THE PHYSIOLOGY OF EXERCISE Melissa J Arkinstall (Exercise Physiologist)

LECTURE OUTLINE I METABOLISM: Muscle Fuel Sources II ENERGY SYSTEMS: Sport Specific III STRATEGIES: Performance Enhancement

I METABOLISM: Muscle Fuel Sources All physical activities require energy to be released to the muscles in order to fuel contraction and prevent fatigue. Important Questions How is energy released? What fuels sources exist in our body? Where does ATP come from? What pathways can we use to make ATP? Why do we slow down?

METABOLISM: Energy Release ATP or adenosine triphosphate is the energy currency used by our body everyday to perform a number of tasks: Maintain body temperature Repair damaged cells Digestion of food Mechanical work – movement ATP ↔ ADP + Energy

METABOLISM: Our need for Energy Fact: Our muscles already contain ATP molecules Problem: There are not enough! Result: Find other ways to provide our body with ATP In order to maintain exercise we need to supply our muscles with an adequate amount of ATP or energy. Physiology Principle: ATP Supply = ATP Demand Energy

METABOLISM: Sources of Energy Question: Where does the additional ATP come from? The chemical breakdown of the fuel sources in our body: Muscle Glycogen Blood Glucose (Liver) Fats (Adipose Tissue) Proteins (Amino Acids)* ATP ↔ ADP + Energy

ATP Triacylglycerol Intramuscular Glycogen (5,000 grams) Triglyceride ADIPOSE TISSUE BLOOD PLASMA MUSCLE Triacylglycerol (5,000 grams) Glycerol FFA Mitochondria O2 ATP Acetyl -CoA Krebs Cycle & Electron Transport Glycogen (600 grams) Glucose (25 grams) Intramuscular Triglyceride (350 grams) FFA-Albumin FFA Fatty Acids LIVER (100-120 grams)

METABOLISM: Sources of Energy Important: two factors determine the amount of ATP required to perform exercise and the types of fuel used: Exercise Intensity – rate of ATP production Exercise Duration – amount of ATP production In the next section we will learn to categorise sporting events using exercise intensity and duration to determine the energy systems that are being used to provide our bodies with ATP. ATP ↔ ADP + Energy

METABOLISM: Effects of Exercise Intensity EFFECTS OF EXERCISE INTENSITY ON FUEL SELECTION DURING EXERCISE 1500 1200 900 600 300 Plasma glucose Plasma FFA IMTG Muscle glycogen Energy expenditure (J/kg/min) 25 65 85 Relative exercise intensity (% of VO2max) Romijn et al. Am. J Phsyiol. Endocrin. Metab. 265: E380-E391, 1993.

METABOLISM: ATP Production ATP is able to be produced by more than one system/ pathway A system can be categorised as either: Anaerobic “O2 independent” Does not require oxygen 2. Aerobic “O2 dependent” Requires oxygen O2 O2

METABOLISM: Anaerobic Pathways Remember: these pathways generate energy without using O2 ATP is produced by these energy systems: ATP-CP system ATP ‘reservoir’ Immediate energy system 2. Anaerobic Glycolysis system Exclusively uses CHO Short-term “lactic acid” system

METABOLISM: Aerobic Pathways Remember: these pathways require O2 to generate energy ATP is produced by these energy systems: 3. Aerobic glycolytic (CHO) system Moderate- to high-intensity exercise Finite energy source (CHO →ATP) 4. Aerobic lipolytic (Fat) system Prolonged low-intensity exercise Unlimited energy source (Fat →ATP) CHO ATP O2 FAT

II ENERGY SYSTEMS: Athletic Events This section will enable you to identify the energy demands of a sporting event and the pathways used. Important Questions How can we categorise sporting events? What energy systems match these categories? What systems provide energy for 800 m sprint? What system has the greatest capacity? What systems fuel an Hawaii Ironman triathlon?

ENERGY SYSTEMS: Athletic Events Sporting events can be classified into 4 main categories listed below: 1. Power Events 2. Speed Events 3. Endurance Events 4. Ultra-Endurance Events

ENERGY SYSTEMS: Pathways The body has 4 distinct systems it can use to supply energy for these different types of events: Event Energy System 1. Power (Jump) 2. Speed (Sprint) 3. Endurance (Run) 4. Ultra-Endurance ATP-CP system (phosphagen) Anaerobic system (O2 independent) Aerobic glycolytic (CHO) system Aerobic lipolytic (Fat) system

ENERGY SYSTEMS: ATP-CP (Phosphagen) Event Type: Maximal strength & speed Event Duration: 0 - 6 sec (Dominant System) Energy Sources 1. ATP ↔ ADP + Pi + H+ 2. CP + ADP + H+ ↔ ATP + Cr Availability: Immediate- stored in muscle Anaerobic Power: Large Anaerobic Capacity: Small The best way to describe energy is to define what it does.

ENERGY SYSTEMS: Anaerobic (O2 independent) Event Type: Maximal speed or high-intensity efforts Event Duration: 6 - 60 sec (Dominant System) Energy Sources Muscle Glycogen ↔ 2 ATP + 2 Lactate + 2H+ Availability: Rapid- via glycogen breakdown (glycolysis) Anaerobic Power: Moderate Anaerobic Capacity: Larger than ATP-CP The best way to describe energy is to define what it does.

ENERGY SYSTEMS: Aerobic Glycolytic (CHO) Event Type: Moderate and High-intensity exercise Event Duration: 2 min – 1.5 hours (Dominant System) Energy Sources Carbohydrates + O2 ↔ 38 ATP + by-products Availability: Fast- via breakdown CHO Aerobic Power: Large Aerobic Capacity: Large but limited The best way to describe energy is to define what it does.

ENERGY SYSTEMS: Aerobic Lipolytic (Fat) Event Type: Low-intensity exercise Event Duration: 4 hours+ (Dominant System) Energy Sources Fats + O2 ↔ 456 ATP + by-products Availability: Slow- via fat breakdown (lipolysis) Aerobic Power: Low Aerobic Capacity: Unlimited The best way to describe energy is to define what it does.

ENERGY SYSTEMS: Effects of Exercise Duration So we can see that the contribution of these systems to energy production will depend on the event type: 49.6% 44.1% 6.3% 60% 50% 35% 40% 65% 92% 8% ATP CP Anaerobic Glycolytic Aerobic Glycolytic Aerobic Lipolytic 6 sec 30 sec 60 sec 2 min 1 hour 4 hours Peak Performance, Hawley & Burke (1998)

III STRATEGIES: Performance Enhancement Athletes and coaches are constantly searching for new ways to improve training and competition performance. Sports science has been very successful in identifying many strategies that are currently used by athletes and coaches.

STRATEGIES: To increase fat availability Fasting 2. Caffeine ingestion (6-9 mg/kg BM) 3. Fat ingestion Medium-chain fatty acids (MCFA) Long-chain fatty acids (LCFA) Intralipid (& heparin) infusion 5. Short-term fat-adaptation

ATP Triacylglycerol Intramuscular Glycogen (5,000 grams) Triglyceride ADIPOSE TISSUE BLOOD PLASMA MUSCLE Triacylglycerol (5,000 grams) Glycerol FFA Mitochondria O2 ATP Acetyl -CoA Krebs Cycle & Electron Transport Glycogen (600 grams) Glucose (25 grams) Intramuscular Triglyceride (350 grams) FFA-Albumin FFA Fatty Acids LIVER (100-120 grams)

STRATEGIES: Effect of 3-d diet on capacity Time to exhaustion (min) HIGH-CHO HIGH-FAT 250 200 150 100 50 Christensen EH & E Hansen. Scand. Arch. Physiol. 81: 161-171, 1939.

STRATEGIES: Effect of 3-d diet on capacity “There is clear evidence that short-term (1-3 days) “fat loading” is detrimental to endurance capacity and the performance of prolonged exercise. Such impairment is likely to result from a combination of the premature depletion of (lowered) muscle glycogen stores and the absence of any worthwhile increase in the capacity for fat utilization during exercise to compensate for the reduction in available carbohydrate fuel.” Burke LM & JA Hawley. Med. Sci. Sports & Exerc. (in press 2002)

Ratings of Perceived Exertion during intense exercise 18 16 14 12 10 High-CHO High-fat * Ratings of perceived exertion D-1 D-1 D-4 D-4 Stepto NK et al. Med. Sci Sports Exerc. 34: 449-455, 2002.

STRATEGIES: Effect of 3-d diet on metabolism “Optimal performance in endurance and ultra-endurance events might be obtained if an athlete trains for most of the year on a high-CHO diet, then undergoes a short-term period of fat-adaptation followed by the traditional CHO-loading regimen in the final days before an event. Nutritional periodisation for ultra-endurance events should aim to enhance the contribution from fat to oxidative energy metabolism, thus potentially sparing endogenous carbohydrate stores.” Hawley JA et al. Sports Med. 25: 241-257, 1998

Overview of Experimental Protocol: Burke et al. 2000 Day 1 Day 2 Day 3 Day 4 Day 5 Day 6 Day 7 LAB Easy ride Hills HIT Easy ride LAB EXP HIT 3-4 h 2-3 h 3-4 h High-CHO or High-Fat diet LAB 20 min @ 70% VO2max High-CHO diet (11 g/kg BM) HIT 8 x 5 min @ 85% VO2max Experimental trial (2 h @ 70% VO2max then 7 kJ/kg BM time-trial)

Fuel metabolism during 2 h cycling at 70% VO2max 100 80 60 40 20 Glucose Glucose Muscle glycogen * Muscle glycogen Energy expenditure (%) Fat * Fat HIGH-CHO HIGH-FAT Burke LM et al. J. Appl. Physiol. 89: 2413-2421, 2000.

Time trial performance after 2 h cycling at 70% VO2max 2 HR CYCLING AT 70% VO2max 50 40 30 20 10 Time to complete 7 kJ/kg BM (min) 34:10 ±2:37 30:44 ±1:07 HIGH-CHO HIGH-FAT Burke LM et al. J. Appl. Physiol. 89: 2413-2421, 2000.

Effect of fat adaptation (6-d) on gene expression 50 40 30 20 10 * FAT/CD36 (Arbitrary units) PRE HIGH-CHO HIGH-FAT

Conclusions: Burke et al. 2000 1. Five days exposure to a high-fat, low-CHO diet caused marked changes in fuel substrate oxidation during 2 h of submaximal exercise at 70% of VO2max

Conclusions: Burke et al. 2000 1. Five days exposure to a high-fat, low-CHO diet caused marked changes in fuel substrate oxidation during 2 h of submaximal exercise at 70% of VO2max These changes were independent of CHO availability because enhanced rates of fat oxidation persisted despite the restoration of muscle glycogen stores

Conclusions: Burke et al. 2000 1. Five days exposure to a high-fat, low-CHO diet caused marked changes in fuel substrate oxidation during 2 h of submaximal exercise at 70% of VO2max These changes were independent of CHO availability because enhanced rates of fat oxidation persisted despite the restoration of muscle glycogen stores Despite promoting “glycogen sparing” during submaximal exercise, fat-adaptation strategies did not provide a clear benefit to the performance of a 30 min time-trial undertaken after 2 h of continuous cycling at 70% of VO2max

Conclusions: Burke et al. 2000 1. Five days exposure to a high-fat, low-CHO diet caused marked changes in fuel substrate oxidation during 2 h of submaximal exercise at 70% of VO2max These changes were independent of CHO availability because enhanced rates of fat oxidation persisted despite the restoration of muscle glycogen stores Despite promoting “glycogen sparing” during submaximal exercise, fat-adaptation strategies did not provide a clear benefit to the performance of a 30 min time-trial undertaken after 2 h of continuous cycling at 70% of VO2max The results of this study do not support the use of fat-adaptation strategies by endurance athletes competing in events lasting 2-3 h in duration

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