Muscle Strength and Physiology

Muscle Strength Assessment and Anatomy

  • Concept of Muscle Strength:
    • Definition of hand grip strength (HGS) assessment.
    • Importance of muscle strength in physical assessment.

Anatomy of Strength

  • Main Components (MVPs):

    • Muscles
    • Bones
    • Connective Tissue
    • Nerves
    • Hormones

  • Mechanical System:

    • Role of bones as levers within a pulley system.
    • Muscle attachments affecting strength during movements.

  • Example of Hand Grip Strength (HGS) Components:

    • Prime movers: Biceps, Triceps, Pecs, and Lats.
    • Stabilizers:
    • Extrinsic wrist flexors and extensors.
    • Intrinsic hand muscles.
    • Other contributing muscles: elbow, shoulder, abdominals, contralateral lumbar/pelvic/lower extremity.

Detailed Overview of the Intrinsic Muscles of the Hand

  • Muscles:

    • Lumbricals
    • Palmar interossei
    • First dorsal interosseus
    • Abductor digiti minimi
    • Flexor digiti minimi brevis
    • Opponens digiti minimi
    • Palmaris brevis (cut)

  • Tendons:

    • Flexor retinaculum
    • Tendon of flexor carpi ulnaris
    • Tendon of flexor digitorum profundus
    • Tendon of flexor digitorum superficialis
    • Tendon of flexor pollicis longus

  • Muscles Involving Thumb Movement:

    • Adductor pollicis
    • Flexor pollicis brevis
    • Opponens pollicis
    • Abductor pollicis brevis

  • Other Important Muscles:

    • Biceps brachii
    • Brachialis
    • Medial and Lateral epicondyles of humerus
    • Similar to earlier muscle categories, with inclusion of various tendon structures and interrelations.

Muscle Contraction and Strength Mechanics

  • Summation of Muscle Strength:
    • Isometric Contraction:
    • Rectus and transversus abdominis stiffening to stabilize trunk posture.
    • Example: Right quadriceps in isometric contraction for stability.
    • Concentric Contraction:
    • Quadriceps and sartorius shortening to move resistance.
    • Eccentric Contraction:
    • Quadriceps and sartorius controlling motion against gravity.

Properties of Skeletal Muscle

  • General Characteristics:
    • Striated appearance due to a structural component known as sarcomeres.
    • Multinucleated structure unlike cardiac muscle.
    • Composition:
    • 75% water
    • 20% protein
    • Contains 50-75% of the body’s protein and constitutes 40% of total body mass.

Connective Tissue in Muscle Structure

  • Types of Connective Tissue:
    • Epimysium: Surrounds the entire muscle.
    • Perimysium: Surrounds bundles of muscle fibers known as Fascicles.
    • Endomysium: Surrounds individual muscle fibers.
    • Sarcolemma: Refers to the muscle cell membrane.

Muscle Structure and Functionality

  • Hierarchy of Muscle Components:

    • Muscle Fiber → Myofibril (contains contractile proteins) → Myofilaments (Actin and Myosin).

  • Sarcomere:

    • Defined as the contractile unit of the muscle fiber, from Z-line to Z-line.
    • Key Zones within Sarcomere:
    • A band (Myosin - Dark): Overlaps with Actin.
    • M line: Anchors Myosin.
    • H zone: Myosin without overlap with Actin.
    • I band: Pure Actin filament.

  • Sarcoplasmic Reticulum:

    • Stores Calcium ions, which are critical for muscle contraction, guarded by terminal cisternae.

Electrochemical Process of Muscle Contraction

  • Resting Membrane Potential:

    • Sodium Potassium ATPase Channels contribute to excitability and polarization within the cells.

  • Process of Muscle Activation:

    • Excited motor signal leads to the release of acetylcholine across the synaptic cleft to the motor end plate.
    • Acetylcholine binds to receptors, opening Sodium channels, exciting the membrane (sarcolemma) with a positive charge.
    • Sodium influx opens T-tubules which cause depolarization of the sarcoplasmic reticulum to release Calcium, critical for initiating muscle contraction.

Structure of the Motor Unit and Neuromuscular Junction

  • Components of a Motor Unit:

    • Muscle fiber
    • Myofibrils
    • Neuromuscular Junction (where the motor neuron meets the muscle fiber).

  • Transmission Events:

    • Acetylcholine's role via synaptic vesicles in facilitating contraction through the motor neuron.

Muscle Contraction Mechanism - Sliding Filament Theory

  • Key Steps in Contraction:
    • Excitation-contraction coupling initiated by a motor neuron signal.
    • Acetylcholine opens Sodium channels generating an action potential that propagates throughout muscle fibers.
    • Depolarization leads to Calcium release from the sarcoplasmic reticulum.
    • Calcium binds to Troponin, moving Tropomyosin, exposing binding sites on Actin for Myosin heads to attach.
    • Hydrolysis of ATP provides the energy for myosin to bind to actin and perform a power stroke.
    • Myosin head releases ADP + P, resulting in muscle contraction as Actin moves toward the M line.
    • Note: One Calcium ion allows for multiple Myosin head binding instances (Approximately 7 binding sites per Calcium ion).

Tension and Force Characteristics in Muscle

  • Length-Tension Relationship:
    • Explains that maximal force is produced near the muscle's normal resting length.
    • Graphical representation demonstrating the relationship between muscle length (um) and tension (%).

Relaxation Phase in Muscle Contraction

  • Mechanism:
    • Removal of impulse from the motor neuron leads to Calcium being returned to the sarcoplasmic reticulum.
    • Without sufficient Calcium, muscle fatigue ensues, inhibiting further contraction.
    • Tropomyosin re-covers the actin binding site, ending contraction.

Adaptations to Endurance and Resistance Training

  • Key Adaptations:
    • Increase in the size of the neuromuscular junction.
    • Increase in the number of synaptic vesicles.
    • Increase in Acetylcholine receptors.
    • Enhanced capillary networks and myoglobin content.
    • Increase in mitochondrial density through satellite cells via resistance training/hypertrophy.
    • Neural adaptations reducing the threshold for action potentials.
    • Enhanced protein synthesis resulting from hypertrophy, especially in myofilaments, and increased engagement of previously dormant muscle fibers.

Energy Systems in Muscle Functionality

  • Sources of ATP for Contraction:
    • Phosphocreatine, Glucose, Glycogen, and Fatty acids through metabolic processes.
    • Role of Glycolysis and oxidative phosphorylation in ATP replenishment.

Power Stroke Clarification

  • Single Power Stroke Dynamics:
    • A single contraction cycle shortens the muscle only by 1% of its resting length, necessitating continuous cycling for effective contraction throughout a muscle's range.

Muscle Fiber Composition in Athletic Performance

  • Typical Muscle Fiber Composition in Elite Athletes:
    • Distance runners: 70-80% Slow Fibers (Type 1), 20-30% Fast Fibers (Type 2)
    • Track sprinters: 25-30% Slow Fibers, 70-75% Fast Fibers
    • Nonathletes: 47-53% Slow Fibers, 47-53% Fast Fibers

Muscle Fiber Mechanics

  • Types of Muscle Fibers:
    • Type 1 (Slow-Oxidative):
    • High mitochondrial content.
    • Fatigue-resistant almost.
    • Increased levels of myoglobin.
    • Slower Vmax and lower force output.
    • Greater efficiency with ATP requirements.
    • Type II (Fast-Twitch):
    • Characterized by rapid fatigue, less myoglobin, and favorable conditions for short bursts of strength and speed.

Factors Affecting Muscle Force

  • Strength Measurement Factors:
    • Length-Tension (relative position of Actin and Myosin).
    • Force-Velocity (relationship between force produced and speed of contraction).
    • Neural stimulation (importance of motor neuron recruitment).
    • Type of muscle fibers involved in the contraction.

Force Regulation and Production in Muscle

  • Aspects Influencing Muscle Force:
    • Number/type of motor units recruited: More units equate to greater force.
    • Muscle length and optimal contraction length for enhanced cross-bridge formation.
    • Firing rate of motor neurons influencing force via simple twitch, summation, and tetanus.
    • Results of warmup exercises leading to post-activation potentiation.

Impact of Firing Rate on Muscle Force Output

  • Effects of Stimulation on Force Production:
    • Variability of force in response to firing rates: simple twitches, summations, and tetanus.
    • Illustrative graphs to represent force output as it relates to neuron firing.

Force Velocity Relationship

  • Dynamics of Contraction Speed:
    • At higher velocities, actin and myosin can separate without binding, decreasing force output.
    • Maximum power output correlates directly with speed of movement and muscle fiber composition.
    • Calculation: Maximum Power Output = Force x Velocity.

Force Velocity Curve and Strength Categories

  • Defined Force-velocity Relationships:
    • Absolute strength, Accelerative strength, and Speed/Strength categories based on movement speed.

Aging and Muscle Functionality

  • Hormonal Influences in Aging:

    • Growth Hormone: Influences tissue growth, body composition, and metabolism.
    • Testosterone: Facilitates muscle protein synthesis, especially increased post-puberty in males. - Female muscle strength stabilizes at about 60-65% that of males post-puberty.

  • Aging Impact on Muscle Mass:

    • Sarcopenia denotes age-related muscle loss, noting a 10% reduction in muscle mass from age 25-50, and an additional 40% loss between ages 50-80.
    • Associated experiences include loss of fast fibers and the relative gain of slow fibers.
    • Resistance training shown to mitigate some age-related muscle loss, bolstering the body’s resilience to chronic stress and illness.