Muscle Fiber Arrangement

Muscle Fiber Arrangement

• Muscle fiber arrangement affects muscle function in skeletal muscle
• Fascicles are arranged differently and divided into four basic structures

Circular

• Fascicles are arranged into concentric rings that surround external body openings

Converging

• Fascicles converge toward a single tendon of insertion and are triangular shaped

Parallel

• Long muscles whose fibers align parallel to the line of pull and can shorten over greater distances
  • Strap muscles are a type of parallel muscle with a narrow, strap-like belly
  • Fusiform muscles are spindle-shaped with an extended belly

Pennate

• Muscles with shorter fibers not aligned with the line of pull, enabling greater force exertion
  • Unipennate muscle fascicles insert into one side of the tendon
  • Bipennate muscle fascicles insert into the tendon from opposite sides
  • Multipennate muscle fascicles insert into one tendon
• PSA (Physiological cross-sectional area) runs perpendicular to the direction of muscle fibers
• ASCA (Anatomical cross-sectional area) is perpendicular to the line of action

Muscle Tissue Characteristics

• Muscle tissue is excitable, responding to stimuli
• Muscles are contractile, extensible, and elastic
• Muscles produce movement, maintain posture, stabilize joints, and produce heat

Muscle Connective Tissue

• Epimysium wraps around the muscle
• Fascicles are bundles of muscle fibers
• Perimysium surrounds fascicles
• Endomysium surrounds individual muscle fibers/cells
• Sarcolemma lies under the endomysium layer

Tendons and Muscle Attachment

• Tendons are collagen-rich and connect muscle to bone
• Muscle contraction moves the tendon, which moves the bone
• The bone that doesn't move during muscle contraction is the origin
• The bone that moves during muscle contraction is the insertion
• Connective sheets contribute to contractile plasticity, promoting elasticity
• Nerve fibers and blood vessels run through these connective sheets

Direct Connection

• The epimysium fuses with the bone by fusing with the periosteum/perichondrium

Indirect Connection

• Tendons connect to the bone, which is more common
• Tendons can resist rubbing against the bone effectively

Tendon Plate

• Tendons are dense connective tissue with collagen fibers in parallel bundles
• Tendons provide high tensile strength to withstand muscle contraction forces

Muscle Fiber Types

• The three main types of muscle fibers are type 1, type 2a, and type 2b

Type 1 Fibers

• Type 1 skeletal muscle fibers are red slow oxidative fibers
• They have the smallest fiber diameter and are highly vascularized
• They are red colored due to high vascularity and contain many mitochondria
• These fibers produce ATP via mitochondria and depend on the respiratory system
• Their rate of contraction is slow due to decreased myosin ATPase
• They are fatigue resistant
• They store energy in triglycerides and contain myoglobin to store oxygen

Type 2a Muscles

• Type 2a muscles are fast oxidative fibers and are the largest of the skeletal muscle fibers
• They depend on oxygen and have a capillary network
• They have a lot of mitochondria and produce ATP, but do not have a red color
• These fibers store glycosomes, breaking down glycogen into glucose
• They can function anaerobically and aerobically and do glycolysis
• They contain creatine kinase to make ATP
• They have moderate fatigue resistance, fast contractility, fast myosin ATPase, and myoglobin

Type 2b Fibers

• Type 2b are fast glycolytic fibers with a middle size
• They receive little oxygen, resulting in low capillary density and a white appearance
• They contain few mitochondria due to low oxygen concentration
• There is almost no aerobic respiration, it mainly depends on glycolysis
• Glyoxysomes are abundant, breaking down glycogen into ATP
• They contain creatine kinase and little myoglobin
• They have very fast contractility, produce the largest amount of power, and fatigue rapidly
• They are the third recruited order, and lifting weights recruits them
• Genetics influence the density of fiber types

Calcium's Role and Components

• Calsequestrin is the primary protein causing calcium release in the sarcoplasmic reticulum (SR)
• Calcium binds to the Tn-c side of troponin in the sarcoplasm
• Troponin pulls on tropomyosin, which unblocks actin binding sites for myosin

Steps of Sliding Filament/Cross-Bridge Cycling

• Tropomyosin is removed and ATP binds to myosin, hydrolyzing ATP
• ADP on myosin uses the made energy to move (cocked position)
• Myosin attaches to actin: myosin 1 to actin 2, myosin 2 to actin 3, etc
• Phosphate is released, causing the myosin to pull on the actin and generating a power stroke
• Myosin pulls the actin to the M-line
• Myosin moves to another position through ADP displacement and ATP addition
• This repeats until the H-zone disappears by overlapping filaments
• Titin uncoils, helping the process and pulling on the Z-disk to bring them closer
• The A-band stays the same

Relaxation Process

• Voltage-gated potassium channels allow cells to release potassium until the resting membrane potential is reached, and inactivated channels will activate
• Calcium leaves troponin, allowing tropomyosin back in place to block myosin heads
• Calcium ions return to the SR by active transport, involving calcium and proton transport
• Sodium-calcium exchangers send calcium back to the SR
• During repolarization, calcium is pushed out of the cell by transporters
• Filaments move away from each other back to their original positions, closing sodium channels

Graded Muscle Response Stimuli

• Two types of stimuli graded muscle response depend on: frequency, and strength of the neural stimuli
• A muscle twitch is a single contraction one neural stimulus initiates
• A motor unit includes a neuron and its fibers

Diet Effect on Muscles

• Diets impact muscles differently and diet for endurance relies on carbohydrate loading

Endurance Training Diet

• Endurance training diets are mainly carbohydrates because the body can store fat
• Carbohydrate intake should increase to 45-65% or 70%
• Athletes should use a moderate super-compensate diet and increase carb intake while reducing physical activity prior to exercise
• 24-36 hours before, athletes eat a carbohydrate-rich meal with 6-12g/kg body mass
• Two hours before, athletes consume a small carb-rich meal
• During exercise, athletes can use sports drinks or energy bars
• Replenishing muscle glycogen storage is important after training

Resistance Training Diet

• Resistance-based training focuses on protein intake to increase muscle mass
• Macronutrient balance with carbohydrates is still important for energy storage
• High-intensity athletes benefit from leucine-rich proteins found in meat, dairy, and eggs to activate the mTOR pathway

Energy Consumption Pathways

• Human body has three major energy consumption pathways: anaerobic-alactic, anaerobic-lactic, and aerobic

Anaerobic-Alactic System

• The anaerobic-alactic system, or ATP/PCr mechanism, is primarily used for type IIb fibers
• It is oxygen-independent and serves as the first energy source
• Generates fast, powerful movements, but is quickly exhausted, resulting in "all-out" exercise

Energy Usage

• Uses free-floating ATP and phosphocreatine (PCr) from the body’s circulation
• Readily and rapidly provides energy for quick ATP reduction

Anaerobic-Lactic System

• The anaerobic-lactic system utilizes glycolysis without oxygen
• It is mostly used by type II fibers and results in lactate production
• It is the second energy source, faster than the ATP/PCr system but slower to deplete
• Produces fast, powerful movements (400-800m sprint)
• Uses carbohydrates for energy and glucose in muscles

Energy Production Mechanism

• Glucose enters glycolysis from glycogenolysis or the bloodstream within muscle cells
• Production of total 4 ATP, but 2 ATP are re-used, resulting in net outcome of 2 ATP
• Glycolysis' end products are two pyruvate molecules
• Pyruvate is transformed into lactate to release into bloodstream
• Lactate release allows NAD+, reduced during glycolysis, to be regenerated

Lactate Usage

• Lactate can travel to well-oxygenated cells and oxidize back into pyruvate, producing ATP
• Lactate travel to the liver and convert to glucose via gluconeogenesis through Cori Cycle

Aerobic System

• The aerobic system combines aerobic glycolysis, pyruvate oxidation, lipolysis, beta-oxidation, TCA cycle, and ETC reaction
• It relies on interconnected oxygen-dependent reactions dependent on the final step of the ETC reaction
• Muscle fibers need interconnection with capillaries, more myoglobin, and mitochondria
• Type IIa fibers can perform aerobic metabolism
• Fuelled by multiple macronutrients, this system can stand longer and be used for long duration

Endurance Training Recruitment

• Type I fiber recruitment is important because they provide most of the energy required
• Type I fibers are only active until 50% Max VO2 is reached
• Type II fiber recruitment depends on training intensity
• Type IIa fibers are recruited if the intensity is greater than 50% Max VO2 and type IIb fibers are recruited when above 70%
• Decrease intensity to maintain energy for a longer period, which uses type I fibers

Endurance Fuels

• Fat and carbohydrates are the major fuel sources

Fat Metabolism

• Fat metabolism begins with increased lipoprotein lipase and lipid hydrolysis
• Free fatty acids move into muscles by FABP and the transporter CD36 or FAT
• Fatty acids are transported to the mitochondria with the transporter CPT, and the enzymes in the TCA cycle increase beta-oxidation

Carbohydrate Metabolism

• The body spares stored carbohydrates during endurance
• With a lower intensity, the focus is on lipid metabolism and glucose
• The lower glycolic flux decreases lactic effects in the circulation
• Lactate is released less with clearance and improved reuse
• Continous excursive leads to increased lactate threshold

Resistance Training Metabolism

• Anaerobic systems are ATP's main source of supply
• Training is split into all-out and submaximal exercises
• Type II fibers are used in these types of exercises

All-Out Exercise

• Peak power for All-out that declines after 30 seconds
• Phosphocreatine and glycogen are its main fuel
• With no aerobic activity it is completed quickly with high-intensity, decreasing quickly

Submaximal Exercise

• Exercise doesn't go as fast as all-out capacity
• Glycogen is fuel derived from liver's stores of carbohydrates or glycogen
• Characterized by increased hepatic gluconeogenesis and muscle glycogenolysis
• Fatty acid oxidation comes into play if it is longer than 30 min

Resistance and Metabolism and Training

• Athletes in resistance training benefit from increased basal metabolic rate during rest
• Muscle growth and energy, glycogen capacity, and fat metabolism improves
• Carbohydrates are used during exercise and fat is used during rest

Acidosis Pathway

• Acidosis lowers blood pH that causes a decrease in bicarbonate
• Traumas happen in muscles that are mini-tears
• Explosive Training shifts to more anaerobic respiration by producing lactate
• Lactate turns pyruvate to regulate hydrogen levels

Muscle Fiber Adaptation to Training

• Different types of training affect athletes differently based on resulting adaptations

Abdirahman (Endurance Training)

• Primarily recruits Type I (slow oxidative) muscle fibers, which are fatigue-resistant, but produce less power
• These fibers rely on aerobic metabolism, using oxygen efficiently to produce ATP
• Long-distance training increases capillaries, as well as how much and what concentration of myoglobin oxygen delivers to enable endurance
• Maintains lean muscles for sustained energy over long durations

Crave (Explosive Training)

• Relies on Type IIb (fast glycolytic) muscle fibers for high force and speed
• Uses anaerobic metabolism via ATP-PCr system and allows for fast ATP production
• High-intensity, short sprints maximize power output
• Mass and strength increasing, muscles can hypertrophy

Metabolic Pathways and Energy Use for Marathon Training

• Abdirahman uses aerobic metabolism for breaking down fats and carbohydrates
• Carbohydrate loading maximizes the stores of glycogen
• Fat has a higher efficiency at lower intensities to conserve glycogen
• Delayed Muscle fatigue, producing lower power

Metabolic Pathways and Energy Use for Sprint Training

• Crave uses anaerobic-alactic and anaerobic-lactic systems
• Creates lactic acid, that causes a burning sensation in muscles
• Recovery requires, while the Cori cycle turns lactate back to glucose
• Elevated creatine phosphate enables explosive performance

Impact on Body Composition

• Abdirahman builds lean muscle optimized for endurance
• Crave builds more muscle that powers speed

Who Would Win the Marathon?

• Abdirahman outperforms Crave because he yields to endurance and and fuell efficiency
• Crave has muscle fibers that cause the body to fatigue quickly, while relying on early anaerobic energy
• Crave can dominate because of reaction and power over a 100m reaction