🪃Principles of Strength and Conditioning Unit 4 – Exercise Physiology

Key Concepts and Definitions

• Homeostasis maintains stable internal conditions necessary for optimal functioning of the body's systems
• Metabolism encompasses all chemical reactions involved in converting nutrients into energy (catabolism) and building new tissues (anabolism)
• Adenosine Triphosphate (ATP) serves as the primary energy currency for cellular processes
  • Consists of adenosine and three phosphate groups
  • Hydrolysis of ATP to ADP + Pi releases energy for cellular work
• Mitochondria are the powerhouses of the cell where aerobic energy production occurs (cellular respiration)
• Glycolysis is the anaerobic breakdown of glucose to pyruvate in the cytoplasm
  • Produces 2 ATP and 2 NADH per glucose molecule
• Krebs cycle (citric acid cycle) oxidizes acetyl-CoA derived from carbohydrates, fats, and proteins to generate NADH and FADH2 for the electron transport chain
• Oxidative phosphorylation is the aerobic production of ATP in the mitochondria using the electron transport chain and chemiosmosis

Energy Systems and Metabolism

• Three energy systems contribute to ATP production during exercise phosphagen system, glycolytic system, and oxidative system
• Phosphagen system provides immediate energy using stored ATP and creatine phosphate
  • Predominates in high-intensity, short-duration activities (weightlifting, sprints)
  • Replenishes ATP rapidly but has limited capacity
• Glycolytic system produces ATP through anaerobic breakdown of glucose or glycogen
  • Predominates in moderate to high-intensity activities lasting 30 seconds to 2 minutes (400m sprint)
  • Produces lactic acid as a byproduct, leading to muscle fatigue
• Oxidative system generates ATP through aerobic metabolism in the mitochondria
  • Predominates in low to moderate-intensity activities lasting several minutes or longer (marathon running)
  • Utilizes carbohydrates, fats, and proteins as substrates
  • Most efficient energy system but has a slower rate of ATP production
• Lactate threshold is the exercise intensity at which lactate begins to accumulate in the blood
  • Training can improve lactate threshold, allowing for higher exercise intensities before fatigue sets in

Cardiovascular Adaptations to Exercise

• Cardiac output ($Q$) increases during exercise to meet increased oxygen and nutrient demands of working muscles
  • $Q = HR × SV$, where $HR$ is heart rate and $SV$ is stroke volume
• Heart rate increases due to sympathetic nervous system activation and parasympathetic withdrawal
• Stroke volume increases due to enhanced contractility and increased venous return (Frank-Starling mechanism)
• Redistribution of blood flow during exercise shunts blood away from inactive tissues (gastrointestinal tract) to active muscles
• Regular aerobic exercise leads to cardiovascular adaptations
  • Increased left ventricular size and wall thickness (eccentric hypertrophy)
  • Increased capillary density in trained muscles
  • Decreased resting heart rate (bradycardia) due to increased vagal tone
• Cardiovascular drift is the gradual increase in heart rate and decrease in stroke volume during prolonged exercise, maintaining constant cardiac output

Respiratory Responses During Exercise

• Ventilation ($\dot{V}_E$) increases during exercise to meet increased oxygen demands and remove carbon dioxide
  • $\dot{V}_E = f × V_T$, where $f$ is breathing frequency and $V_T$ is tidal volume
• Tidal volume increases due to increased contraction of inspiratory muscles (diaphragm, external intercostals)
• Breathing frequency increases due to stimulation of respiratory centers in the brainstem
• Respiratory exchange ratio (RER) is the ratio of carbon dioxide production to oxygen consumption ($\dot{V}CO_2 / \dot{V}O_2$)
  • RER reflects the relative contribution of carbohydrates and fats to energy production
  • RER of 1.0 indicates primarily carbohydrate metabolism, while 0.7 indicates primarily fat metabolism
• Ventilatory threshold is the point during incremental exercise where ventilation increases disproportionately to oxygen consumption
  • Reflects the onset of anaerobic metabolism and lactate accumulation
• Maximal oxygen uptake ($\dot{V}O_{2max}$) is the maximum rate at which an individual can consume and utilize oxygen during exercise
  • Determined by the Fick equation $\dot{V}O_2 = Q × (a-\bar{v})O_2$, where $(a-\bar{v})O_2$ is the arteriovenous oxygen difference

Muscular Adaptations to Training

• Resistance training leads to muscle hypertrophy, an increase in muscle cross-sectional area
  • Hypertrophy results from increased protein synthesis and satellite cell activation
• Neural adaptations precede hypertrophy in the early stages of resistance training
  • Increased motor unit recruitment and firing frequency
  • Improved coordination and skill acquisition
• Fiber type-specific adaptations occur with different training modalities
  • High-intensity resistance training promotes growth of type II (fast-twitch) fibers
  • Endurance training enhances oxidative capacity of type I (slow-twitch) fibers
• Endurance training increases mitochondrial density and capillary density in trained muscles
  • Mitochondrial biogenesis is stimulated by PGC-1α, a transcriptional coactivator
• Delayed onset muscle soreness (DOMS) is the pain and stiffness felt 24-72 hours after unaccustomed or intense exercise
  • Caused by microtrauma to muscle fibers and inflammation
• Repeated bout effect is the adaptive response where muscles become less susceptible to DOMS after an initial bout of exercise

Thermoregulation and Exercise

• Core body temperature increases during exercise due to heat production from metabolic processes
• Heat dissipation mechanisms include radiation, conduction, convection, and evaporation (sweating)
  • Sweating is the primary means of heat loss during exercise
  • Evaporation of sweat cools the skin, which in turn cools the blood
• Cutaneous vasodilation increases blood flow to the skin to facilitate heat exchange with the environment
• Dehydration impairs thermoregulation and exercise performance
  • Decreases plasma volume, leading to reduced stroke volume and cardiac output
  • Increases heart rate to compensate for reduced stroke volume (cardiovascular drift)
• Heat acclimatization improves the body's ability to regulate temperature in hot environments
  • Increases sweat rate and decreases sodium concentration in sweat
  • Expands plasma volume, improving cardiovascular stability
• Hyponatremia is a potentially dangerous condition caused by excessive fluid intake and electrolyte imbalance
  • Symptoms include headache, confusion, nausea, and in severe cases, seizures and coma

Fatigue and Recovery Mechanisms

• Fatigue is the acute impairment of exercise performance, characterized by a decrease in muscular force or power output
• Central fatigue originates in the central nervous system and involves reduced neural drive to working muscles
  • Caused by factors such as decreased motivation, altered neurotransmitter levels (serotonin, dopamine), and afferent feedback from fatigued muscles
• Peripheral fatigue occurs within the muscle itself and involves impaired excitation-contraction coupling
  • Caused by factors such as metabolite accumulation (lactate, H+), glycogen depletion, and impaired calcium release from the sarcoplasmic reticulum
• Recovery processes aim to restore homeostasis and adapt to the stress of exercise
  • Replenishment of energy stores (ATP, creatine phosphate, glycogen)
  • Removal of metabolic byproducts (lactate)
  • Repair of damaged tissues (muscle fibers, connective tissue)
• Post-exercise nutrition plays a crucial role in recovery
  • Carbohydrate intake replenishes glycogen stores
  • Protein intake provides amino acids for muscle protein synthesis and tissue repair
• Sleep is essential for optimal recovery and adaptation
  • Growth hormone release during deep sleep promotes tissue repair and growth
  • Inadequate sleep can impair cognitive function, mood, and exercise performance

Practical Applications in Strength and Conditioning

• Periodization is the systematic planning of training programs to optimize performance and minimize the risk of overtraining
  • Involves manipulating training variables such as volume, intensity, and exercise selection over time
  • Common periodization models include linear, undulating, and block periodization
• Resistance training programs should be designed to target specific goals (strength, hypertrophy, power)
  • Strength-focused programs emphasize high loads (>85% 1RM) and low repetitions (1-5)
  • Hypertrophy-focused programs use moderate loads (67-85% 1RM) and higher repetitions (6-12)
  • Power-focused programs incorporate explosive movements (Olympic lifts, plyometrics) and emphasize high velocity
• Aerobic training programs should be designed to improve cardiovascular fitness and endurance
  • Continuous training involves sustained, moderate-intensity exercise (60-80% HRmax) for extended durations
  • Interval training alternates high-intensity work periods with lower-intensity recovery periods
    • High-intensity interval training (HIIT) uses short, near-maximal intensity work periods (90-100% HRmax) with active recovery
• Warm-up and cool-down routines are important for injury prevention and recovery
  • Warm-up prepares the body for exercise by increasing muscle temperature, enhancing range of motion, and improving neuromuscular function
  • Cool-down promotes gradual return to resting state and may aid in lactate removal
• Flexibility training, including stretching and mobility exercises, helps maintain joint range of motion and reduces the risk of injury
  • Static stretching is most effective when performed after exercise when muscles are warm
  • Dynamic stretching is preferred before exercise as part of the warm-up routine
• Proper exercise technique and form are essential for maximizing training effects and minimizing the risk of injury
  • Strength and conditioning professionals should provide guidance and feedback to ensure safe and effective execution of exercises