Exercise Physiology Notes

1) Excitation-contraction coupling:

• Sequence of events where nerve signal (action potential) leads to muscle contraction.

• Involves the sarcolemma, T-tubules, sarcoplasmic reticulum (SR), and myofibrils.

• Steps:

1. Action potential travels along motor neuron to neuromuscular junction released.

2. Acetylcholine (ACh) is released, binds to receptors on muscle fiber, causing depolarization.

3. Action potential propagates along the sarcolemma and down T-tubules.

4. Dihydropyridine receptors (DHPR) in T-tubules detect voltage change and communicate with ryanodine receptors (RyR) on the SR.

5. RyR opens, releasing $Ca^{2+}$ from the SR into the sarcoplasm.

6. $Ca^{2+}$ binds to troponin, causing tropomyosin to shift and expose myosin-binding sites on actin.

7. Myosin heads bind to actin, forming cross-bridges.

8. Myosin head pivots (power stroke), pulling actin filament toward the center of the sarcomere.

9. ATP binds to myosin, causing it to detach from actin. ATP hydrolysis provides energy for the next power stroke.

10. $Ca^{2+}$ is actively transported back into the SR, tropomyosin blocks myosin-binding sites, and muscle relaxes.

    • More Details:

    • The process begins with a signal from the nervous system. A motor neuron sends an action potential to the neuromuscular junction, which is the interface between the neuron and the muscle fiber.

    • At the neuromuscular junction, the motor neuron releases acetylcholine (ACh). ACh diffuses across the synaptic cleft and binds to ACh receptors on the muscle fiber's sarcolemma (plasma membrane).

    • The binding of ACh causes depolarization of the sarcolemma, generating an action potential that propagates along the sarcolemma and into the T-tubules.
      2) Muscle fiber types:

• Type I (slow-twitch):

  • High oxidative capacity, fatigue-resistant, used for endurance activities.

  • High mitochondrial density, capillary density, and myoglobin content.

  • Low glycolytic capacity and contraction speed.

• Type IIa (fast-twitch oxidative):

  • Intermediate oxidative and glycolytic capacity, moderately fatigue-resistant.

  • Faster contraction speed and higher force production than type I fibers.

• Type IIx (fast-twitch glycolytic):

  • Low oxidative capacity, easily fatigued, used for short-duration, high-intensity activities.

  • High glycolytic capacity and contraction speed.

• Fiber type distribution varies among individuals and muscles.

  • More Details:

  • Type I fibers are efficient at using oxygen to generate ATP, making them ideal for prolonged, low-intensity activities. They contain more mitochondria and myoglobin, which aid in oxygen delivery and utilization.

  • Type IIa fibers can use both aerobic and anaerobic metabolism to generate ATP. They are recruited for activities requiring more force and power than type I fibers can produce.

  • Type IIx fibers primarily use anaerobic metabolism and can generate high force and power but fatigue quickly. They are best suited for short bursts of intense activity.
    3) Satellite cells:

• Muscle stem cells located between the sarcolemma and basement membrane.

• Activated by muscle damage or overload.

• Proliferate and differentiate into myoblasts, which fuse with existing muscle fibers to repair or hypertrophy them.

• Important for muscle growth and adaptation to exercise.

  • More Details:

  • When muscle fibers are damaged due to exercise or injury, satellite cells become activated. They proliferate (multiply) and differentiate into myoblasts.

  • These myoblasts then fuse with existing muscle fibers, contributing nuclei and cytoplasm to repair the damaged fibers or increase their size (hypertrophy).

  • Satellite cells play a crucial role in muscle adaptation to resistance training, enabling muscle growth and strength gains.
    4) Force regulation:

• Factors influencing force production:

  • Number of motor units recruited

  • Type of motor units recruited (fiber type composition)

  • Size of the muscle

  • Initial muscle length

  • Angle of the joint

  • Neural drive (frequency of stimulation)

  • More Details:

  • The nervous system controls force production by varying the number and type of motor units recruited. More motor units and a higher proportion of fast-twitch fibers lead to greater force.

  • Larger muscles can generate more force due to their greater cross-sectional area and number of sarcomeres.

  • The initial length of the muscle affects force production due to the length-tension relationship (discussed later).

  • The angle of the joint influences the leverage and mechanical advantage of the muscle.

  • Neural drive, or the frequency of stimulation, affects the rate at which motor units fire and produce force.
    5) Force-velocity, force-power and length-tension curves:

• Force-velocity curve:

  • Inverse relationship between force and velocity of muscle contraction.

  • As velocity increases, the force decreases because there is less time for cross-bridges to form and exert force.

• Force-power curve:

  • Power = force x velocity.

  • There is an optimal velocity at which maximal power is generated.

  • At very low or very high velocities, power output is low.

• Length-tension curve:

  • Relationship between the length of the muscle fiber and the force it can generate.

  • Optimal length is where there is maximal overlap between actin and myosin filaments and the greatest number of cross-bridges can form.

  • Too short or too long muscle lengths result in reduced force production.

  • More Details:

  • At high velocities, the rate at which cross-bridges can form and exert force is limited, resulting in lower force production.

  • Maximal power is generated at an intermediate velocity, where both force and velocity are reasonably high.

  • When the muscle is too short, actin filaments overlap, hindering cross-bridge formation. When the muscle is too long, there is insufficient overlap between actin and myosin, also reducing cross-bridge formation.
    6) Motor units- innervation ratios and Size Principle:

• Motor unit: A motor neuron and all the muscle fibers it innervates.

• Innervation ratio: The number of muscle fibers per motor neuron. Fine motor control muscles (e.g., eye muscles) have low innervation ratios, while large, powerful muscles (e.g., quadriceps) have high innervation ratios.

• Size principle: Motor units are recruited in order of size, from smallest (type I) to largest (type II).

  • This allows for efficient and controlled force production.

  • More Details:

  • Muscles involved in fine motor control, such as those in the eyes or hands, have small innervation ratios (e.g., 1:10), allowing for precise movements.

  • Large, powerful muscles, such as those in the legs, have high innervation ratios (e.g., 1:1000), enabling them to generate large forces.

  • The size principle allows for efficient and coordinated muscle activation. Smaller, fatigue-resistant motor units are recruited first for low-intensity activities, while larger, more powerful motor units are recruited as the intensity increases.
    7) Macronutrients:

• Carbohydrates:

  • Primary fuel source during high-intensity exercise.

  • Stored as glycogen in muscles and liver.

  • Recommended intake: 3-12 g/kg body weight per day, depending on activity level.

• Fats:

  • Primary fuel source during low-intensity exercise and rest.

  • Stored as triglycerides in adipose tissue and muscles.

  • Recommended intake: 20-35% of total calories.

• Proteins:

  • Important for muscle repair and growth.

  • Not a primary fuel source, but can be used during prolonged exercise.

  • Recommended intake: 1.2-2.0 g/kg body weight per day, depending on training intensity.

  • More Details:

  • During high-intensity exercise, carbohydrates are broken down rapidly to provide ATP through glycolysis. The recommended intake varies depending on the duration and intensity of exercise.

  • Fats are a dense energy source and are primarily used during low-intensity exercise and rest. They are stored in adipose tissue and muscles as triglycerides.

  • Proteins are essential for repairing damaged muscle tissue and building new muscle proteins. The recommended intake increases with training intensity to support muscle growth and repair.
    8) Bioenergetics, metabolism, and fuel selection:

• Bioenergetics: The study of how energy is transferred and utilized in biological systems.

• Metabolism: The sum of all chemical reactions in the body.

• Fuel selection: The choice of fuel (carbohydrates, fats, proteins) used for energy production depends on exercise intensity and duration.

• ATP-PCr system: Immediate energy source for short, high-intensity activities (e.g., sprinting, weightlifting).

• Glycolysis: Breakdown of glucose or glycogen to produce ATP and pyruvate or lactate. Important for moderate to high-intensity activities.

• Oxidative phosphorylation: Use of oxygen to produce ATP from carbohydrates, fats, or proteins. Primary energy source during low to moderate-intensity activities.

  • More Details:

  • The ATP-PCr system provides immediate energy for short bursts of high-intensity activity. It uses creatine phosphate to regenerate ATP rapidly.

  • Glycolysis breaks down glucose or glycogen to produce ATP and pyruvate or lactate. It is important for moderate to high-intensity activities and can occur with or without oxygen.

  • Oxidative phosphorylation uses oxygen to produce ATP from carbohydrates, fats, or proteins. It is the primary energy source during low to moderate-intensity activities and occurs in the mitochondria.
    9) Lactate threshold and lactate shuttle:

• Lactate threshold: The point during exercise at which blood lactate levels begin to rise exponentially.

  • Indicates a shift towards anaerobic metabolism.

• Lactate shuttle: Lactate can be transported from muscle fibers to other tissues (e.g., heart, liver, brain) to be used as fuel.

  • Lactate is not a waste product, but an important energy source.

  • More Details:

  • During low-intensity exercise, lactate production is low, and it is readily cleared by the body. As exercise intensity increases, lactate production exceeds clearance capacity, leading to an exponential rise in blood lactate levels.

  • Lactate can be transported from muscle fibers to other tissues, such as the heart, liver, and brain, where it is used as fuel. This process is known as the lactate shuttle.

  • Lactate is not a waste product, but an important energy source that can be oxidized to produce ATP.
    10) Hormonal regulation of metabolism:

• Insulin:

  • Released by pancreas in response to high blood glucose levels.

  • Promotes glucose uptake and storage, inhibits fat breakdown.

• Glucagon:

  • Released by pancreas in response to low blood glucose levels.

  • Promotes glycogen breakdown and glucose release, stimulates fat breakdown.

• Epinephrine and norepinephrine:

  • Released by adrenal medulla during exercise.

  • Stimulate glycogen breakdown, increase heart rate and blood pressure.

• Cortisol:

  • Released by adrenal cortex during prolonged exercise and stress.

  • Promotes protein breakdown and glucose production.

  • More Details:

  • Insulin is released by the pancreas in response to high blood glucose levels. It promotes glucose uptake and storage in muscles and the liver, while inhibiting fat breakdown.

  • Glucagon is released by the pancreas in response to low blood glucose levels. It promotes glycogen breakdown and glucose release from the liver, while stimulating fat breakdown to provide energy.

  • Epinephrine and norepinephrine are released by the adrenal medulla during exercise. They stimulate glycogen breakdown, increase heart rate and blood pressure, and enhance alertness.

  • Cortisol is released by the adrenal cortex during prolonged exercise and stress. It promotes protein breakdown and glucose production, helping to maintain blood glucose levels during prolonged activity.
    11) Regulation of heart rate, cardiac output and stroke volume:

• Heart rate (HR): Number of heart beats per minute.

  • Regulated by autonomic nervous system (sympathetic increases HR, parasympathetic decreases HR).

• Stroke volume (SV): Amount of blood ejected per heart beat.

  • Influenced by preload (venous return), afterload (arterial blood pressure), and contractility.

• Cardiac output (Q): Amount of blood pumped per minute (Q = HR x SV).

  • More Details:

  • The autonomic nervous system regulates heart rate. The sympathetic nervous system increases heart rate during exercise, while the parasympathetic nervous system decreases heart rate during rest.

  • Stroke volume is influenced by preload, afterload, and contractility. Preload refers to the amount of blood filling the ventricles before contraction, afterload is the resistance the heart must pump against, and contractility is the forcefulness of ventricular contraction.

  • Cardiac output is the product of heart rate and stroke volume. It represents the amount of blood pumped by the heart per minute.
    12) Cardiovascular drift:

• Gradual increase in heart rate and decrease in stroke volume during prolonged, steady-state exercise.

  • Primarily due to dehydration and increased body temperature.

  • Reduced plasma volume decreases venous return, leading to lower stroke volume and higher heart rate to maintain cardiac output.

  • More Details:

  • During prolonged, steady-state exercise, cardiovascular drift occurs due to dehydration and increased body temperature. Dehydration reduces plasma volume, which decreases venous return and lowers stroke volume.

  • To compensate for the decrease in stroke volume, heart rate gradually increases to maintain cardiac output and meet the body's oxygen demands.
    13) Respiration during exercise:

• Increased ventilation (breathing rate and tidal volume) to meet oxygen demands.

  • Regulated by chemoreceptors (detect changes in blood pH, $CO_2$, and $O_2$) and neural input from the brain.

  • Ventilation increases linearly with exercise intensity up to the ventilatory threshold, then increases exponentially.

  • More Details:

  • Ventilation increases linearly with exercise intensity to meet the body's oxygen demands and remove carbon dioxide. As exercise intensity increases, ventilation increases exponentially beyond the ventilatory threshold.

  • Chemoreceptors detect changes in blood pH, carbon dioxide levels, and oxygen levels, stimulating an increase in ventilation to maintain homeostasis.

  • Neural input from the brain also contributes to the regulation of ventilation during exercise.
    14) Carbon dioxide transport in the blood:

• Dissolved in plasma (7-10%).

• Bound to hemoglobin (20-33%).

• As bicarbonate ions ($HCO_3^−$) (60-70%).This reaction is catalyzed by carbonic anhydrase.

  • More Details:

  • Carbon dioxide is transported in the blood in three primary forms: dissolved in plasma, bound to hemoglobin, and as bicarbonate ions ($HCO_3^−$).

  • A small amount of carbon dioxide is dissolved directly in the plasma.

  • Some carbon dioxide binds to hemoglobin, forming carbaminohemoglobin.

  • The majority of carbon dioxide is transported as bicarbonate ions ($HCO_3^−$). This reaction is catalyzed by the enzyme carbonic anhydrase, which is found in red blood cells.
    15) Factors leading to fatigue:

• Depletion of energy substrates (glycogen, ATP).

• Accumulation of metabolic byproducts (lactate, hydrogen ions, inorganic phosphate).

• Neuromuscular fatigue (impaired nerve transmission, reduced motor unit recruitment).

• Central fatigue (reduced motivation, impaired brain function).

  • More Details:

  • Fatigue can result from various factors, including depletion of energy substrates (glycogen, ATP), accumulation of metabolic byproducts (lactate, hydrogen ions, inorganic phosphate), neuromuscular fatigue (impaired nerve transmission, reduced motor unit recruitment), and central fatigue (reduced motivation, impaired brain function).

  • Depletion of glycogen and ATP limits the availability of energy for muscle contraction.

  • Accumulation of metabolic byproducts, such as lactate and hydrogen ions, can disrupt muscle function and contribute to fatigue.

  • Neuromuscular fatigue involves impaired nerve transmission and reduced motor unit recruitment, limiting the ability to generate force.

  • Central fatigue involves reduced motivation and impaired brain function, decreasing the drive to continue exercising.
    16) Dose-response relationship of exercise and how to improve VO2max:

• Dose-response relationship: The amount of exercise needed to elicit a specific health outcome (e.g., improved $VO_2max$, reduced blood pressure).

• $VO_2max$: The maximum amount of oxygen the body can consume during exercise.

  • Factors influencing $VO_2max$: Genetics, age, sex, training status.

• Ways to improve $VO_2max$:

  • High-intensity interval training (HIIT)

  • Endurance training

  • Cross-training

  • More Details:

  • The dose-response relationship refers to the amount of exercise needed to elicit a specific health outcome, such as improved $VO_2max$ or reduced blood pressure. The optimal dose of exercise varies depending on the individual and the desired outcome.

  • $VO_2max$ is the maximum amount of oxygen the body can consume during exercise. It is influenced by genetics, age, sex, and training status.

  • High-intensity interval training (HIIT), endurance training, and cross-training are effective ways to improve $VO_2max$. HIIT involves short bursts of high-intensity exercise followed by periods of rest or low-intensity exercise. Endurance training involves prolonged, sustained exercise at a moderate intensity. Cross-training involves engaging in a variety of different types of exercise.
    17) Principles of training and exercise prescription:

• Progression: Gradually increasing the intensity, frequency, or duration of training to avoid plateaus and reduce the risk of injury.

• Specificity (SAID principle): The body adapts specifically to the demands placed upon it. Training should be relevant to the desired outcome.

  • SAID: Specific Adaptations to Imposed Demands

• Overload: Exposing

• Individuality: Recognizing that individuals will respond differently to the same training stimulus, and tailoring programs accordingly.

• Reversibility: The benefits of training are lost when training is discontinued.

  • More Details:

  • Progression involves gradually increasing the intensity, frequency, or duration of training to avoid plateaus and reduce the risk of injury. This allows the body to adapt and improve over time.

  • Specificity (SAID principle) states that the body adapts specifically to the demands placed upon it. Training should be relevant to the desired outcome. For example, if the goal is to improve endurance, training should focus on endurance activities.

  • Overload involves exposing the body to stress that is greater than what it is normally accustomed to. This stimulates adaptation and improvement.

  • Individuality recognizes that individuals will respond differently to the same training stimulus, and programs should be tailored accordingly.

  • Reversibility states that the benefits of training are lost when training is discontinued. To maintain the benefits of training, it is important to continue exercising regularly.
    18) Adaptations to resistance and endurance exercise:

• Resistance Exercise:

  • Neural Adaptations: Increased motor unit recruitment, firing rate, and synchronization.

  • Hypertrophy: Increase in muscle fiber size (primarily type II fibers).

  • Metabolic Adaptations: Increased anaerobic enzyme activity, increased glycogen storage.

• Endurance Exercise:

  • Cardiovascular Adaptations: Increased stroke volume, cardiac output, and capillary density in muscles.

  • Respiratory Adaptations: Increased ventilatory efficiency and increased $VO_2max$.

  • Muscular

• Muscular Adaptations: Increased mitochondrial density and increased oxidative enzyme activity.

  • More Details:

  • Resistance exercise leads to neural adaptations, such as increased motor unit recruitment, firing rate, and synchronization. It also causes hypertrophy, which is an increase in muscle fiber size, primarily in type II fibers. Metabolic adaptations to resistance exercise include increased anaerobic enzyme activity and increased glycogen storage.

  • Endurance exercise results in cardiovascular adaptations, such as increased stroke volume, cardiac output, and capillary density in muscles. Respiratory adaptations include increased ventilatory efficiency and increased $VO_2max$. Muscular adaptations include increased mitochondrial density and increased oxidative enzyme activity, enhancing the muscle's ability to use oxygen to generate ATP.