Muscle Physiology Chapter 3

Skeletal Muscle Structure

• Sarcomeres are the smallest functional unit of muscle tissue, comprised of thin (actin) and thick (myosin) filaments.
• Myosin heads form cross-bridges with actin filaments, contracting muscles by bringing Z-lines closer together.
• ATP serves as the energy source, activating ATPase sites on myosin heads.

Muscle Fiber Types

• Type I fibers are slow, oxidative, and ideal for repeated low-force contractions over extended periods; they contain high mitochondria and capillaries.
• Type II fibers, including IIa (fast, oxidative-glycolytic) and IIx (fast, glycolytic), are suited for quick, intense contractions but have fewer mitochondria and weaker capillary supply.
• Muscle composition varies: elite marathoners may have ~20% Type II fibers in vastus lateralis, while elite sprinters may reach ~60%.
• Postural muscles consist predominantly of Type I fibers (>70%).

Energy Production (ATP)

• ATP is continuously regenerated during exercise through:
  • Phosphocreatine hydrolysis.
  • Anaerobic metabolism of glycogen or glucose.
  • Aerobic metabolism of acetyl-CoA, primarily from carbohydrate and fat breakdown.
• Keto-acids from some amino acids can fuel oxidative metabolism but contribute minimally during exercise.

Spirometry and Oxygen Consumption Analysis

• Closed-circuit spirometry measures energy expenditure (EE) under resting conditions, showing decreased oxygen volume with consumption.
• Open-circuit spirometry compares inhaled air with exhaled air (Douglas Bag technique) to assess gas exchange during exercise.
• Breath-by-breath systems provide accurate EE assessments and frequent feedback but are primarily lab-based.

Respiration Chamber and Tracer Methods

• Respiration chambers accurately measure energy balance and gas exchange, used for controlled dietary studies.
• Doubly labeled water determines energy expenditure over days or weeks via isotope detection, though it's costly.
• Labeled bicarbonate provides a less expensive estimation of EE through changes in CO2 production.

Heart Rate Monitoring

• Heart rate (HR) correlates linearly with oxygen consumption during submaximal exercise, aiding in EE estimation.
• Rest-related movements can influence HR without impacting oxygen consumption.

Accelerometer and Activity Records

• Accelerometers detect body movements, estimating activity levels and energy expenditure, though they often underestimate actual EE. Triaxial devices are more effective than uniaxial.
• 24-hour activity records provide a rough EE estimation, but can be biased due to personal reporting inaccuracies.

Components of Energy Expenditure

• Resting metabolic rate (RMR) accounts for 60% to 75% of daily energy expenditure; varies by fat-free mass, age, gender, and genetics.
• Diet-induced thermogenesis contributes ~10% of energy expenditure from food digestion and metabolism.
• Thermic effect of exercise constitutes 15% to 30% of energy expenditure, increasing significantly in active populations.

Energy Balance

• Energy balance is the difference between energy intake and energy expenditure over days or weeks.
• Positive energy balance leads to weight gain, while negative energy balance results in weight loss.

Energy Expenditure in Sports

• Intermittent sports like tennis have lower average energy outputs due to short bursts of high intensity followed by recovery.
• Continuous sports like cycling and running require higher energy outputs with elite females needing >3000 kcal/day and males >4000 kcal/day.

Considerations for Energy Intake in Endurance Events

• Athletes in events such as the Tour de France face challenges in energy intake during competition; significant refeeding occurs post-event.
• Gastrointestinal issues complicate absorption of large calorie volumes during intensive training.

Extreme Energy Expenditures

• High energy expenditures can reach 8,600 kcal/day for cross-country skiers and 10,750 kcal/day for ultraendurance runners without weight loss, provided intake matches expenditure.
• Historical expeditions (e.g., Scott and Shackleton) illustrate extremes, with significant weight loss due to limited energy intake despite high energy expenditure (~1,000,000 kcal over the expedition).

Explore the structure of skeletal muscle in Chapter 3, focusing on sarcomeres as the fundamental units of muscle contraction. This quiz delves into the interaction between actin and myosin filaments, as well as the role of ATP in muscle contraction.