NEUROMUSCULAR NEUROMUSCULAR FATIGUEFATIGUE

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

Importance of Importance of Neuromuscular FatigueNeuromuscular Fatigue

Does ODoes O22 delivery alone limit exercise delivery alone limit exercise performance?performance? Is it just OIs it just O22 transport and O transport and O22 fuel utilization? fuel utilization? Have we adequately explored other areas Have we adequately explored other areas

relating to muscle contractile function?relating to muscle contractile function? TD Noakes – South AfricaTD Noakes – South Africa

Only 50% of VOOnly 50% of VO22 max trials result in a max trials result in a plateau – is there really a plateau?plateau – is there really a plateau?

Is fatigue biochemical or CNS controlled Is fatigue biochemical or CNS controlled anticipatory response?anticipatory response?

Loss of Strength with Loss of Strength with FatigueFatigue

Any volitional Any volitional loss of strength loss of strength during a during a sustained sustained exercise is the exercise is the basis of fatigue.basis of fatigue.

Effect of Fatigue on Effect of Fatigue on Reflexes and Reflexes and CoordinationCoordination

A reflex arc is fatigable.A reflex arc is fatigable. If a reflex arc is stimulated repeatedly – it will If a reflex arc is stimulated repeatedly – it will

eventually fail to elicit any type of expected eventually fail to elicit any type of expected reflex response.reflex response. The more interneurons and synapses involved, The more interneurons and synapses involved,

the more quickly it may become fatigued.the more quickly it may become fatigued.

Coordination can be viewed the same Coordination can be viewed the same wayway Irradiation of motor impulses to neighboring Irradiation of motor impulses to neighboring

motor nerve centers – coordination is lost.motor nerve centers – coordination is lost.

Effect of Fatigue on Effect of Fatigue on Industrial WorkersIndustrial Workers

How much work can How much work can be done in an 8-hour be done in an 8-hour time period without time period without fatigue?fatigue? Static work is more Static work is more

fatiguing than fatiguing than dynamic workdynamic work Blood flowBlood flow Rest periodsRest periods

Basic Nature of FatigueBasic Nature of Fatigue

Relationship between intensity of work Relationship between intensity of work and endurance appears to be a and endurance appears to be a fundamental characteristic of fundamental characteristic of performance…performance… Is there some equation that can be Is there some equation that can be

universally applied to calculate the highest universally applied to calculate the highest sustainable workload?sustainable workload? Physical Working Capacity at Fatigue ThresholdPhysical Working Capacity at Fatigue Threshold PWCPWCFTFT

Central versus PeripheralCentral versus Peripheral

Where does fatigue occur?Where does fatigue occur? Central fatigue Central fatigue

Proximal to the motor unitProximal to the motor unit

Peripheral fatiguePeripheral fatigueResiding within the motor unitResiding within the motor unit

Central FatigueCentral Fatigue

Brain and spinal cord; CNS fatigueBrain and spinal cord; CNS fatigue Studies that used voluntary exhaustion and Studies that used voluntary exhaustion and

then additional electrical stimulationthen additional electrical stimulation After voluntary exhaustion, electrical stimulation After voluntary exhaustion, electrical stimulation

evoked sizable force productionevoked sizable force production Central location of fatigueCentral location of fatigue

Peripheral FatiguePeripheral Fatigue

Fatigue occurring within the local motor Fatigue occurring within the local motor unit; local fatigueunit; local fatigue Studies that fatigued a muscle with electrical Studies that fatigued a muscle with electrical

stimulation to the point of no muscle twitchstimulation to the point of no muscle twitch Muscle action potentials were relatively Muscle action potentials were relatively

unaffectedunaffected Peripheral location of fatigue (but not at the NMJ)Peripheral location of fatigue (but not at the NMJ)

So, where does fatigue So, where does fatigue occur?occur?

In both central and peripheral locations.In both central and peripheral locations. The location of fatigue is intensity-dependentThe location of fatigue is intensity-dependent

Lower-intensity, longer duration fatigue will primarily occur Lower-intensity, longer duration fatigue will primarily occur centrallycentrally

Higher-intensity, short duration fatigue will primarily occur Higher-intensity, short duration fatigue will primarily occur peripherallyperipherally

Example Example Why does pedaling rate decrease during Why does pedaling rate decrease during the Wingate test?the Wingate test?

Example Example Why can’t we do another repetition after a Why can’t we do another repetition after a 5RM lift?5RM lift?

Example Example Why do we slow down during the course of Why do we slow down during the course of a 1600 m race? Do we slow down?a 1600 m race? Do we slow down?

What Causes Fatigue?What Causes Fatigue?

There are two hypotheses:There are two hypotheses: The Accumulation hypothesisThe Accumulation hypothesis The Depletion hypothesisThe Depletion hypothesis

The origin of fatigue is exercise-The origin of fatigue is exercise-dependent and may be due to either dependent and may be due to either accumulation, depletion, or both.accumulation, depletion, or both.

Accumulation HypothesisAccumulation Hypothesis There is a buildup of metabolic by-products in There is a buildup of metabolic by-products in

the muscle fiberthe muscle fiber Lactic acid (lactate)Lactic acid (lactate) Hydrogen ions (HHydrogen ions (H++)) AmmoniaAmmonia Inorganic phosphateInorganic phosphate

Lactate is the primary marker associated with Lactate is the primary marker associated with the accumulation hypothesisthe accumulation hypothesis

If you exercise at a high enough intensity, HIf you exercise at a high enough intensity, H++ accumulation interferes with force productionaccumulation interferes with force production Applies to maximal exercise for 20 sec Applies to maximal exercise for 20 sec 3 minutes 3 minutes

Four Factors Associated with the Four Factors Associated with the Decrease in Force Production Due Decrease in Force Production Due to Hto H++ Accumulation Accumulation

1.1. HH++ interferes with Ca interferes with Ca++++ release from the release from the sarcoplasmic reticulum.sarcoplasmic reticulum.

2.2. HH++ interferes with actin-myosin binding interferes with actin-myosin binding affinityaffinity

3.3. HH++ interferes with ATP hydrolysis interferes with ATP hydrolysis4.4. HH++ interferes with ATP production interferes with ATP production

1. Ca1. Ca++++ release from the release from the sarcoplasmic reticulumsarcoplasmic reticulum

Lactic acid (HLactic acid (H++) accumulation disrupts the ) accumulation disrupts the release of Carelease of Ca++++ from the sarcoplasmic from the sarcoplasmic reticulum, in part, by changing the reticulum, in part, by changing the membrane potential (ICF vs. ECF)membrane potential (ICF vs. ECF)

When CaWhen Ca++++ is not released as effectively, is not released as effectively, less is available to bind with troponin-C.less is available to bind with troponin-C.

2. Actin-myosin binding 2. Actin-myosin binding affinityaffinity

Actin and myosin do not bind as readily Actin and myosin do not bind as readily or as “tightly” in an increased acidic or as “tightly” in an increased acidic cellular environment (i.e., cellular environment (i.e., microenvironment).microenvironment).

3. ATP hydrolysis3. ATP hydrolysis

HH++ accumulation decreases the accumulation decreases the effectiveness of mATPase.effectiveness of mATPase.

Why?Why?

4. ATP production4. ATP production

HH++ accumulation interferes with enzymes accumulation interferes with enzymes that catalyze reactions that produce ATP.that catalyze reactions that produce ATP. What is the rate limiting step in glycolysis?What is the rate limiting step in glycolysis? Allosteric inhibition:Allosteric inhibition:

Acid RemovalAcid Removal

What are the two primary ways to clear What are the two primary ways to clear HH++ accumulation? accumulation? Increased blood flowIncreased blood flow BufferingBuffering

What is the body’s primary blood buffer?What is the body’s primary blood buffer?

Depletion HypothesisDepletion Hypothesis

2 aspects to the depletion hypothesis:2 aspects to the depletion hypothesis: Neural depletionNeural depletion

Depletion of acetylcholineDepletion of acetylcholine

Depletion of energy substratesDepletion of energy substrates Phosphagen depletionPhosphagen depletion Glycogen depletionGlycogen depletion

Neural DepletionNeural Depletion Neural fatigue that is caused by a Neural fatigue that is caused by a

depletion of the stimulatory depletion of the stimulatory neurotransmitter ACh.neurotransmitter ACh. You can induce neural depletion in an excised You can induce neural depletion in an excised

muscle, but can this happen muscle, but can this happen in vivoin vivo?? Two possible instances where it might have Two possible instances where it might have

occurred:occurred: East German woman completing the final lap of a East German woman completing the final lap of a

marathonmarathon Ironman Triathalon competition in Hawaii (same Ironman Triathalon competition in Hawaii (same

occurance)occurance)

Depletion of Energy Depletion of Energy SubstratesSubstrates

2 aspect of substrate depletion:2 aspect of substrate depletion: Phosphagen depletionPhosphagen depletion Glycogen depletionGlycogen depletion

Phosphagen DepletionPhosphagen Depletion 2 aspects to phosphagen depletion:2 aspects to phosphagen depletion:

1.1. Reduction in ATPReduction in ATP Small ATP stores in skeletal muscleSmall ATP stores in skeletal muscle Enough to provide 2 – 3 seconds of maximal Enough to provide 2 – 3 seconds of maximal

muscular contractionmuscular contraction Used quicklyUsed quickly

2.2. Depletion of phosphocreatine (PC)Depletion of phosphocreatine (PC) Enough PC stored to provide up to 20 – 30 Enough PC stored to provide up to 20 – 30

seconds of maximal muscular contractionseconds of maximal muscular contraction Nearly completely depleted during maximal exercise Nearly completely depleted during maximal exercise

Glycogen DepletionGlycogen Depletion Glycogen is a polymer of glucose that is created Glycogen is a polymer of glucose that is created

with glycogen synthasewith glycogen synthase Glycogen is stored in relatively large amounts in Glycogen is stored in relatively large amounts in

skeletal muscle.skeletal muscle. About 2,000 kcals of energy stored in the form of glycogen About 2,000 kcals of energy stored in the form of glycogen

(skeletal muscle)(skeletal muscle) Where are the two primary locations for glycogen storage in the Where are the two primary locations for glycogen storage in the

body?body? It takes approximately 100 kcals to run a mile, so we have It takes approximately 100 kcals to run a mile, so we have

enough glycogen stored for about 20 miles of running.enough glycogen stored for about 20 miles of running. Glycogen depletion occurs during long-term activities Glycogen depletion occurs during long-term activities

that are done at a medium to moderate intensitythat are done at a medium to moderate intensity When this occurs, the body is forced to use alternative When this occurs, the body is forced to use alternative

energy sources (that are not as powerful as glucose energy sources (that are not as powerful as glucose metabolism)metabolism)

Example: “Hitting the runner’s wall”Example: “Hitting the runner’s wall” What about glycogen supercompensation??What about glycogen supercompensation??

Muscle Temperature Muscle Temperature Effect on FatigueEffect on Fatigue

Optimal deep muscle temperature Optimal deep muscle temperature between 80 - 86between 80 - 86 F F At 103, the endurance time decreased 65%At 103, the endurance time decreased 65%

Due to metabolite accumulation or temperature Due to metabolite accumulation or temperature effects of protein/enzyme function (titration).effects of protein/enzyme function (titration).

At 68, the endurance time decreased 80%At 68, the endurance time decreased 80% Due to interference with neuromuscular Due to interference with neuromuscular

transmissiontransmission

Electromyographic Electromyographic Observations of FatigueObservations of Fatigue

EMG Amplitude (submaximal workloads)EMG Amplitude (submaximal workloads) Increases linearly with exhaustionIncreases linearly with exhaustion PWCPWCFTFT

EMG Amplitude (maximal workload)EMG Amplitude (maximal workload) Remains constant or decreases with exhaustionRemains constant or decreases with exhaustion

““Muscle Wisdom” hypothesisMuscle Wisdom” hypothesis

EMG Frequency (max and submax)EMG Frequency (max and submax) Decreases…Decreases… Why?Why?

Assignment for next Assignment for next weekweek

Read handoutRead handout deVries & HoushdeVries & Housh

Read Enoka, 2003 pgs. 374-389.Read Enoka, 2003 pgs. 374-389. Prepare for questions next week over this Prepare for questions next week over this

lecture.lecture.

Course ProjectsCourse Projects

Pick one of the five neuromuscular Pick one of the five neuromuscular disorders:disorders: ParkinsonismParkinsonism Muscular/Myotonic DystrophyMuscular/Myotonic Dystrophy Cerebral PalsyCerebral Palsy Low Back PainLow Back Pain Peripheral neuropathy (generic)Peripheral neuropathy (generic)

Course ProjectsCourse Projects

Give a 50-min lecture on the neuromuscular Give a 50-min lecture on the neuromuscular disorder that you chosedisorder that you chose EtiologyEtiology PathologyPathology Common signs / symptomsCommon signs / symptoms How does it affect motor unit function?How does it affect motor unit function? Describe how we could investigate this disorder with Describe how we could investigate this disorder with

surface EMG and MMG:surface EMG and MMG: Collect pilot data and report your results on 4 or 5 healthy Collect pilot data and report your results on 4 or 5 healthy

subjectssubjects Extrapolate your findings to the diseased subjectsExtrapolate your findings to the diseased subjects

Course ProjectsCourse Projects

Lectures given on:Lectures given on: April 18April 18 April 25April 25 May 2May 2

Choice must be made by next week.Choice must be made by next week.