ASSIGNMENT 1: ENERGY SYSTEMS IN EXERCISE

Title: Bioenergetics of Muscular Exercise - A Critical Analysis of the Three Energy Systems

Introduction (Brief - for student)

Part A: The ATP-Phosphocreatine (Phosphagen) System

1. ATP as the universal energy currency. Each mole of ATP yields ~7,300 calories upon hydrolysis of a terminal phosphate bond. Muscular ATP stores are finite; at maximal power output they are exhausted in under 3 seconds (Guyton & Hall, Ch. 85).
2. Phosphocreatine (PCr) as the immediate buffer. PCr carries a higher-energy phosphate bond (~10,300 cal/mol) compared to ATP. The reaction PCr + ADP → Cr + ATP is catalyzed by creatine kinase and is virtually instantaneous. Muscle cells contain 2-4 times as much PCr as ATP.
3. Capacity and power of the phosphagen system. Together, ATP + PCr (the phosphagen system) sustain maximal muscle power for 8-10 seconds - sufficient for events like the 100-metre dash, explosive jumping, weight lifting, and diving (Guyton & Hall, Table 85.1).
4. AMPK signalling. As the AMP:ATP ratio rises during exercise, AMP-activated protein kinase (AMPK) is activated, which phosphorylates and upregulates ATP-generating pathways while suppressing anabolic ATP-consuming pathways - a key molecular switch linking energy status to metabolic control.

Part B: The Glycogen-Lactic Acid (Glycolytic) System

1. Anaerobic glycolysis pathway. Muscle glycogen is split into glucose, which undergoes glycolysis (10-step Embden-Meyerhof pathway) to yield 4 ATP per glucose (net 2 ATP from free glucose) without oxygen consumption. The process produces pyruvate; under conditions of oxygen insufficiency, pyruvate is reduced to lactate by lactate dehydrogenase (LDH).
2. Power and capacity. This system generates ATP approximately 2.5 times faster than the oxidative system but at only half the rate of the phosphagen system. It sustains maximal or near-maximal effort for an additional 1.3-1.6 minutes beyond the phosphagen window (Guyton & Hall, Ch. 85) - making it the dominant energy source for the 200 m-400 m dash, 100 m swim, and team sports with repeated high-intensity bursts (basketball, soccer, ice hockey).
3. Lactate accumulation and fatigue. Lactate, along with intracellular H⁺ (acidosis), Pi (inorganic phosphate), and impaired Ca²⁺ handling, contributes to peripheral fatigue. The lactate threshold (LT) - the exercise intensity at which blood lactate begins to accumulate non-linearly above resting levels (~2 mmol/L) - and the onset of blood lactate accumulation (OBLA) at 4 mmol/L are critical performance markers.
4. Lactate clearance. Post-exercise, lactic acid is removed by: (a) oxidative metabolism in less active or cardiac muscle (reconverted to pyruvate), and (b) gluconeogenesis in the liver (Cori cycle), converting lactate back to glucose for glycogen resynthesis (Guyton & Hall, Ch. 85).

• Estimate which energy system(s) dominate at the 10-second, 25-second, and 45-second marks.
• Explain the physiological basis of the "burning" sensation experienced in the final 100 m.
• Describe the recovery timeline for lactic acid clearance and what an active cool-down achieves biochemically.

Part C: The Aerobic (Oxidative) System

1. Substrates and pathways. The aerobic system oxidises glucose (via glycolysis + pyruvate dehydrogenase + TCA cycle + electron transport chain), free fatty acids (via beta-oxidation + TCA cycle), and, to a minor extent, amino acids. The net yield from one mole of glucose under aerobic conditions is approximately 30-32 ATP, versus 2-4 ATP anaerobically.
2. Fat vs. carbohydrate utilisation. At rest and low-intensity exercise, fat is the dominant fuel. As exercise intensity increases, there is a progressive shift toward carbohydrate. By the time of exhaustion in prolonged exercise (>3-4 hours), 60-85% of energy may come from fat (Guyton & Hall, Fig. 85.5). A high-carbohydrate diet favours faster glycogen repletion and better high-intensity performance.
3. Maximal oxygen uptake (VO₂max). VO₂max is the gold-standard index of the aerobic system's capacity. Resting O₂ consumption is ~250 mL/min; in elite male marathon runners, this rises to ~5,100 mL/min (~71 mL/kg/min). Both pulmonary ventilation and O₂ consumption increase approximately 20-fold from rest to maximal exercise (Guyton & Hall, Ch. 85).
4. Oxygen debt (EPOC - Excess Post-exercise Oxygen Consumption). Approximately 11.5 litres of extra O₂ must be consumed after heavy exercise to: replenish stored O₂ (bound to haemoglobin and myoglobin), reconstitute the phosphagen system (~3.5 L, the "alactacid" component), and clear lactic acid (~8 L, the "lactacid" component) (Guyton & Hall, Ch. 85, Fig. 85.3).
5. Glycogen repletion. Full muscle glycogen restoration requires approximately 48 hours on a high-carbohydrate diet. A high-fat or protein diet delays this significantly and may prolong recovery for days (Guyton & Hall, Fig. 85.4). Co-ingestion of protein with carbohydrate post-exercise can accelerate glycogen resynthesis rates.

• Compare and contrast the power output and sustainability of all three energy systems using a clearly labelled table (include: dominant fuel, ATP yield per mole, rate of ATP production, time course, representative sport/event, and limiting factor).
• Explain the "crossover concept" in fat-carbohydrate utilisation during graded exercise.
• A long-distance runner trains 5 days/week at 70% VO₂max. Critically evaluate the nutritional strategies (timing, macronutrient composition) they should adopt to optimise glycogen availability and recovery, citing current evidence.

Part D: Integration Across the Energy Continuum

1. No system operates in isolation. All three systems contribute simultaneously; what changes with intensity is their relative contribution. The phosphagen system peaks within the first few seconds; glycolysis ramps up within 10-15 seconds; the aerobic system reaches full capacity at approximately 2-4 minutes.
2. Sport-specific energy system profiling. Using Guyton & Hall's Table 85.1, students should analyse energy system demands in sports ranging from 100-metre sprint (phosphagen-dominant) to marathon (aerobic-dominant), and mixed-demand sports like boxing, football, and rowing.
3. Muscle fibre type and energy system matching. Fast-twitch (Type II) fibres, with high glycolytic enzyme activity, are suited to the phosphagen and glycolytic systems. Slow-twitch (Type I) fibres, rich in mitochondria and capillary density, are the primary effectors of the aerobic system. Data from Guyton & Hall (Table in Ch. 85) shows marathoners have ~82% slow-twitch fibres vs. ~37% in sprinters/jumpers.

References for Assignment 1

• Guyton AC, Hall JE. Textbook of Medical Physiology, 14th ed. Elsevier, 2021. Chapter 85: Sports Physiology.
• McArdle WD, Katch FI, Katch VL. Exercise Physiology: Nutrition, Energy, and Human Performance, 8th ed. Wolters Kluwer, 2015.
• Powers SK, Howley ET. Exercise Physiology: Theory and Application to Fitness and Performance, 10th ed. McGraw-Hill, 2018.
• Brooks GA. The science and translation of lactate shuttle theory. Cell Metabolism, 2018; 27(4): 757-785.

ASSIGNMENT 2: MEASUREMENT OF ENERGY SYSTEMS IN EXERCISE

Title: Laboratory and Field-Based Methods for Quantifying Energy System Contribution During Exercise

Introduction (Brief - for student)

Part A: Measurement of the Aerobic Energy System

A1. Direct Calorimetry

A2. Indirect Calorimetry (Open-Circuit Spirometry)

• VO₂ (mL/min or mL/kg/min): volume of O₂ consumed per unit time
• VCO₂ (mL/min): CO₂ produced
• Respiratory Exchange Ratio (RER): RER = VCO₂/VO₂
  • RER = 0.70 indicates pure fat oxidation
  • RER = 1.00 indicates pure carbohydrate oxidation
  • RER >1.00 indicates non-oxidative CO₂ production (exceeds aerobic capacity; buffering of lactic acid by bicarbonate)
• Caloric equivalent: At RER = 0.82 (mixed diet), 1 L O₂ = ~4.825 kcal

1. Subject exercises at progressively increasing workloads (each stage: 3 min / or 15-25 W ramp/min)
2. Expired gas is collected via metabolic cart (mouthpiece + nose clip)
3. VO₂max criteria: plateau in VO₂ despite increase in workload, RER >1.10, HR within 10 bpm of age-predicted maximum, Borg RPE ≥ 19
4. Results expressed as absolute (L/min) or relative (mL/kg/min)

A3. Excess Post-exercise Oxygen Consumption (EPOC)

• Total oxygen debt after maximal 4-minute exercise: ~11.5 litres
• Alactacid component (~3.5 L): rapid phase (first 3-5 min recovery); reflects PCr resynthesis and replenishment of O₂ stores (myoglobin, haemoglobin, dissolved plasma O₂)
• Lactacid component (~8 L): slow phase (up to 60 min); reflects lactic acid removal via oxidation and gluconeogenesis

Part B: Measurement of the Glycolytic (Lactic Acid) System

B1. Blood Lactate Analysis

• Resting blood lactate: 0.5-2.0 mmol/L
• Lactate Threshold (LT₁/Aerobic Threshold): First non-linear rise in blood lactate, typically ~2 mmol/L; corresponds to ~55-65% VO₂max in untrained, ~75-80% in trained subjects
• Onset of Blood Lactate Accumulation (OBLA / LT₂ / Anaerobic Threshold): Blood lactate = 4 mmol/L; corresponds to the maximal lactate steady state; gold-standard marker of endurance performance

1. Incremental exercise test (3-5 min stages on cycle ergometer or treadmill)
2. Blood sample at end of each stage
3. Plot blood [lactate] vs. VO₂ or workload
4. Identify LT₁ by visual inspection/mathematical modelling (log-log transformation, Dmax method, or fixed 4 mmol/L criterion)

B2. Wingate Anaerobic Test (WAnT) - Measuring Peak Anaerobic Power and Glycolytic Capacity

• Protocol: 30-second all-out sprint on a cycle ergometer against a resistance of 0.075 kg/kg body mass (standard) or sport-specific load
• Variables derived:
  • Peak Power (PP): highest power output achieved in any 5-second segment (~5-10 s); reflects phosphagen system
  • Mean Power (MP): average power over 30 seconds; reflects integrated anaerobic capacity (phosphagen + glycolytic)
  • Fatigue Index (FI): [(PP - Lowest Power) / PP] × 100; reflects the rate of power decrement, reflecting glycolytic system depletion rate
• Normative values: Elite male athletes: PP ~900-1200 W; FI ~40-60%

Part C: Measurement of the Phosphagen (ATP-PCr) System

C1. Phosphorus Magnetic Resonance Spectroscopy (³¹P-MRS)

• PCr depletion rate during exercise correlates with exercise intensity
• PCr resynthesis half-time (t½PCr) during recovery: ~20-30 seconds in healthy skeletal muscle; prolonged in mitochondrial disease or deconditioning
• pH from chemical shift of Pi relative to PCr: normal exercising muscle pH ~6.5-6.8 at fatigue

C2. The Phosphocreatine Recovery Test (Indirect Field Estimation)

Part D: Metabolic Equivalents (METs) and Indirect Estimation Methods

D1. MET concept

• Walking 3.5 km/h ≈ 3 METs
• Jogging 9 km/h ≈ 9 METs
• Vigorous cycling ≈ 10-12 METs
• Elite sprint ≈ 20+ METs

D2. Heart Rate-VO₂ Relationship

• Individual HR-VO₂ regression equation (from submaximal test)
• FLEX HR method
• Polar fitness equations

Summary Table for Students (Comparison of Methods)

References for Assignment 2

• Guyton AC, Hall JE. Textbook of Medical Physiology, 14th ed. Elsevier, 2021. Chapter 85: Sports Physiology (pp. 1059-1070).
• McArdle WD, Katch FI, Katch VL. Exercise Physiology: Nutrition, Energy, and Human Performance, 8th ed. Wolters Kluwer, 2015.
• Powers SK, Howley ET. Exercise Physiology: Theory and Application to Fitness and Performance, 10th ed. McGraw-Hill, 2018.
• Bangsbo J. Oxygen deficit: a measure of the anaerobic energy production during intense exercise? Can J Appl Physiol. 1996; 21(5): 350-363.
• Beaver WL, Wasserman K, Whipp BJ. A new method for detecting anaerobic threshold by gas exchange. J Appl Physiol. 1986; 60(6): 2020-2027.