Muscle Anatomy, Function, and Adaptation: Skeletal Muscle Overview

Skeletal Muscle Functions

Force production for movement and breathing; postural support; heat generation during cold stress.

Percentage of Body Weight from Skeletal Muscle

40-50% of total body weight.

Satellite Cells

Responsible for muscle growth and repair; add nuclei to fibers allowing greater protein synthesis; activated by strength training.

Myofibrils

Contractile elements of muscle fibers containing actin (thin) and myosin (thick) filaments.

Sarcomere

Smallest functional unit of a muscle fiber; area between Z-lines.

Sarcoplasmic Reticulum

Stores calcium ions (Ca++); releases them to initiate contraction.

Transverse Tubules

Transmit action potentials from sarcolemma to sarcoplasmic reticulum.

Neuromuscular Junction

Junction between motor neuron and muscle fiber; site where acetylcholine (ACh) triggers muscle depolarization.

Sliding Filament Model

Muscle shortens as actin slides over myosin; cross-bridges form, perform a power stroke, and shorten the sarcomere.

Excitation-Contraction Coupling

Process linking muscle fiber depolarization (excitation) to contraction through Ca++ release and binding to troponin.

Muscle Relaxation

Occurs when stimulation stops, ACh is no longer released, Ca++ returns to SR, and tropomyosin blocks binding sites.

Muscle Fatigue

Decline in muscle power output due to lactate, H+, ADP, Pi, free radicals, or glycogen depletion.

Exercise-Associated Muscle Cramps

Involuntary contractions; likely caused by excessive motor neuron firing, not dehydration alone; relieved by stretching.

Pickle Juice Theory

Pickle juice may stop cramps via acetic acid activating oropharyngeal reflexes that inhibit motor neurons.

Muscle Fiber Types

Type I (slow-twitch, oxidative), Type IIa (fast-oxidative glycolytic), Type IIx (fast-glycolytic).

Type I Fiber Characteristics

High mitochondria, capillaries, myoglobin; low force, high endurance.

Type IIx Fiber Characteristics

Few mitochondria, low capillary density, high force, low endurance.

Type IIa Fiber Characteristics

Intermediate properties between Type I and IIx.

How Fiber Type Is Determined

Muscle biopsy and myosin ATPase staining or gel electrophoresis.

Athletes and Fiber Type

Endurance athletes → more Type I; Power athletes → more Type II.

Types of Muscle Contraction

Isometric (no length change), Concentric (shortening), Eccentric (lengthening).

Force-Velocity Relationship

Higher force at low velocities; fast fibers perform faster movements.

Force-Power Relationship

Peak power higher in fast fibers; increases with velocity until ~200-300°/sec.

Post-Activation Potentiation

Temporary increase in muscle force output after previous contraction or warm-up.

Aging and Muscle (Sarcopenia)

10% loss from 25-50 yrs, 40% more by 80 yrs; resistance training can delay loss.

Muscle Disease: Diabetes

Causes progressive loss of muscle mass; aerobic and resistance training are protective.

Overload Principle

Training must exceed normal load to produce adaptations.

Specificity Principle

Training adaptations are specific to muscle fibers, energy systems, velocity, and contraction type used.

Reversibility

Training gains are lost when overload is removed.

Muscular Strength

Maximal force a muscle can generate; measured by 1-RM.

Muscular Endurance

Ability to sustain repeated submaximal contractions.

High-Resistance Training (2-10 RM)

Promotes strength gains.

Low-Resistance Training (20+ RM)

Promotes endurance gains.

Anaerobic Exercise

Short-duration (10-30s) high-intensity efforts using ATP-PC and glycolysis.

Energy System Use in Anaerobic Work

≤10s → ATP-PC; 20-30s → 80% anaerobic, 20% aerobic.

Anaerobic Training Adaptations

Increases peak anaerobic power (3-28% in 4-10 weeks), improves buffering capacity and H+ transporters.

Hypertrophy

Increase in muscle fiber size due to more myofibrils and cross-bridges; occurs in both Type I and II fibers.

Hyperplasia

Increase in muscle fiber number; limited evidence in humans.

Neural Adaptations

Early (first 8-20 weeks) strength gains from improved motor unit recruitment, firing rate, synchronization, and less inhibition.

mTOR Pathway

Protein kinase regulating muscle protein synthesis via mRNA and ribosome production.

Leucine Supplementation

BCAA that slightly activates mTOR; limited effect in untrained individuals.

Satellite Cell Activity

Increases during resistance training; assists hypertrophy and repair.

Fiber Type Shift

Resistance training causes small Type IIx → IIa conversion; less shift than endurance training.

Concurrent Training

Combining strength and endurance training may impair strength gains but not endurance.

Causes of Strength Impairment in Concurrent Training

Neural interference, low glycogen, and reduced mTOR activity from endurance work.

Overtraining and Strength Gains

Limited evidence; may reduce protein synthesis.

Detraining Effects

Strength decreases ~31% after 30 weeks off; Type I -2%, Type IIa -10%, Type IIx -14%.

Retraining Effects

Rapid regain of strength and size within 6 weeks.

Strength Maintenance

Can maintain with reduced training for up to 12 weeks.

Aging and Strength

Decline after age 50 due to sarcopenia, fiber loss, and fewer motor units.

Benefits of Resistance Training in Aging

Increases muscle size, strength, balance, and reduces fall risk.

Key Difference: Hypertrophy vs. Hyperplasia

Hypertrophy = fiber enlargement; Hyperplasia = new fiber formation; hypertrophy is the main mechanism.