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1. OVERVIEW OF EXERCISE METABOLISMLecture content provided by GSSI, a division of PepsiCo, Inc. Any opinions or scientific interpretations expressed in this presentation are those of the author and do not necessarily reflect the position or policy of PepsiCo, Inc.
2. Lecture OutlineMuscle contraction energeticsRest to Exercise transitionMetabolic responses during recoveryInfluence of intensity and durationEstimation of fuel utilization
3. Energy for Muscle ContractionAdenosine Triphosphate (ATP) Molecule which provides energy for muscle fibers to contractThe amount stored in the muscle is limitedHargreaves, M & Spriet, L. Nat Metab. 2020; https://doi.org/10.1038/s42255-020-0251-4.
4. Energy for Muscle ContractionFuel Sources to Generate ATP for Muscle ContractionPCr(phosphocreatine)Anaerobic GlycolysisAerobic MetabolismHigh intensity~5 s or lessHigh intensity~30 s – 3 minLower intensityLong durationGlucoseGlucoseFatty AcidsRegenerated during recoveryNotes:Creatine is made in the body and consumed from meat. The PCr system is also the mechanism through which creatine supplements provide energy for high intensity muscle contractionGlucose is supplied mainly from the breakdown of GLYCOGEN, the storage form of glucose in the muscle and liverAmino acids can enter the aerobic glycolysis system, but their contribution to ATP generation is very smallHargreaves, M & Spriet, L. Nat Metab. 2020; https://doi.org/10.1038/s42255-020-0251-4.
5. CAR ENGINEONE KIND OF FUELHUMAN ENGINEFAT = ENDURANCE FUELCARB = HIGH INTENSITY FUELPCr = FAST ACCELERATIONS
6. Energy for Muscle Contraction - ByproductsAnaerobic glycolysisLactateAerobic MetabolismCO2As oxygen is used in the process to convert fuels to ATP, carbon dioxide is producedRespiration increases to remove the excess CO2 from the bodyPreviously thought to cause fatigue and muscle soreness. No longer believed to be the case, may provide an additional fuel sourceHargreaves, M & Spriet, L. Nat Metab. 2020; https://doi.org/10.1038/s42255-020-0251-4.
7. TRANSITIONS
8. Rest to Exercise TransitionImagine a football player is standing on the sideline. On coach’s whistle, he is to begin sprinting. What is happening in the body to ensure the muscle has the fuel to go from rest to quick contractions?
9. Rest to Exercise TransitionWithin the first step muscles must increase their rate of ATP productionAnaerobic energy sources (PCr + anaerobic glycolysis) are used first, giving time for the aerobic system to contributeTake Home Message: several energy systems are involved in rest-to-work transitionsMeasurement of O2 consumption can be used as an index of aerobic ATP productionProvides information about aerobic metabolism during exerciseVO2 max = maximal oxygen consumption, often used as an indicator of fitnessOxygen deficit = lag in O2 update at the beginning of exercise, indicating the reliance on anaerobic systemsHargreaves, M & Spriet, L. Nat Metab. 2020; https://doi.org/10.1038/s42255-020-0251-4.
10. Recovery from Exercise: Metabolic ResponsesMetabolism (indicated by oxygen uptake) remains elevated for several minutes after exerciseThe magnitude of elevation depends on the intensity of exerciseEPOC = Excess Post-Exercise Oxygen ConsumptionWhy does EPOC occur?Resynthesize PCReplace O2 in muscle & bloodElevated body temperatureConvert lactic acid to glucoseElevated epinephrine & norepinephrineSport Nutrition 3rd Ed. Jeukendrup & Gleeson, Human Kinetics.
11. Influence ofIntensity andDuration onEnergy PathwaySelection
12. Influence of Duration on Source of ATPPCrPCrAnaerobic glycolysis1-5 sPCrAnaerobic glycolysisAerobic Systems*> 1 min~5-10 sMost sports use a combination of anaerobic and aerobic pathways to produce ATPSport Nutrition 3rd Ed. Jeukendrup & Gleeson, Human Kinetics.Hargreaves, M & Spriet, L. Nat Metab. 2020; https://doi.org/10.1038/s42255-020-0251-4. *Major pathway, muscle glycogen dominant fuel sourceSeveral minutes to hoursAerobic SystemsCHO and Fat provide almost all ATP
13. Influence of DurationSecondsMinutes10306024103060120Percent Aerobic102030406585959899Percent Anaerobic908070603515521Duration of MAXIMAL ExerciseContribution of Aerobic/Anaerobic Production of ATP During MAXIMAL Exercise as a Function of the Duration of the EventRecreated from:Powers & Howley. McGraw-Hill Education. 2017
14. Influence of Duration & IntensityShort-Term Intense ExerciseEnergy comes primarily from anaerobic pathwaysLength of the activity determines if PCr or glycolysis is the predominant energy sourceTransition from PCr to glycolysis is a gradual shiftExample50 m dash or single play in a football game: PCr400 m dash (~55 s): PCr, glycolysis, and aerobic, with anaerobic glycolysis predominatingPowers & Howley. McGraw-Hill Education. 2017
15. Influence of Duration & IntensityProlonged ExerciseEnergy comes primarily from aerobic metabolismSteady-state oxygen update can be maintained during prolonged low-intensity exerciseHowever, a hot/humid environment or higher intensity increases oxygen consumption over timeReminderOxygen consumption is an indicator of aerobic metabolismPowers & Howley. McGraw-Hill Education. 2017
16. Assessing Metabolism during Exercise
17. Incremental Exercise: VO2 MaxIncremental exercise = gradually increasing exercise intensity over timeAs intensity increases, oxygen uptake increases in a linear fashion until VO2 max is reachedVO2 max is the “physiological ceiling” for the ability to deliver O2 to contracting musclesTraining can result in an increased VO2 max, and sometimes training intensity is set at a %VO2 maxIntensityVO2 (L/min)VO2 maxPowers & Howley. McGraw-Hill Education. 2017
18. Incremental Exercise: VO2 MaxVO2 max is influenced by: The maximal ability of the cardiorespiratory system to deliver oxygen to the muscleThe muscles ability to take up the oxygen and produce ATP aerobicallyGenetics“Excellent” VO2 max values for ages 18-25 yo:> 60 mL/kg/min> 56 mL/kg/minVO2 max values of some of the best US male and female endurance athletesJoan BenoitDistance Runner1984 Olympic Marathon Champion78.6 mL/kg/minLance ArmstrongCyclist84.0 mL/kg/minhttps://www.topendsports.com/testing/records/vo2max.htmPowers & Howley. McGraw-Hill Education. 2017
19. Intermittent Exercise: Lactate ThresholdMost of the ATP production in the early part of an incremental exercise test (lower intensities) comes from aerobic sourcesAs intensity increases, blood levels of lactic acid begin to riseWhile there is debate, it is thought that the rise in lactic acid indicates an increasing reliance on anaerobic metabolismLactate threshold (LT) = exercise intensity where there is an increase in lactic acid in the bloodIntensityBlood lactate concentrationLTPowers & Howley. McGraw-Hill Education. 2017
20. Intermittent Exercise: Lactate ThresholdWhile the exact cause of the LT is unclear, it may occur due to:Low muscle oxygenAccelerated glycolysisReduced rate of lactate removalIn a practical setting, lactate threshold can be used to set training intensityIntensityBlood lactate concentrationLTPowers & Howley. McGraw-Hill Education. 2017
21. Intermittent Exercise: Lactate ThresholdLactate is now known NOT to cause fatigueHowever, the LT test is still utilizedClick here to watch a YouTube video by Dr. Laurent Bannock explaining lactate and the utility of VO2 max vs LT testingIntensityBlood lactate concentrationLT
22. Estimation of Fuel UtilizationRespiratory Exchange Ratio (RER)=VCO2/VO2=Ratio of carbon dioxide produced to oxygen consumedBased on their chemical structures, fat and carbohydrate differ in the amount of O2 used and CO2 produced during oxidationFat oxidation requires more O2 than carbohydrateIgnores protein, since protein contributes little as a fuel source Determined using a metabolic cart during steady state exercisePowers & Howley. McGraw-Hill Education. 2017
23. Estimation of Fuel UtilizationPercentage of Carbohydrate and Fat Utilization by RER value (R)R0.700.851.0% Fat100500% CHO050100Powers & Howley. McGraw-Hill Education. 2017
24. Aerobic Metabolism: Influence of Intensity and Duration on Fuel Selection
25. Intensity & Fuel SelectionSchematic of Energy Sources Based on Exercise IntensityFat is the primary energy source for low-intensity exercise ( < ~30% VO2 max)Carbohydrate is the primary energy source for high intensity exercise (> 70% VO2 max)Athletes are almost always using a mixture of carbohydrate and fat% Energy from Fuel SourcePowers & Howley. McGraw-Hill Education. 2017
26. Intensity & Fuel SelectionSpriet L and Randell R. Sports Science Exchange. 2020;29(205)1-6SSE #205
27. Intensity & Fuel SelectionSchematic of Energy Sources Based on Exercise IntensityMYTH: Individuals exercising to lose weight should work out at a lower intensity, in the “fat burning zone.TRUTH: Weight loss depends on overall calories expended. Exercising at a lower intensity results in lower calorie burn (given the same duration). It is more beneficial to work out at a higher intensity, to get a greater EPOC and use more fat for energy during recovery.% Energy from Fuel Source
28. Duration & Fuel SelectionAt a given intensity, as exercise duration increases there is a gradual shift from carbohydrate utilization to a greater reliance on fat as a fuel sourceExercise Time% Energy from Fuel SourceCarbohydrateFatPowers & Howley. McGraw-Hill Education. 2017
29. TRAINING
30. Impact of Training on Fuel Metabolism for Improved PerformanceRegular training can improve fatigue resistance and performance, partly through changes in muscle metabolismHigh Intensity Interval Training (HIIT)Endurance TrainingIncreases capacity for anaerobic energy productionEnhances tolerance to metabolic acidosis (increased buffering capacity)Increased VO2 max, leading to greater fatigue resistanceIncreased skeletal muscle mitochondrial densityIncreased fat oxidationIncreased capacity for carbohydrate oxidation, allowing for maintenance of higher power outputHargreaves, M & Spriet, L. Nat Metab. 2020; https://doi.org/10.1038/s42255-020-0251-4.
31. SUMMARY
32. Link to Summary Video