Exercise Physiology: Key Principles, Energy Metabolism, and Neuromuscular Control

Comprehensive vocabulary flashcards covering Modell's physiological principles, carbohydrate and fat bioenergetics, metabolic thresholds, hormonal regulation, and neuromuscular physiology.

Modell's 'Big 3'

The three core principles—conservation of mass, control systems, and mass flow—that provide a unified conceptual framework for understanding physiological systems.

Gradient ($G$)

The difference ($\Delta$) in mass, energy, pressure, or concentration between two compartments.

Conductance ($K$)

The property or pathway mechanism that permits flow between compartments.

Mass Flow Equation

$\text{FLOW} = \Delta G \times K$, stating that flow is directly proportional to the driving gradient multiplied by conductance.

Integrator

A central control system component that processes incoming sensory information and coordinates an appropriate regulatory response.

Effector

A control system component that causes the required physical change in flow (inflow or outflow) to restore mass balance.

Feedback Sensing

A control mechanism that detects a change in mass balance after it has already occurred, triggering corrective actions to restore homeostasis.

Feedforward Sensing

A control mechanism that anticipates an impending disturbance in mass balance before it occurs, initiating preventive actions.

Afferent Pathway

The communication pathway transmitting sensory information from the sensor to the integrator.

Efferent Pathway

The communication pathway carrying coordinated regulatory instructions from the integrator to the effector.

Catabolism

The breakdown of complex molecules into simpler compounds to release and produce usable energy.

Anabolism

The synthesis of complex molecules from simpler precursors using energy.

Bioenergetics

The biochemical process of converting macronutrient substrates from food into usable cellular energy ($\text{ATP}$).

Glycogenolysis

The enzymatic breakdown of stored glycogen into glucose (in liver) or glucose 6-phosphate (in skeletal muscle).

Gluconeogenesis

The liver pathway that synthesizes glucose from non-carbohydrate precursors, such as lactate, amino acids, and glycerol.

De Novo Lipogenesis

The hepatic metabolic pathway that synthesizes fatty acids from excess dietary carbohydrates (glucose).

Re-esterification

The metabolic process of combining three fatty acids with a glycerol backbone to form a triglyceride for storage.

Lipolysis

A 3-step enzymatic process in which lipases sequentially cleave fatty acids from a triglyceride glycerol backbone.

Hormone-Sensitive Lipase (HSL)

The key lipolytic enzyme in adipose tissue that mobilizes stored triglycerides into free fatty acids and glycerol; stimulated by catecholamines/glucagon and inhibited by insulin.

Brown Adipose Tissue

Metabolically active adipose tissue ($3\text{--}7\%$ of total adipose tissue in adults) containing high mitochondrial and capillary density that expends energy via thermogenesis for heat production.

White Adipose Tissue

Primary fat storage tissue ($\sim 15\%$ of body mass) consisting of adipocytes with a single large lipid droplet designed for energy storage.

Intramuscular Triglycerides (IMTG)

Local lipid droplets stored directly within skeletal muscle fibers that provide rapidly accessible fatty acids for lipolysis during exercise.

Insulin

An anabolic peptide hormone secreted by pancreatic $\beta$-cells in the fed state that promotes nutrient uptake and storage into tissues while inhibiting catabolic pathways.

Glucagon

A catabolic peptide hormone secreted by pancreatic $\alpha$-cells in the fasted state that stimulates liver glycogenolysis and gluconeogenesis to maintain blood glucose.

Catecholamines

Catabolic amino-acid derivative hormones (epinephrine and norepinephrine) released by the adrenal medulla and sympathetic nerves that mobilize glycogen and fatty acids during exercise.

Phosphocreatine Depletion Kinetics

During high-intensity exercise, PCr concentration drops precipitously within seconds to buffer and resynthesize ATP before exhaustion occurs.

Mitochondrial Creatine Kinase (mtCK)

An enzyme located in the mitochondrial intermembrane space that utilizes mitochondrial ATP to resynthesize PCr, bypassing spatial transport limitations of large ATP molecules.

Adenylate Kinase

A backup emergency enzyme that catalyzes the reaction $2\,\text{ADP} \rightarrow \text{ATP} + \text{AMP}$ when PCr is depleted and other energy systems have not yet matched demand.

Hexokinase (HK)

The glycolytic enzyme that catalyzes the irreversible phosphorylation of blood-derived glucose into glucose 6-phosphate, trapping it inside the cell at the cost of $1\,\text{ATP}$.

Phosphofructokinase (PFK)

The primary allosteric, rate-limiting enzyme of glycolysis; activated by ADP, AMP, and $\text{P}_i$, and inhibited by ATP, citrate, and $\text{H}^+$.

Glycogen Phosphorylase

The key enzyme that breaks down glycogen into glucose 1-phosphate without consuming ATP, activated by AMP, ADP, $\text{Ca}^{2+}$, and catecholamines.

Lactate Dehydrogenase (LDH)

The reversible enzyme that reduces pyruvate to lactate while oxidizing $\text{NADH} + \text{H}^+$ to $\text{NAD}^+$, replenishing the $\text{NAD}^+$ necessary to sustain glycolytic ATP generation.

Lactate Shuttling

The process whereby lactate generated in fast-twitch Type II fibers is transported via circulation or diffusion to oxidative Type I fibers, heart, or liver for conversion back to pyruvate and aerobic oxidation.

Allosteric Regulation

Enzymatic regulation occurring when an effector molecule binds to a regulatory site outside the active site, inducing a conformational change that alters catalytic activity.

Covalent Modification

The reversible alteration of an enzyme's activity through the covalent addition or removal of a chemical group, most commonly phosphorylation or dephosphorylation.

GLUT4

An insulin- and muscle-contraction-responsive glucose transporter protein that translocates from intracellular vesicles to the sarcolemma to enhance muscle glucose uptake.

Pyruvate Dehydrogenase (PDH)

A multienzyme mitochondrial complex that decarboxylates pyruvate to form acetyl-CoA, bridging glycolysis and the TCA cycle under aerobic conditions.

Oxidative Phosphorylation (OXPHOS)

The process coupling electron transport along inner mitochondrial protein complexes to the phosphorylation of ADP to ATP by ATP synthase using a proton gradient.

Fatty Acyl-CoA Synthetase (FACS)

A cytosolic enzyme that activates incoming free fatty acids by attaching coenzyme A (forming fatty acyl-CoA), preparing them for mitochondrial entry.

Carnitine Palmitoyltransferase (CPT)

A two-part mitochondrial transport system (CPT-I on outer membrane, CPT-II on inner membrane) that exchanges CoA for carnitine to shuttle long-chain fatty acids into the mitochondrial matrix.

$\beta$-Oxidation

A cyclical mitochondrial pathway where fatty acyl-CoA is broken down, cleaving 2 carbons per turn to yield acetyl-CoA, $\text{NADH}$, and $\text{FADH}_2$.

Three-Zone Exercise Intensity Model

A physiological framework dividing exercise into low (Zone 1), moderate (Zone 2), and high (Zone 3) intensity using lactate and ventilatory thresholds ($\text{LT}_1/\text{VT}_1$ and $\text{LT}_2/\text{VT}_2$).

Lactate Threshold

The highest exercise workload or oxygen uptake achieved with less than a $1.0\,\text{mmol/L}$ rise in blood lactate concentration above resting baseline.

Ventilatory Threshold (VT)

The workload during progressive exercise where minute ventilation ($\dot{V}_E$) begins to increase disproportionately relative to oxygen consumption ($\dot{V}\text{O}_2$) to blow off excess $\text{CO}_2$ generated by carbonic anhydrase buffering $\text{H}^+$.

FATMAX

The specific exercise intensity (typically around $60\text{--}65\%\,\dot{V}\text{O}_{2\text{max}}$) at which the whole-body rate of fat oxidation reaches its maximal level.

Nonprotein Respiratory Exchange Ratio (RER)

The ratio of carbon dioxide production to oxygen consumption ($\dot{V}\text{CO}_2 / \dot{V}\text{O}_2$) measured at the mouth, used to estimate the relative percentages of fat and carbohydrate oxidized and caloric energy equivalent per liter of $\text{O}_2$.

Myokines

Cytokines and hormone-like peptides synthesized and secreted by skeletal muscle fibers in response to contraction that exert local and systemic endocrine effects.

Interleukin-6 (IL-6)

A contraction-induced myokine that acts systemically during exercise to exert anti-inflammatory effects, stimulate lipolysis, and promote hepatic glucose release.

Sarcomere

The functional repeating contractile unit of skeletal muscle striated myofibrils, delimited from Z-line to Z-line.

Satellite Cells

Quiescent undifferentiated muscle stem cells located between the basal lamina and sarcolemma that proliferate and donate nuclei to muscle fibers during hypertrophy and repair.

Cross-Bridge Cycling

The cyclical, ATP-dependent interaction wherein energized myosin heads bind to actin, perform a power stroke pulling the thin filament toward the M-line, detach upon ATP binding, and recock.

Dihydropyridine Receptor (DHPR)

A voltage-sensing L-type calcium channel located in the T-tubule sarcolemma that undergoes a conformational shift upon membrane depolarization to mechanically open RyR1.

Ryanodine Receptor (RyR1)

The calcium-release channel situated in the terminal cisternae of the sarcoplasmic reticulum membrane that rapidly releases stored $\text{Ca}^{2+}$ into the sarcoplasm upon activation by DHPR.

SERCA Pump

An ATP-dependent calcium pump (sarco/endoplasmic reticulum $\text{Ca}^{2+}$-ATPase) that actively transports cytosolic $\text{Ca}^{2+}$ back into the sarcoplasmic reticulum lumen to induce muscle relaxation.

Henneman's Size Principle

The neurophysiological principle stating that motor units are systematically recruited in order of increasing size, from smallest (low-threshold, fatigue-resistant Type I) to largest (high-threshold, high-force Type IIx).

Eccentric Contraction

A dynamic muscle action where muscle fibers lengthen under load, generating higher peak force than isometric or concentric actions due to mechanical strain on bound cross-bridges.

Central Fatigue

A reduction in voluntary muscular force production originating proximal to the neuromuscular junction, caused by impaired central motor drive, reduced motor unit recruitment, or psychological factors.

Peripheral Fatigue

A decline in muscle force generation caused by mechanical or biochemical failure at or distal to the neuromuscular junction, including excitation-contraction uncoupling, substrate depletion, or metabolite accumulation ($\text{P}_i$, $\text{H}^+$).

Neural Adaptations vs. Hypertrophy Timeline

During resistance training, rapid early strength gains (first few weeks) are mediated primarily by neural drive and motor unit recruitment, whereas muscle hypertrophy accounts for strength increases later in training.