Lecture 10 ExPhys

Key Terminology in Exercise Training

Muscular Strength: * Defined as the maximal force that a muscle or muscle group can generate. * Can also be defined as the maximal weight an individual can lift with a single effort. * Gains in muscular strength involve changes in both the structure of the muscle and its neural control.
Muscular Power: * Defined as the rate at which work is performed. * It is the product of force and velocity ( $\text{Power} = ext{Force} imes ext{Velocity}$ ). * It represents the explosive aspect of strength and includes a speed component.
Muscular Endurance: * Defined as the capacity to perform repeated muscle contractions or to sustain a single contraction over time. * Endurance is characterized by high fatigue resistance. * Improvements are achieved through gains in muscular strength and changes in local blood flow and metabolic function. * Testing: Can be tested using a load equal to $75\%$ of a person's 1-repetition maximum ( $1RM$ ).
Aerobic Power: * Defined as the rate of energy release by cellular metabolic pathways that depend on the continued availability of oxygen. * Synonymous with aerobic capacity and maximal oxygen uptake ( $VO_2max$ ). * Limited primarily by the central cardiovascular system and, to a lesser extent, by respiration and metabolism. * Likely ultimately limited by the inability of Cardiac Output ( $CO$ ) to increase further.
Anaerobic Power: * Defined as the rate of energy release by cellular metabolic processes that function without the involvement of oxygen.

Principles of Training

Principle of Individuality: Individuals do not all possess the same inherent ability to respond to an acute exercise bout or the same capacity to adapt to exercise training.
Principle of Specificity: * Training adaptations are highly specific to the type of activity performed and to the volume and intensity of the exercise. * Adaptations are specific to the mode, intensity, and duration of training.
Principle of Reversibility: Any gains achieved with training will eventually be lost if training is discontinued ("use it or lose it").
Principle of Progressive Overload: To maximize benefits, the training stimulus must be progressively increased as the body adapts to the current stimulus.
Principle of Variation (Periodization): The systematic process of changing one or more variables in an exercise training program (mode, volume, or intensity) over time to keep the stimulus challenging and effective.

Neural Control of Strength Gains

Foundational Concept: Strength is not solely a property of muscle but is a property of the neuromotor system. Neural adaptations are especially important in the early stages of resistance training (RT).
Mechanism of Gains: * Motor Unit Recruitment: Resistance training results in an increase in motor unit recruitment. * Synchronization: Motor units are generally recruited asynchronously. Training may change connections in the spinal cord, allowing units to act more synchronously. This facilitates the rate of force development and the ability to exert steady forces, though it does not necessarily produce a more forceful contraction. * Rate Coding: Refers to the frequency of impulses sent to a muscle. Training can increase the frequency of discharge (rate coding) of $\alpha$ -motor neurons. * Neural Drive: The combination of motor unit recruitment and rate coding. It begins in the Central Nervous System (CNS) and spreads to muscle fibers via peripheral nerves. * Autogenic Inhibition: Inhibitory mechanisms (e.g., Golgi Tendon Organs) prevent muscles from exerting force that could damage bones or connective tissue. Training can reduce these inhibitory impulses, allowing for greater force production. * Coactivation: Resistance training can lead to a reduction in the coactivation of antagonist muscles, allowing the agonist to become effectively stronger. * Neuromuscular Junction: Changes also occur at the junction to facilitate strength.

Muscle Hypertrophy and Structural Changes

Hypertrophy: An increase in muscle size.
Atrophy: A decrease in muscle size.
Transient Hypertrophy: The "pump" felt during and immediately after exercise, resulting from fluid accumulation (edema) in the interstitial and intracellular spaces from blood plasma. This lasts only a few hours.
Chronic Hypertrophy: Long-term increase in muscle size resulting from structural changes. * Fiber Hypertrophy: Increase in the size of existing individual muscle fibers. Caused by an increased number of myofibrils and actin and myosin filaments, providing more cross-bridges. The size of existing myofibrils does not change, but new sarcomeres are added in parallel. * Fiber Hyperplasia: Increase in the total number of fibers. While splitting of fibers occurs in animals, it is believed to contribute only $5\%$ to $10\%$ of muscle size increase in humans under very high-intensity training. This involves the activation of satellite cells.
Role of Eccentric Training: Important for maximizing increases in muscle fiber cross-sectional area. High-velocity eccentric training is particularly effective, likely due to disruptions in sarcomere $Z$ -lines (fiber protein remodeling).
Intensity Thresholds: Traditionally thought to require $60\%$ to $85\%$ of $1RM$ , but more recent research shows intensities $< 50\%$ of $1RM$ can produce equal hypertrophy if performed to failure.
Hormonal Role: Anabolic hormones like testosterone, growth hormone ( $GH$ ), and insulin-like growth factor 1 ( $IGF-1$ ) increase transiently after exercise, but experimental data suggests these acute increases are not strictly required for strength or mass gains.
Timeline of Gains: * Week 2 to Week 10: Primarily Neural Activation. * Week 10 and beyond: Primarily Muscle Fiber Hypertrophy.

Muscle Atrophy and Inactivity

Immobilization: Major changes begin within hours of muscle inactivity. * Protein synthesis starts to decrease within $6$ hours. * Strength decreases most dramatically in the first week at a rate of $3\%$ to $4\%$ per day. * Results in atrophy and decreased neuromuscular activity.
Cessation of Training: Strength can be maintained for at least $12$ weeks with reduced training frequency.

Fiber Type Alterations

Recent research indicates fiber types can transition based on training: * High-intensity RT and speed work can convert Type I fibers to Type IIa. * Type IIx fibers typically decrease as Type IIa fibers increase.
Muscle Fiber Distribution Estimates: * Mostly Fast Twitch: Pectorals, Deltoids, Traps, Biceps, Lats, Glutes. * Near $50$ - $50$ Split: Triceps, Hamstrings. * Mostly Slow Twitch: Soleus, Quadriceps, Gastrocnemius, Core.

Nutrition and Protein Synthesis

Protein Balance: Hypertrophy requires synthesis to exceed breakdown (net positive balance).
Daily Requirements: * General public: $0.8\,g/kg$ or $lb$ per day. * Athletes: $1.0$ to $1.7\,g/kg$ or $lb$ per day.
Timing: $20$ to $25\,g$ of protein immediately after exercise is recommended.
Type of Protein: Rapidly digested, rich in essential amino acids (e.g., Whey Protein).
mTOR: The mammalian target of rapamycin ( $mTOR$ ) primarily controls the rate of protein synthesis within myofibrils. Hypertrophy cannot occur without $mTOR$ .
Synthesis Post-Exercise: A single bout of resistance exercise can elevate net protein synthesis for up to $24$ hours.

Adaptations for Special Populations

Older Adults: Experience Sarcopenia (loss of muscle mass). RT helps increase independence and prevent injuries. They require larger amounts of protein than younger individuals.
Children: RT is safe and effective for increasing strength and mass provided proper precautions and lifting forms are used; potential for growth plate injuries is a common debate.
Athletes: Training is sport-specific and used for performance improvement and injury prevention.

Cardiovascular Adaptations to Aerobic Training

VO2max: The highest rate of oxygen consumption during maximal exercise. Endurance training increases $VO_2max$ by an average of $15\%$ to $20\%$ over $20$ weeks.
Heart Size: * "Athlete's Heart" is nonpathological enlargement. * The left ventricle undergoes the greatest adaptation. Left ventricular chamber size and wall thickness increase, leading to increased filling and stroke volume.
Stroke Volume (SV): Increases at rest, submaximal, and maximal efforts. * Untrained SV Max: $80$ to $109\,ml/beat$ . * Trained SV Max: $110$ to $149\,ml/beat$ . * Highly Trained SV Max: $150$ to $220+\,ml/beat$ .
Hemodynamic Factors: SV increases due to increased contractility, increased preload (plasma volume, diastolic filling time), and decreased afterload (reduced total peripheral resistance).
Heart Rate (HR): * Resting HR decreases (Bradycardia is $< 60\,bpm$ ; elite athletes can be in the $30$ s or $40$ s) due to increased parasympathetic vagal tone. * Submaximal HR is lower at any absolute intensity. * Maximal HR remains stable or decreases slightly.
Cardiac Output (CO): * Resting and submaximal CO remains largely unchanged; maximal CO increases significantly.
Blood Flow: * Increased capillarization ( $> 15\%$ increase in number of capillaries). * Greater recruitment of existing capillaries and better redistribution (shunting) away from inactive regions.
Blood Volume: Increases with training. * Plasma volume increases through increased plasma proteins (oncotic pressure) and release of ADH/Aldosterone. * Red Blood Cell ( $RBC$ ) count increases, leading to higher hemoglobin and oxygen-carrying capacity.

Pulmonary and Metabolic Adaptations

Pulmonary Ventilation: Unchanged at rest, decreases up to $30\%$ at submaximal intensities, and increases at maximal intensities (up to $180\,L/min$ ).
Arterial-Venous Oxygen Difference (a-v)O2: Increases with training due to greater oxygen extraction by active tissues.
Muscle Changes: * Type I fibers increase in cross-sectional area. * Myoglobin content increases by $75\%$ to $80\%$ . * Mitochondrial number and size increase. * Oxidative enzyme activity increases, leading to "glycogen sparing" (using more fat for fuel).
Lactate Threshold: Increases due to reduced lactate production and increased clearance, which is closely linked to improved performance.
Respiratory Exchange Ratio (RER): Decreases at submaximal intensities, reflecting increased reliance on free fatty acids rather than carbohydrates ( $CHO$ ).

Limits and Factors of Aerobic Power

Limitations: Evidence strongly supports that central and peripheral cardiovascular factors (delivery of oxygen) limit endurance capacity rather than mitochondrial oxidative enzymes.
Influencing Factors: * Heredity: Genetics determine a "genetic window" for $VO_2max$ . * Sex: Untrained females typically have lower $VO_2max$ ; estrogen may impact vasculature in post-menopausal women. * Training Status: Highly conditioned individuals see smaller relative improvements.

Anaerobic Training and HIIT

Testing: Wingate anaerobic test is the most widely used measurement.
Muscle Adaptations: Higher recruitment and cross-sectional area of Type II fibers, particularly Type IIa.
Energy Systems: * ATP-PCr System: Minimal changes; some increase in enzymes if bouts are $< 6\,s$ . * Glycolytic System: Increased activity of key enzymes with $30\,s$ bouts.
HIIT: High-intensity interval training induction mirrors aerobic benefits in a time-efficient manner.
Cross-training: Refers to training for multiple sports or fitness components simultaneously.

Physical Activity Guidelines and Monitoring

Minimum Requirements for Health: * Option 1: $150$ minutes moderate intensity exercise per week + $2$ days RT. * Option 2: $75$ minutes vigorous intensity exercise per week + $2$ days RT. * Option 3: A mix of both + $2$ days RT.
Monitoring Intensity: * RPE Scale: Rate of Perceived Exertion (1 to 10). * 10: Max effort/Impossible. * 7-8: Vigorous/Borderline uncomfortable. * 4-6: Moderate/Breathing heavily. * METS (Metabolic Equivalents): Light ( $3.2$ to $5.3$ ), Moderate ( $5.4$ to $7.5$ ), Hard ( $7.6$ to $10.2$ ).

Flexibility Training

Proprioceptors: * Muscle Spindles: Trigger stretch reflex, causing muscle to contract when stretched. * Golgi Tendon Organs (GTOs): Located at muscle/tendon junctions; detect tension and cause muscle to relax (autogenic inhibition).
Methodology: Stretch slowly to minimize the stretch reflex and activate GTOs. Holds should be a minimum of $20$ seconds, though some suggest $30$ to $90$ seconds for flexibility enhancement.