Exercise Physiology – Chapter 4: Exercise Metabolism

Vocabulary flashcards covering major terms and concepts from Chapter 4: Exercise Metabolism, including oxygen kinetics, energy systems, lactate dynamics, and fuel selection during exercise.

Oxygen Deficit

The lag in oxygen uptake at the beginning of exercise when ATP demand is met primarily by anaerobic pathways.

Oxygen Debt

Outdated term for the elevated post-exercise oxygen uptake originally thought to repay the earlier oxygen deficit.

Excess Post-exercise Oxygen Consumption (EPOC)

The elevated oxygen intake after exercise used to restore the body to resting conditions.

Rapid Portion of EPOC

The first 2–3 min of recovery from exercise, there is a steep decline in oxygen and ATP and PCr are resynthesized and muscle/blood O2 stores are replenished.

Slow Portion of EPOC

Later recovery phase driven by elevated heart rate, breathing, temperature, epinephrine and norepinephrine, and lactic acid-to-glucose conversion through gluconeogenesis

ATP-PCr System

Immediate energy pathway for the first 1-5 s of intense activity.

Glycolysis

Anaerobic breakdown of glucose/glycogen to pyruvate or lactate yielding ATP for efforts from 5 seconds to 2 minutes

Aerobic Metabolism

ATP production in mitochondria using oxygen; creating ATP used during exercise lasting >3 min or at low intensity.

Phosphocreatine (PCr)

High-energy compound that donates a phosphate to ADP to rapidly form ATP during explosive movements.

Lactic Acid

Product formed when excess NADH converts pyruvic acid under anaerobic conditions; can later be oxidized or converted to glucose.

Gluconeogenesis

Formation of glucose from non-carbohydrate precursors (e.g., lactate, glycerol, amino acids) mainly in the liver.

Lactate Threshold

Exercise intensity at which blood lactate rises exponentially; ~50-60% VO2max untrained, 65-80% trained.

VO2max

Maximum rate of oxygen consumption; gold-standard index of cardiorespiratory fitness.

Incremental Exercise

Exercise test where workload increases progressively until exhaustion to measure VO2max and lactate threshold.

Short-Term Intense Exercise

Efforts up to ~45 s relying on ATP-PCr, then glycolysis, with aerobic contribution increasing beyond 60 s.

Prolonged Exercise

Activity >10 min where ATP is generated mainly aerobically; may show upward VO2 drift from heat/catecholamines.

Steady-State Oxygen Uptake

Condition during submaximal exercise when VO2 plateaus and energy needs are met aerobically.

Upward Drift

Gradual increase in VO2 during prolonged exercise in a hot environment or at high catecholamine levels.

Respiratory Exchange Ratio (RER)

VCO2 / VO2; indicates fuel use (0.70 = 100% fat, 1.00 = 100% carbohydrate) in steady-state exercise.

Direct Calorimetry

Technique that measures heat production in a sealed chamber to determine energy expenditure.

Indirect Calorimetry

Estimates energy use by analyzing O2 consumption and CO2 production during breathing.

Cross-Over Concept

Shift from fat use at low intensities to carbohydrate use at high intensities (>70% VO2max).

Muscle Glycogen

Primary carbohydrate store in muscle; main CHO fuel during moderate-to-high intensity and early exercise.

Blood Glucose

CHO derived from hepatic glycogenolysis that supplies low-intensity or long-duration exercise when muscle glycogen falls.

Intramuscular Triglycerides

Fat droplets inside muscle fibers; principal fat source during moderate-intensity exercise.

Plasma Free Fatty Acids (FFA)

Fatty acids released from adipose lipolysis; increasingly oxidized as exercise duration lengthens.

Lipolysis

Breakdown of triglycerides into glycerol and free fatty acids for energy.

Hormone-Sensitive Lipase

Enzyme activated by epinephrine/norepinephrine/glucagon that catalyzes lipolysis in adipose and muscle.

Epinephrine (Exercise Context)

Catecholamine whose rising levels accelerate glycogen breakdown and glycolysis during high-intensity work.

Fast-Twitch Fibers

Muscle fibers recruited during intense exercise; contain LDH isoform favoring lactate formation.

Lactate Dehydrogenase (LDH)

Enzyme that interconverts pyruvate and lactate; fast-fiber isozyme promotes lactate production.

Cori Cycle

Pathway where lactate from muscle is converted to glucose in liver and returned to muscle.

Lactate Shuttle

Transport of lactate from producing cells to other tissues for oxidation or gluconeogenesis.

VO2max Test

Progressive treadmill or cycle protocol measuring maximal oxygen uptake; depends on subject motivation.

McArdle’s Syndrome

Genetic disorder lacking muscle phosphorylase, preventing glycogen breakdown and lactate production.

Training Effect on Oxygen Deficit

Endurance training lowers oxygen deficit via improved cardiovascular and muscular aerobic capacity.

FATmax

Exercise intensity just below lactate threshold where absolute fat oxidation rate peaks.

Excess NADH Effect

Situation during accelerated glycolysis where NADH accumulates, driving pyruvate to lactate conversion.

Delayed-Onset Muscle Soreness (DOMS)

Post-exercise pain due to muscle damage and inflammation, not lactic acid accumulation.

Branch-Chain Amino Acids

Valine, leucine, isoleucine; can be oxidized by muscle for small ATP contribution, especially late in long exercise.

Glycogen Phosphorylase

Key enzyme initiating glycogenolysis; activated by epinephrine-cAMP pathway and Ca2+/calmodulin.

Glycogenolysis

Process of breaking glycogen into glucose-1-phosphate for energy during exercise.

Calmodulin Activation

Calcium-binding that stimulates phosphorylase kinase, enhancing glycogen breakdown during muscle contraction.

Hyperventilation Impact on RER

Sudden over-breathing raises VCO2 disproportionately, making RER unreliable until steady state resumes.

Fuel Selection Factors

Exercise intensity, duration, diet, training status, and hormonal milieu that determine CHO vs. fat use.

Adenosine Triphosphate (ATP)

Universal energy currency; hydrolysis powers muscle contraction and cellular work.

Non-protein RER Table

Values (0.70–1.00) correlating RER to percentage of energy from fat vs. carbohydrate, excluding protein.

Energy Systems Continuum

Interaction of ATP-PCr, glycolytic, and oxidative pathways contributing varying proportions based on activity demand.