
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

13
Four Factors Associated with the Decrease in
Force Production Due to H Accumulation
  1. H interferes with Ca release from the
    sarcoplasmic reticulum.
  2. H interferes with actin-myosin binding affinity
  3. H interferes with ATP hydrolysis
  4. H interferes with ATP production

14
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.

15
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).

16
3. ATP hydrolysis
  • H accumulation decreases the effectiveness of
    mATPase.
  • Why?

17
4. ATP production
  • H accumulation interferes with enzymes that
    catalyze reactions that produce ATP.
  • What is the rate limiting step in glycolysis?
  • Allosteric inhibition

18
Acid Removal
  • What are the two primary ways to clear H
    accumulation?
  • Increased blood flow
  • Buffering
  • What is the bodys primary blood buffer?

19
Depletion Hypothesis
  • 2 aspects to the depletion hypothesis
  • Neural depletion
  • Depletion of acetylcholine
  • Depletion of energy substrates
  • Phosphagen depletion
  • Glycogen depletion

20
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)

21
Depletion of Energy Substrates
  • 2 aspect of substrate depletion
  • Phosphagen depletion
  • Glycogen depletion

22
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

23
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 runners wall
  • What about glycogen supercompensation??

24
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

25
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?

26
(No Transcript)
27
Assignment for next week
  • Read handout
  • deVries Housh
  • Read Enoka, 2003 pgs. 374-389.
  • Prepare for questions next week over this lecture.

28
Course Projects
  • Pick one of the five neuromuscular disorders
  • Parkinsonism
  • Muscular/Myotonic Dystrophy
  • Cerebral Palsy
  • Low Back Pain
  • Peripheral neuropathy (generic)

29
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

30
Course Projects
  • Lectures given on
  • April 18
  • April 25
  • May 2
  • Choice must be made by next week.
About PowerShow.com