Exercise and Metabolism: Human Physiology Practice Flashcards

Bioenergetics and the Concept of Energy

• The Upper Limit of Performance:

  • All sports involve muscular activity. When physical limits are reached, performance declines through loss of power, speed, skill, and coordination.

  • Protein formation is dictated by genes, which control cell metabolism, governing cell/tissue structure and function (speed, strength, stamina, skill).

• Thermodynamics in the Human Body:

  • The body requires a continuous supply of energy for biological work.

  • Macronutrients: Energy is released from fats, carbohydrates, and proteins via step-wise reactions. Bonds are split to release energy and formed to store it.

  • Free Energy Equation Example: $Glucose + Oxygen + Activation\,energy \rightarrow Carbon\,dioxide + Water + Free\,energy$.

  • Body heat and enzymes help overcome the required activation energy.

• ATP: The Universal Energy Currency:

  • Adenosine Triphosphate (ATP): Consists of Adenine, Ribose, and three Phosphate groups.

  • Chemical energy is stored in phosphate bonds; potential energy from food is transferred to ATP to power biological work.

  • Reaction: $ATP \rightarrow ADP + P_i + energy$ .

Macronutrient Energy Density

• Energy Content per Gram:

  • Carbohydrate: $16.0\,kJ/g$ ($3.8\,kcal/g$).

  • Fat: $37.0\,kJ/g$ ($9.0\,kcal/g$).

  • Protein: $17.0\,kJ/g$ ($4.0\,kcal/g$).

  • Alcohol: $29.0\,kJ/g$ ($7.0\,kcal/g$).

Overview of Energy Systems

• System Classes:

  • Anaerobic: Can occur in the absence of oxygen (e.g., ATP-PCr, Glycolysis).

  • Aerobic: Requires the presence of oxygen (e.g., Citric Acid Cycle, Oxidative Phosphorylation).

• Energy System Characteristics:

  • ATP Hydrolysis: Powers exercise for $1-2\,sec$. Intensity: Maximal. $100\%$ anaerobic.

  • Phosphocreatine (PCr) System: Powers exercise for $10-15\,sec$. Intensity: Very high. Fuel: PCr. Examples: $100\,m$ sprint. $85\%$ anaerobic, $15\%$ aerobic.

  • Glycolysis: Powers exercise for $15\,sec$ to $3-5\,min$. Intensity: High. Fuel: Glycogen. Examples: $400-800\,m$. Approximately $70\%$ anaerobic, $30\%$ aerobic.

  • Citric Acid Cycle & Electron Transport Chain: Powers exercise from $2-3\,min$ onwards. Intensity: Low to Moderate. Fuel: Glycogen and Fat. Example: Marathon. <3\% anaerobic, >97\% aerobic.

Detailed Energy Pathways

• The Phosphagen System (ATP-PCr):

  • PCr acts as an "energy reservoir" for rapid ATP recycling.

  • Cells store $4-6$ times more PCr than ATP.

  • Reaction: $PCr + ADP \xrightarrow{Creatine\,Kinase} ATP + Creatine$.

  • Rate of ATP re-synthesis is instantaneous (index of $9$).

• Glycolysis:

  • Breakdown of glucose ($C_6H_{12}O_6$) or glycogen into pyruvate ($C_3H_3O_3$).

  • Consists of 10 enzymatic reactions in the cell cytoplasm. Converts one 6-carbon molecule to two 3-carbon molecules.

  • Aerobic/Low Intensity: Pyruvate enters the Citric Acid Cycle; Hydrogen is accepted by NAD to become NADH for the Electron Transport Chain.

  • Anaerobic/High Intensity: Hydrogen binds to pyruvate to form Lactate ($C_3H_5O_3$) via Lactate Dehydrogenase (LDH).

  • Energy Gain: Net gain of $2 \times ATP$ per glucose molecule ($2$ invested, $4$ produced).

• Citric Acid Cycle (Krebs Cycle):

  • Occurs in the mitochondrial matrix.

  • Pyruvate converted to Acetyl-CoA ($C_2H_3O$) via Pyruvate Dehydrogenase.

  • Acetyl portion joins oxaloacetate to form citrate (catalyzed by citrate synthase).

  • Produces $1 \times ATP$ per cycle ($2$ per glucose), plus $CO_2$, NADH, and $FADH_2$.

• Electron Transport Chain (Oxidative Phosphorylation):

  • Uses NADH and $FADH_2$ as hydrogen/electron donors.

  • Involves four complexes (1-4) in the inner mitochondrial membrane.

  • Creates a concentration gradient and electric potential via active pumping of $H^+$ ions into the outer compartment.

  • ATP Synthase: Uses the diffusion of $H^+$ ions to synthesize ATP.

  • Yield: $30-34 \times ATP$ molecules. Total maximum ATP from one glucose molecule: $38$.

Muscle Structure and Contraction Mechanism

• Hierarchical Structure: Whole muscle —> Muscle fiber —> Fibril —> Myofilaments (Myosin and Actin).

• Sliding Filament Theory:

  1. ATP attaches to the myosin head.

  2. ATP hydrolysis provides energy for myosin to detach and "cock" back.

  3. Myosin binds to actin to form a cross-bridge.

  4. Energy release causes the "pull" or power stroke (sliding).

  5. A new ATP must bind for the myosin to detach and repeat the cycle.

Sprinting and High-Intensity Exercise

• Performance and Fatigue:

  • Power output declines rapidly after the first few seconds of maximal exercise.

  • Fatigue in short sprints is primarily attributed to the depletion of muscle ATP and PCr stores.

  • Recovery: PCr regeneration follows a curvilinear path, requiring several minutes to return to pre-exercise levels.

• Creatine Supplementation:

  • Found naturally in meat, fish, and poultry ($4-5\,g/kg$).

  • $95\%$ of body creatine is in the muscle.

  • Supplementation Protocol: $20-30\,g/day$ of creatine monohydrate for $5$ days can increase muscle Cr by up to $30\%$.

  • Mechanisms of Improvement: Increased rate of ATP re-synthesis, delayed PCr depletion, potential reduction in lactate accumulation, and ability to train at higher intensities.

Middle-Distance Performance and Acidosis

• The Oxygen Deficit:

  • Calculated as the difference between oxygen required for a task and oxygen actually consumed.

  • Example: A $1500\,m$ run requiring $350\,ml\,O_2/kg$. If $VO_2max$ provides $280\,ml$ over 4 minutes, the oxygen debt is $70\,ml$, which must be met by anaerobic systems.

• Lactic Acid Controversy:

  • Traditional view (A.V. Hill, 1929): Lactic acid accumulation causes fatigue by blocking contractile proteins via acidosis.

  • Contemporary view (Pedersen et al., 2004): Intracellular acidosis may actually preserve muscle excitability.

  • Mechanism: Acidosis decreases Chloride ($Cl^-$) channel activity, which helps sustain action potentials in the T-tubules despite extracellular Potassium ($K^+$) accumulation and depolarization.

• Bicarbonate Supplementation:

  • Dose: $0.3\,g$ of $NaHCO_3$ per kg body mass (e.g., $21\,g$ for a $70\,kg$ individual).

  • Mechanism: Increases extracellular pH, promoting a faster efflux of $H^+$ ions from the muscle cell, delaying intracellular acidification.

  • Side Effects: Can cause gastrointestinal distress due to $CO_2$ production.

Endurance Performance and Carbohydrate Utilization

• Substrate Dynamics:

  • As exercise intensity increases (measured as $$), reliance shifts from fats to muscle glycogen and blood glucose.

  • Fat stores are massive (e.g., adipose triglyceride provides energy for $≈ 92.5\,hours$), but glycogen is limited (muscle stores last $≈ 60\,minutes$ at high intensity).

• Carbohydrate Loading and Supplementation:

  • Loading Goal: To maximize pre-exercise muscle glycogen content.

  • Performance Impact: High-CHO diets significantly increase exercise capacity/time to fatigue compared to normal or low-CHO diets.

  • Post-Exercise: Consuming $50-100\,g$ of CHO immediately after training helps restore stores.

  • During Exercise: $30-60\,g$ of CHO per hour is recommended for sessions lasting over one hour to spare plasma glucose and maintain intensity.

Comparative VO2 Max Scores

• Top Recorded Scores (Men):

  • Oskar Svendsen (Cycling): $97.5\,ml/kg/min$ (recorded at 18 years old).

  • Espen Harald Bjerke (XC Skier): $96.0\,ml/kg/min$.

  • Bj%rn D%hlie (XC Skier): $96.0\,ml/kg/min$.

  • Greg LeMond (Cycling): $92.5\,ml/kg/min$.

  • Matt Carpenter (Runner): $92.0\,ml/kg/min$.

• Health Correlations:

  • VO2 max is a strong predictor of survival. The top $2\%$ of fitness levels show a $97\%$ 10-year survival rate compared to $77\%$ in the bottom $25\%$.