Energy Substrates and Metabolism
Energy Substrates
Carbohydrates
Storage Form:
Muscle glycogen: approximately 300–500 g
Liver glycogen: approximately 80–100 g
Blood glucose: approximately 5 g
Energy Yield:
Roughly 4 kcal per gram
Faster ATP production compared to fat
Used When?
High-intensity exercise
At the beginning of exercise
When oxygen availability is limited
During 400m-type efforts
Important Notes:
Glycogen yields slightly more ATP than blood glucose
Limited storage capacity, leading to performance decline when depleted
Concept: "Hitting the wall" refers to glycogen depletion
Fats (Triglycerides → Free Fatty Acids)
Storage Form:
Triglycerides stored in adipose tissue
Intramuscular triglycerides (IMTG)
Energy Yield:
Approximately 9 kcal per gram
Much larger storage capacity (virtually unlimited amounts)
Used When?
During rest
Low-intensity exercise
Prolonged endurance exercise
Important Notes:
Produces more ATP per molecule
Slower ATP production rate relative to carbohydrates
Requires oxygen for metabolism
Protein
Storage Form:
Functional tissue, primarily in muscle
Energy Yield:
Roughly 4 kcal per gram
Used When?
During starvation
Very prolonged exercise
In low glycogen states
Important Notes:
Minor contributor to energy production (5–10% of total energy)
Not an ideal source for energy
Energy Systems (Bioenergetics)
1. ATP-PCr (Phosphagen System)
Location: Cytosol
Oxygen Requirement: No
Fuel: Phosphocreatine
ATP Yield: 1 ATP per phosphocreatine ()
Speed: Fastest energy system
Duration: 0–10 seconds
Example Activities:
60m sprint
Maximum jump
Limitation:
Depletion of phosphocreatine
2. Glycolysis
Location: Cytosol
Oxygen Requirement:
No (anaerobic glycolysis)
Yes (aerobic glycolysis)
Fuel:
Glucose or glycogen
ATP Yield:
2 ATP from glucose
3 ATP from glycogen
Duration: Approximately 10 seconds to 2 minutes
Example Activities:
400m sprint
Limitation:
Accumulation of hydrogen ions (H⁺), leading to acidosis
3. Oxidative Phosphorylation
Location: Mitochondria
Oxygen Requirement: Yes
Fuel: Primarily carbohydrates and fats (small contributions from protein)
ATP Yield: Approximately 30–32 ATP per glucose ()
Speed: Slowest of the energy systems
Duration: 2 or more minutes
Example Activities:
Distance running
Limitation:
Oxygen delivery and mitochondrial capacity
Regulation of Metabolism
Rate-Limiting Enzymes
Function: Control the speed of metabolic pathways
Examples:
Phosphofructokinase (PFK) → regulates glycolysis
Carnitine palmitoyltransferase-1 (CPT-1) → regulates fat oxidation
Role of ADP
Effect: Increase in ADP levels leads to an increase in ATP production
Significance: ADP is a primary regulator of oxidative metabolism
Feedback Inhibition
Mechanism: End product of a metabolic pathway inhibits an earlier step
Example: ATP inhibits glycolysis, slowing down the pathway when ATP levels are sufficient
Phosphorylation vs Hydrolysis
Phosphorylation: Process of adding a phosphate group to a molecule
Hydrolysis: Process of breaking a bond using water, often releasing energy
Catabolism vs Anabolism
Catabolism: Processes that break down molecules, releasing energy
Anabolism: Processes that build molecules, requiring energy input
Glycolysis & Aerobic Metabolism
Glycolysis
Purpose:
Break down glucose into pyruvate
Produce ATP and NADH ( ext{Nicotinamide adenine dinucleotide})
Aerobic Conditions
If Oxygen is Present:
Pyruvate is converted into Acetyl-CoA, entering the Krebs Cycle
Anaerobic Conditions
If No Oxygen is Present:
Pyruvate is converted into lactate
Krebs Cycle
Location
Occurs in the mitochondria
Outputs
Produces:
NADH
FADH₂
CO₂
Electron Transport Chain (ETC)
Function: Utilizes NADH and FADH₂ to create a proton gradient across the mitochondrial membrane
Final Electron Acceptor: Oxygen
Outcome: Produces the majority of ATP during cellular respiration
Fat Metabolism
Lipolysis
Process: Breaks down triglycerides into glycerol and free fatty acids
Beta-Oxidation
Process: Converts fatty acids into Acetyl-CoA
Reasons for Slower Fat Metabolism
More enzymatic steps are required
Metabolism requires oxygen
Slower transport of fatty acids into mitochondria
When Fat Dominates
Typically occurs during:
Low-intensity exercise
Long-duration activities
When glycogen stores are depleted
Lactate Metabolism
Lactate Formation
Occurs when glycolytic rate exceeds mitochondrial capacity to process pyruvate
Causes of Accumulation
High-intensity exercise leading to increased H⁺ production
Lactate Shuttle
Lactate can be transported to different sites in the body:
Other muscle fibers
Heart
Liver
Cori Cycle
Process: Lactate is converted into glucose in the liver
Important Note: Lactate is a usable fuel source during metabolism.
Glycogen Utilization
Muscle Glycogen
Location: Used locally within the muscle
Liver Glycogen
Purpose: Helps maintain blood glucose levels
"Hitting the Wall"
Condition occurs when liver glycogen is depleted leading to extreme fatigue and decreased performance
Muscle Fiber Types
Type I (Slow-Twitch)
Characteristics:
High mitochondrial density
High concentration of oxidative enzymes
High fatigue resistance
Low glycolytic capacity
Best Suited for: Endurance activities
Type II (Fast-Twitch)
Type IIa:
Mixed oxidative and glycolytic characteristics
Type IIx:
High glycolytic capacity
Low mitochondrial density
Fatigues quickly
Best Suited for: Sprinting and power activities
Oxidative Capacity
Definition: The ability of muscle tissue to utilize oxygen to produce ATP
Indicators:
Mitochondrial density
Concentration of oxidative enzymes (e.g., citrate synthase)
Implication: Higher oxidative capacity corresponds to better endurance performance
Oxygen Kinetics
VO₂ Response
Description: Oxygen consumption increases with exercise intensity
Oxygen Deficit
Occurs at the beginning of exercise when energy is supplied anaerobically
EPOC (Excess Post-Exercise Oxygen Consumption)
Function: Recovery phase post-exercise requiring elevated oxygen levels
Restores:
Phosphocreatine stores
Oxygen stores
Lactate processing
Respiratory Exchange Ratio (RER)
RER values indicate fuel utilization:
0.7: Primarily fat utilization
0.85: Mixed usage of fats and carbohydrates
1.0: Primarily carbohydrate utilization
1.0: Indicative of high-intensity effort
Nervous System
Afferent Nerves
Function: Sensory nerves that carry information to the Central Nervous System (CNS)
Efferent Nerves
Function: Motor nerves that transmit signals from the CNS to muscles
Myelination
Importance: Increases the conduction velocity of nerve impulses
Brain Regions Involved in Movement
Motor Cortex: Controls voluntary movements
Cerebellum: Coordinates movements
Basal Ganglia: Regulates movement initiation and control
Hormonal Regulation
Negative Feedback Mechanism
Description: Hormone release halts when physiological goals are met
Opposing Hormones
Insulin: Decreases blood glucose levels
Glucagon: Increases blood glucose levels
Hormonal Changes During Exercise
Insulin: Decreases
Glucagon: Increases
Epinephrine: Increases
Cortisol: Increases (especially during longer durations of exercise)
Purpose: Maintain blood glucose levels during physical activity
Applied Exercise Scenarios
Short Maximum Sprint (0–10 sec)
Dominated by ATP-PCr system
Fuel from phosphocreatine (PCr)
Primarily involves Type II muscle fibers
400m Effort
Dominated by glycolysis
High accumulation of lactate due to the intensity and duration
Endurance Event
Dominated by the oxidative system
Carbohydrates predominantly used early in the event
More fat utilized as the event continues
Primarily involves Type I muscle fibers
Steady-State Submaximal Exercise
Involves oxidative phosphorylation
Utilizes a mix of fat and carbohydrates as fuel
Stable VO₂ levels are maintained throughout the exercise
BIG PICTURE INTEGRATION
As Intensity Increases:
Fat utilization decreases
Carbohydrate utilization increases
Lactate levels increase
Recruitment of Type II muscle fibers increases
As Duration Increases:
Contribution from fat increases
Glycogen availability decreases
Risk of "hitting the wall" increases