2Objectives Identify factors affecting maximal performance. Provide evidence for and against the central nervous system being a site of fatigue.Identify potential neural factors in the periphery that may be linked to fatigue.Explain the role of cross-bridge cycling in fatigue.Summarize the evidence on the order of recruitment of muscle fibers with increasing intensities of activity and the type of metabolism upon which each is dependent.
3ObjectivesDescribe the factors limiting performance in all-out activities lasting less than ten seconds.Describe the factors limiting performance in all-out activities lasting 10 to 180 seconds.Discuss the subtle changes in the factors affecting optimal performance as the duration of maximal performance increase from three minutes to four hours.
4Outline Sites of Fatigue Factors Limiting All-Out Aerobic Performances Central FatiguePeripheral FatigueFactors Limiting All-Out Anaerobic PerformancesUltra Short-Term Performances (Less than Ten Seconds)Short-Term Performances (10 to 180 Seconds)Factors Limiting All-Out Aerobic PerformancesModerate-Length Performances (Three to Twenty Minutes)Intermediate-Length Performances (Twenty-One to Sixty Minutes)Long-Term Performances (One to Four Hours)Athlete as Machine
5Factors Affecting Performance Sites of FatigueFactors Affecting PerformanceFigure 19.1
6Sites of Fatigue Fatigue Inability to maintain power output or force during repeated muscle contractionsCentral fatigueCentral nervous systemPeripheral fatigueNeural factorsMechanical factorsEnergetics of contraction
8Central Fatigue Reduction in motor units activated Sites of FatigueCentral FatigueReduction in motor units activatedReduction in motor unit firing frequencyCentral nervous system arousal can alter the state of fatigueBy facilitating motor unit recruitmentIncreasing motivationPhysical or mental diversionExcessive endurance training (overtraining)Reduced performance, prolonged fatigue, etc.Related to brain serotonin activity“Central Governor” modelConscious and subconscious brain, not spinal cord or motor unit
9Peripheral Fatigue: Neural Factors Sites of FatiguePeripheral Fatigue: Neural FactorsNeuromuscular junctionNot a site for fatigueSarcolemma and transverse tubulesAbility of muscle membrane to conduct an action potentialInability of Na+/K+ pump to maintain action potential amplitude and frequencyCan be improved by trainingAn action potential block in the T-tubulesReduction in Ca+2 release from sarcoplasmic reticulum
10Sites of FatigueIn SummaryIncreases in CNS arousal facilitate motor unit recruitment to increase strength and alter the state of fatigue.The ability of the muscle membrane to conduct an action potential may be related to fatigue in activities demanding a high frequency of stimulation.Repeated stimulation of the sarcolemma can result in a reduction in the size and frequency of action potentials; however, shifts in the optimal frequency needed for muscle activation preserve force output.Under certain conditions an action potential block can occur in the t-tubule to result in a reduction in Ca+2 release from the SR.
11Peripheral Fatigue: Mechanical Factors Sites of FatiguePeripheral Fatigue: Mechanical FactorsCross-bridge cycling and tension development depends on:Arrangement of actin and myosinCa+2 binding to troponinATP availabilityHigh H+ concentration may contribute to fatigueReduce the force per cross-bridgeReduce the force generated at a given Ca+2 concentrationInhibit Ca+2 release from SRLonger “relaxation time” is a sign of fatigueDue to slower cross-bridge cycling
12Sites of FatigueA Closer Look 19.1 Radical Production During Exercise Contributes to Muscle FatigueExercise promotes free radical formationMolecules that contain unpaired electron in outer orbitalCapable of damaging proteins, lipids, and DNACan contribute to fatigueDamage contractile proteins (myosin and troponin)Limits the number of cross-bridges in strong binding stateDepress sodium/potassium pump activityDisruption of potassium homeostasisOptimal levels of antioxidants can postpone fatigueHigh doses can impair muscle function
13Sites of FatigueIn SummaryThe cross-bridge ability to “cycle” is important in continued tension development. Fatigue may be related to the effect of a high H+ concentration on the ability of troponin to bind to Ca+2, the inability of the sarcoplasmic reticulum to take up Ca+2, or the lack of ATP needed to dissociate the cross-bridge from actin.
14Peripheral Fatigue: Energetics of Contraction Sites of FatiguePeripheral Fatigue: Energetics of ContractionImbalance ATP requirements and ATP generating capacityAccumulation of PiInhibits maximal forceReduces cross-bridge binding to actinInhibits Ca+2 release from SRRate of ATP utilization is slowed faster than rate of ATP utilizationMaintains ATP concentration
15Sites of FatigueIn SummaryFatigue is directly associated with a mismatch between the rate at which the muscle uses ATP and the rate at which ATP can be supplied.Cellular fatigue mechanisms slow down the rate of ATP utilization faster than the rate of ATP generation to preserve the ATP concentration and cellular homeostasis.
16Peripheral Fatigue: Energetics of Contraction Sites of FatiguePeripheral Fatigue: Energetics of ContractionMuscle fiber recruitment in increasing intensities of exerciseType I Type IIa Type IIxUp to 40% VO2 max type I fibers recruitedType IIa fibers recruited at 40–75% VO2 maxExercise >75% VO2 max requires IIx fibersResults in increased lactate production
17Order of Muscle Fiber Type Recruitment Sites of FatigueOrder of Muscle Fiber Type RecruitmentFigure 19.3
18Sites of FatigueIn SummaryMuscle fibers are recruited in the following order with increasing intensities of exercise: Type I Type IIa Type IIxThe progression moves from the most to the least oxidative muscle fiber type. Intense exercise (>75% VO2 max) demands that type IIx fibers be recruited, resulting in an increase in lactate production.
19Ultra Short-Term Performances Factors Limiting All-Out Anaerobic PerformancesUltra Short-Term PerformancesEvents lasting <10 secondsDependent on recruitment of Type II muscle fibersGenerate great forces that are neededMotivation, skill, and arousal are importantPrimary energy source is anaerobicATP-PC system and glycolysisCreatine supplementation may improve performance
20Factors Affecting Fatigue in Ultra Short-Term Events Factors Limiting All-Out Anaerobic PerformancesFactors Affecting Fatigue in Ultra Short-Term EventsFigure 19.4
21Factors Limiting All-Out Anaerobic Performances In SummaryIn events lasting less than ten seconds, optimal performance is dependent on the recruitment of appropriate type II fibers to generate the great forces needed.Motivation or arousal is required, as well as the skill needed to direct the force.The primary energy sources are anaerobic, with the focus on phosphocreatine.
22Short-Term Performances Factors Limiting All-Out Anaerobic PerformancesShort-Term PerformancesEvents lasting 10–180 secondsShift from anaerobic to aerobic metabolism70% energy supplied anaerobically at 10s60% supplied aerobically at 180sAnaerobic glycolysis is primary energy sourceResults in elevated lactate and H+ levelsInterferes with Ca+2 binding with troponinIngestion of buffers may improve performance
23Factors Affecting Fatigue in Short-Term Events Factors Limiting All-Out Anaerobic PerformancesFactors Affecting Fatigue in Short-Term EventsFigure 19.5
24Factors Limiting All-Out Anaerobic Performances In SummaryIn short-term performances lasting 10 to 180 seconds, there is a shift from 70% of the energy supplied anaerobically at 10 seconds to 60% being supplied aerobically at 180 seconds.Anaerobic glycolysis provides a substantial portion of the energy, resulting in elevated lactate levels.
25Moderate-Length Performances Factors Limiting All-Out Aerobic PerformancesModerate-Length PerformancesEvents lasting 3–20 minutes60% ATP generated aerobically at 3 min90% ATP supplied aerobically at 20 minHigh VO2 max is importantHigh maximal stroke volumeHigh arterial oxygen contentHemoglobin contentInspired oxygenRequires energy expenditure near VO2 maxType IIx fibers recruitedHigh levels of lactate and H+ accumulation
27Factors Limiting All-Out Aerobic Performances The Winning Edge 19.1 Is Maximal Oxygen Uptake Important in Distance Running Performance?VO2 max sets the upper limit for ATP production in endurance eventsEven though race is not run at 100% VO2 maxPerformance also determined by:%VO2 max at which runner can performEstimated by the lactate thresholdRunning economy
28Factors Limiting All-Out Aerobic Performances In SummaryIn moderate-length performances lasting three to twenty minutes, aerobic metabolism provides 60% to 90% of the ATP, respectively.These activities require an energy expenditure near VO2 max, with type II fibers being recruited.Any factor interfering with oxygen delivery (e.g., altitude or anemia) would decrease performance, since it is so dependent on aerobic energy production. High levels of lactate accompany these types of activities.
29Intermediate-Length Performances Factors Limiting All-Out Aerobic PerformancesIntermediate-Length PerformancesEvents lasting 21–60 minutesPredominantly aerobicUsually conducted at <90% VO2 maxHigh VO2 max is importantOther important factorsRunning economyHigh percentage of type I muscle fibersEnvironmental factorsHeatHumidityState of hydration
31Factors Limiting All-Out Aerobic Performances In SummaryIntermediate-length activities lasting twenty-one to sixty minutes are usually conducted at less than 90% VO2 max, and are predominantly aerobic.Given the length of the activity, environmental factors such as heat, humidity, and the state of hydration play a role in the outcome.
32Long-Term Performances Factors Limiting All-Out Aerobic PerformancesLong-Term PerformancesEvents lasting 1–4 hoursClearly aerobicEnvironmental factors more importantMaintaining rate of carbohydrate utilizationMuscle and liver glycogen stores declineIngestion of carbohydrateMaintain carbohydrate oxidation by the muscleConsumption of fluids and electrolytesDiet also influences performance
34Factors Limiting All-Out Aerobic Performances In SummaryIn long-term performances of one to four hours duration, environmental factors play a more important role as the muscle and liver glycogen stores try to keep up with the rate at which carbohydrate is used.Diet, fluid ingestion, and the ability of the athlete to deal with heat and humidity all influence the final outcome.
35Athlete as Machine Continuing goal to improve performance Potential to treat elite athletes like machinesCollection of parts evaluated by specialistsImplementation of research to improve performanceMay be exposing athletes to riskIn research or in implementation of techniquesInstitutional Review BoardsMinimize risk to subjects being studied
36Study Questions List the factors influencing performance. Is the limiting factor for strength development located in the CNS or out in the periphery? Support your position.Tracing the path the action potential takes from the time it leaves the motor end plate, where might the “weak link” be in the mechanisms coupling excitation to contraction?When fatigue occurs, there is still ATP present in the cell. What is the explanation for this?Describe the pattern of recruitment of muscle fiber types during activities of progressively greater intensity, and explain them.
37Study QuestionsAs the duration of maximal effort increases from less than ten seconds to 10 to 180 seconds, what factor becomes limiting in terms of energy production?Draw a diagram of the factors limiting maximal running performances of 1,500 m to 10,000 m.While a high VO2 max is essential to world-class performance, what role does running economy play in a winning performance?Given that lactate accumulation will adversely affect endurance, what test might be an indicator of maximal sustained running (swimming, cycling) speed?What is the role of environmental factors, such as altitude and heat, in very long-distance performances of one to four hours’ duration?