LP2A: Skeletal System: Bone Histology & Physiology

Overview and Primary Functions of the Skeletal System

• Systemic Composition:

  • The adult human skeletal system comprises exactly $206$ bones.

  • Bones meet at junctions called joints or articulations, where two or more bones come together.

• Core Functions of the Skeletal System:

  • Support & Shape: Provides a rigid structural framework that gives shape to body structures and supports total body weight.

  • Protection of Vital Organs: Encases and shields internal structures:

  • Cranial bones protect the brain.

  • Thoracic cage (sternum and ribs) protects the lungs and heart.

  • Pelvic bones protect pelvic organs.

  • Body Movement: Serves as a mechanical system of levers acted upon by skeletal muscles.

  • Hemopoiesis / Hematopoiesis: Production of blood cells occurring within red bone marrow.

  • Mineral Homeostasis & Depot Storage: Serves as a depot for storing and withdrawing essential mineral ions ($Ca^{2+}$, $PO_4^{3-}$, $Na^+$, $K^+$, $Mg^{2+}$, and carbonate ions) to maintain body fluid composition and support metabolic activities.

  • Energy / Lipid Storage: Neutral fats are stored in yellow bone marrow as an energy reserve.

Mechanical Principles: Anatomical Levers

• Definition and Components of a Lever System:

  • Bones and muscles interact as mechanical devices known as levers to produce body movement.

  • Every lever system consists of four fundamental components:

  1. Rigid bar or rod: Represented by the bone.

  2. Pivot or fulcrum ($\Delta$): The axis or point on which the bar turns, represented by the joint/articulation.

  3. Resistance / Load: An object or weight moved against resistance.

  4. Force: The input energy supplying effort to move the bar, supplied by skeletal muscle contraction.

• Classes of Levers in Human Anatomy:

  • First-class, second-class, and third-class levers all exist within the human body.

  • The majority of anatomical levers are third-class levers.

• Third-Class Lever Mechanics:

  • Structural Arrangement: The effort force is located directly between the fulcrum (pivot) and the load resistance.

  • Schematic Configuration: Resistance $\leftarrow$ Force $\leftarrow$ Fulcrum ($\Delta$).

  • Anatomical Example (Biceps Curl):

  • Fulcrum ($\Delta$): The elbow joint.

  • Force: Insertion of the biceps brachii muscle onto the radial tuberosity.

  • Resistance: A load held in the hand (e.g., a $5\,lb$ weight) or the weight of the forearm.

  • Additional Anatomical & Clinical Examples: Eyebrow tweezers, surgical forceps, bicep curls, shoulder abduction, and knee extension.

  • Functional Advantage: Provides a broad range and high speed of movement, facilitating rapid dynamic actions and posture maintenance.

Hemopoiesis and Bone Marrow Dynamics

• Developmental Sites of Blood Cell Formation:

  • Early Embryonic Stage: Hemopoiesis begins outside the embryo proper within the yolk sac.

  • Fetal Developmental Stage: Production shifts internally to the liver and spleen.

  • Late Fetal, Infant, and Adult Stages: Blood cell production transitions exclusively to the bone marrow.

• Anatomy and Distribution of Bone Marrow:

  • Marrow is a soft, netlike mass of vascular connective tissue located within:

  • Medullary cavities/canals of long bones.

  • Irregular spaces within spongy (cancellous) bone.

  • Larger central/osteonic canals of compact bone.

• Red Bone Marrow (Hematopoietic Tissue):

  • Function: Active site for the production of red blood cells (erythrocytes), white blood cells (leukocytes), and blood platelets (thrombocytes).

  • Pigmentation: Red color is derived from the iron-containing, oxygen-carrying pigment hemoglobin inside red blood cells.

  • Infant vs. Adult Distribution:

  • In infants, red marrow occupies the medullary cavities of virtually all bones.

  • With advancing age, yellow marrow progressively replaces red marrow.

  • In adults, active red marrow is localized to spongy bone within the skull, ribs, sternum, clavicles, vertebrae, and pelvis.

  • Reversibility: If systemic blood cell supply becomes severely deficient, yellow marrow can revert back into red bone marrow to resume blood cell synthesis.

• Yellow Bone Marrow (Adipose Reserve):

  • Function: Stores neutral lipids/fats and functions strictly as an energy reserve; inactive in blood cell formation.

  • Distribution: Occupies the central medullary canals of mature adult long bones.

Mineral Homeostasis and Chemical Composition of Bone Matrix

• Mineral Storage & Systemic Balance:

  • Bone tissue stores $99\%$ of the body's total calcium content.

  • Bone tissue acts as a continuous reservoir for calcium ($Ca^{2+}$), phosphorus ($PO_4^{3-}$), magnesium ($Mg^{2+}$), sodium ($Na^+$), potassium ($K^+$), and carbonate ions.

  • Metabolic Roles of Calcium: Calcium ions are strictly required for blood clot formation, nerve impulse conduction, and muscle fiber contraction.

  • Accumulation of Heavy Metals: Bone tissue inadvertently acts as a sink for harmful metallic elements accidentally ingested from contaminated paint, soil, or food (e.g., lead, radium, strontium).

• Chemical Composition of Extracellular Matrix:

  • Inorganic Mineral Salts ($\sim 70\%$ of matrix weight; $\sim 2/3$ of total bone weight):

  • Primary constituent: Calcium phosphate [$Ca_3(PO_4)_2$].

  • Calcium phosphate combines with calcium hydroxide [$Ca(OH)_2$] to form hydroxyapatite crystals.

  • Incorporates additional inorganic salts including sodium, magnesium, and carbonate ions.

  • Physical Properties: Hydroxyapatite crystals are extremely hard and rigid; they withstand significant compression without damage, but are inflexible, brittle, and subject to shattering when bent or twisted.

  • Organic Framework ($\sim 1/3$ of total bone weight):

  • Primary constituent: Collagen protein fibers synthesized by osteoblasts.

  • Physical Properties: Collagen fibers are flexible and extraordinarily strong; they possess high tensile strength to withstand pulling, bending, and twisting forces.

  • Composite Matrix Synergy: Hydroxyapatite crystals form small plates and rods that lock tightly onto the collagen fiber framework. This combination produces bone tissue that is strong, moderately flexible, and highly resistant to shattering.

Morphological Classification of Bones

• Flat Bones:

  • Anatomical Examples: Skull bones (occipital, parietal, frontal, nasal, lacrimal, vomer), thoracic cage (sternum, ribs), and pelvic girdle (ilium, ischium, pubis).

  • Characteristics & Function: Somewhat flattened and broad; function as protective shields for underlying internal organs (brain, heart, pelvic viscera) and provide expansive surface areas for muscle attachment.

• Long Bones:

  • Anatomical Examples: Lower limbs (femur, tibia, fibula, metatarsals, phalanges) and upper limbs (humerus, radius, ulna, metacarpals, phalanges).

  • Characteristics & Function: Longer than they are wide (the femur is the longest bone in the human body). Located primarily in the appendicular skeleton to support body weight and serve as levers for locomotion.

• Short Bones:

  • Anatomical Examples: Wrist carpals (scaphoid, lunate, triquetral, hamate, pisiform, capitate, trapezoid, trapezium) and ankle tarsals (calcaneus, talus, navicular, cuboid, lateral cuneiform, intermediate cuneiform, medial cuneiform).

  • Characteristics & Function: Cube-shaped, with roughly equal length and width; provide local stability and limited sliding movement.

• Irregular Bones:

  • Anatomical Examples: Vertebrae of the vertebral column and pelvic bones (pubis, ilium, ischium).

  • Characteristics & Function: Possess complex, irregular shapes that do not fit standard categories; customized to protect specialized internal structures (e.g., vertebrae protect the spinal cord).

• Sesamoid Bones:

  • Anatomical Examples: Patella (kneecap); also small nodular bones in tendons of hands and feet.

  • Characteristics & Function: Small, rounded bones embedded directly within tendons; function to protect tendons from excessive mechanical stress, wear, and friction.

Macroscopic Anatomy and Histology of Long Bones

• Anatomical Structural Regions:

  • Diaphysis: The extended, straight central shaft of a long bone.

  • Epiphysis (plural: Epiphyses): Expanded terminal ends (proximal epiphysis and distal epiphysis) that articulate with adjacent bones.

  • Metaphysis: The transitional region connecting the diaphysis to each epiphysis.

• Internal Micro-Anatomy & Tissue Types:

  • Compact Bone (Cortical Bone):

  • Dense, hard, light, tightly packed outer wall of the diaphysis and thin shell covering epiphyses.

  • Contains a solid, continuous matrix with no gaps or spaces.

  • Formulated as a hollow tube to maximize strength while reducing total bone weight.

  • Spongy Bone (Cancellous Bone):

  • Fills the interior of the epiphyses and lines interior diaphyseal borders.

  • Composed of an open meshwork of bony plates and rods called trabeculae (singular: trabecula).

  • Trabeculae are oriented directly along lines of stress and cross-braced extensively to withstand multi-directional mechanical compression.

  • Reduces overall bone mass while protecting embedded marrow tissue.

  • Medullary Cavity / Marrow Canal:

  • A continuous hollow space running through the length of the diaphysis.

  • Continuous with the interstitial spaces of spongy bone.

  • Contains bone marrow (dominated by yellow marrow in adults).

• Cartilaginous & Fibrous Membrane Structures:

  • Articular Cartilage: Thin layer of hyaline cartilage covering the outer articulating surface of each epiphysis; provides a frictionless sliding surface for joint motion; completely avascular and aneural.

  • Periosteum:

  • Tough, highly vascular, and nerve-rich fibrous connective tissue membrane covering all outer surfaces except articular cartilage.

  • Firmly anchored to bone tissue via perforating fibers (thick collagen bundles/Sharpey's fibers extending into extracellular matrix).

  • Contains pain-sensitive sensory nerves and blood vessels vital for bone formation and repair.

  • Fibers are continuous with attached tendons and ligaments.

  • Endosteum:

  • Thin membrane lining the interior medullary cavity, central canals, and trabecular spaces.

  • Contains a single layer of bone-forming cells (osteoblasts) and a minimal amount of connective tissue.

• Vascularization:

  • Bone tissue is dynamic and highly vascularized.

  • Blood vessels enter from the periosteum to supply diaphyses (nutrient artery/vein), metaphyses, epiphyses, and periosteal tissues.

  • Continuous blood flow supplies oxygen and nutrients, removes $CO_2$ and acid waste, and transports newly synthesized blood cells into circulation.

• Anatomy of Non-Long Bones:

  • Short, flat, and irregular bones typically consist of a core of spongy bone sandwiched between two protective outer plates of compact bone.

• Longitudinal Bone Growth (Epiphyseal Plate Physiology):

  • In immature bones, the epiphyseal disk/plate—a layer of hyaline cartilage—persists at the metaphysis between the diaphysis and epiphysis.

  • Cartilage cells actively divide, thickening the disk and driving lengthening of the bone.

  • Calcium salts accumulate in the matrix adjacent to older cartilage cells; as calcification occurs, cartilage cells die.

  • Osteoclasts break down the calcified cartilage matrix; osteoblasts then invade and deposit fresh bone tissue in its place.

  • Epiphyseal Closure: At approximately $18$ years of age in females and $21$ years of age in males, cartilage cells stop dividing and are completely replaced by bone tissue, leaving an epiphyseal line and permanently ending longitudinal growth.

Cellular Components of Osseous Tissue

• Osteoprogenitor Cells:

  • Mesenchymal stem cells located within the endosteum, inner periosteum, and vascular canals.

  • Capable of division; differentiate directly into osteoblasts.

• Osteoblasts:

  • Bone-forming cells responsible for osteogenesis (matrix deposition).

  • Synthesize and secrete organic matrix components (proteins), forming uncalcified osteoid.

  • Elevate local concentrations of calcium phosphate to promote mineral salt deposition in the osteoid.

  • Transition into osteocytes once fully surrounded by calcified matrix.

• Osteocytes:

  • Mature, non-dividing bone cells; the most abundant cell type in bone tissue.

  • Reside in matrix pockets called lacunae (singular: lacuna), situated between concentric matrix layers called lamellae.

  • Extend cytoplasmic processes through microscopic channels called canaliculi (singular: canaliculus) that link neighboring lacunae to central vascular canals.

  • Function to maintain mineral and protein content of surrounding matrix and assist in localized bone repair.

• Osteoclasts:

  • Giant, multinucleated bone-destroying cells containing $50$ or more nuclei.

  • Derived from hematopoietic stem cell lines (monocyte/macrophage fusion).

  • Perform osteolysis (resorption): Adhere to endosteal or periosteal surfaces via a specialized ruffled border, forming a leakproof seal.

  • Secrete lysosomal enzymes (which digest organic collagen fibers) and organic acids (which dissolve inorganic hydroxyapatite minerals).

  • Mobilize calcium and phosphate into body fluids for metabolic utilization.

Physiology of Bone Remodeling

• Dynamics and Turnover Rates:

  • Bone is dynamic living tissue undergoing continuous replacement via remodeling (balanced resorption and deposition).

  • Approximately $100\%$ of an infant's skeleton and $10\text{--}30\%$ of an adult's skeleton is replaced annually.

  • Rates of remodeling vary across individual bones and regional zones within a bone.

• Step-by-Step Cellular Mechanism of Osteoclastic Resorption:

  1. Sealing Zone Formation: Osteoclast attaches tightly to endosteum or periosteum, establishing a leakproof seal around its ruffled border.

  2. Secretion: Lysosomal enzymes and acids are pumped into the sealed pocket.

  3. Matrix Dissolution: Acids dissolve inorganic bone minerals ($Ca^{2+}$, $PO_4^{3-}$), while lysosomal enzymes digest organic collagen fibers.

  4. Endocytosis: Degraded matrix proteins and dissolved minerals enter the osteoclast via endocytic vesicles at the ruffled border.

  5. Transcellular Transport: Vesicles cross the osteoclast cytoplasm.

  6. Exocytosis: Contents are released via exocytosis on the basolateral membrane (opposite the ruffled border) into interstitial fluid.

  7. Capillaries: Released calcium and phosphate ions diffuse into blood capillaries.

  8. Rebuilding: Osteoclasts vacate the site, allowing osteoblasts to invade and deposit new bone tissue.

• Functional Significance:

  • Maintains systemic blood calcium and phosphate homeostasis.

  • Replaces old or micro-damaged bone tissue to maintain skeletal integrity.

  • Adapts skeletal structure to physical strain: Increased mechanical loading causes localized increases in bone mass and volume at the site of stress.

Endocrine Regulation of Skeletal Homeostasis

• Parathyroid Hormone (PTH):

  • Secreted by parathyroid glands in response to low blood calcium levels.

  • Target Actions:

  • Stimulates osteoclasts to resorb bone matrix, releasing $Ca^{2+}$ into blood.

  • Enhances calcium reabsorption in the kidney tubules (reducing urinary calcium loss).

  • Enhances intestinal absorption of calcium via activation of calcitriol.

  • Outcome: Restores blood calcium levels to normal homeostatic values.

• Calcitonin:

  • Secreted by C-cells of the thyroid gland in response to high blood calcium levels.

  • Target Actions:

  • Directly inhibits osteoclasts while osteoblastic activity continues uninterrupted.

  • Increases renal excretion rate of calcium ions into urine.

  • Outcome: Decreases blood calcium levels and increases bone mineral density.

• Sex Steroids (Estrogens & Androgens/Testosterone):

  • Secreted by ovaries and testes; strongly stimulate osteoblast activity and matrix synthesis.

  • Responsible for accelerating growth and triggering epiphyseal plate closure.

  • Estrogens induce faster epiphyseal plate closure than testosterone, causing females to stop growing earlier and generally achieve shorter average height than males.

  • Estrogen promotes intestinal calcium retention and directly inhibits PTH action on osteoclasts. Loss of estrogen (menopause, amenorrhea) accelerates osteoclastic resorption.

  • Decreased testosterone in aging males (notably by age $65$) contributes to osteoporosis development.

• Other Regulatory Hormones:

  • Growth Hormone (Pituitary Gland): Stimulates protein synthesis and longitudinal bone growth.

  • Thyroxine (Thyroid Gland): Works with growth hormone to stimulate osteoblast activity and bone matrix synthesis.

  • Calcitriol (Kidneys / Active Vitamin D): Stimulates calcium and phosphate absorption along the digestive tract.

Lifecycle Dynamics and Peak Bone Mass Development

• Lifespan Trajectory of Bone Mass:

  • 0–20 Years: Active growth phase characterized by rapid accretion of bone mass.

  • Early Adulthood: Skeletal consolidation phase; peak bone mass is attained between 30–35 years of age.

  • Age 40+: Involutary loss phase begins; bone resorption rate begins to exceed deposition rate.

  • Middle/Late Adulthood: Sex hormone decline (especially post-menopause) accelerates demineralization.

• Aging Effects on Bone Structure:

  1. Demineralization: Loss of calcium and mineral salts from matrix.

  2. Brittleness: Decreased protein synthesis leads to reduced collagen fiber production, rendering bone brittle and easily broken.

• Sex Differences in Bone Mineral Density (BMD):

  • Males achieve higher absolute peak BMD than females.

  • Post-menopausal females experience rapid acceleration of bone loss, potentially losing up to $20\%$ of total bone mass within the first $5\text{--}7$ years following menopause due to estrogen withdrawal.

• Determinants of Peak Bone Mass:

  • Mechanical Strain: Physical activity and gravity act as essential osteogenic stimuli. Youth should engage in high-impact activities.

  • Nutritional Status:

  • Calcium: Optimal daily adult intake is $1000\text{--}1500\,mg/day$ (varies by age, sex, pregnancy/lactation).

  • Vitamin C: Required for collagen synthesis.

  • Vitamin A: Stimulates osteoblast activity.

  • Vitamin D: Required for intestinal calcium absorption.

  • Vitamins K and $B_{12}$: Necessary for synthesis of bone proteins.

  • Hormonal Levels: Adequate sex steroids and growth hormone during growth years.

  • Genetics: Sets upper limit for attainable BMD (e.g., the $tt$ genotype of the Vitamin D receptor gene increases risk of spinal bone loss by impairing calcium absorption).

Exercise Science and Skeletal Adaptation Principles

• Principle of Specificity:

  • Skeletal adaptation is specific to the exact bone stressed, tissue composition (cortical vs. trabecular), and exercise modality.

  • Weight-bearing Exercise: Movement where body weight is supported against gravity (impact-loading); highly osteogenic.

  • Non-weight-bearing Exercise: Movement where body is supported/suspended (e.g., swimming, cycling); minimal osteogenic stimulus.

  • Resistance Training: High strain via muscle contraction; full-body program benefits entire skeleton.

• Principle of Overload:

  • Skeletal tissue must be loaded beyond normal daily thresholds to adapt.

  • Strain is manipulated via volume (repetitions) or intensity (ground reaction forces / joint forces).

  • Mechanical strain intensity is more critical than repetition/volume for bone density gains.

  • Progression must be gradual.

• Principle of Individualization:

  • Bone adaptations vary widely between individuals based on genetics, baseline BMD, age, nutrition, and hormonal status.

• Principle of Reversibility:

  • Discontinuation of regular exercise results in progressive loss of accumulated BMD gains.

Pathology of Low Bone Density: Osteopenia and Osteoporosis

• Clinical Classifications & T-Score Definitions:

  • Bone Mass: Absolute quantity of bone tissue.

  • Bone Mineral Density (BMD): Relative value of hydroxyapatite per measured bone area.

  • Osteopenia: Decreased BMD between $1.0$ and $2.5$ standard deviations ($1.0\text{--}2.5\text{ SD}$) below the young adult normal mean. Normal age-related decline starting between ages $30\text{--}40$

  • Osteoporosis: Pathological porosity and loss of BMD greater than $2.5$ standard deviations ($>2.5\text{ SD}$) below young adult normal mean.

  • Established Osteoporosis: Osteoporosis ($>2.5\text{ SD}$ below normal) combined with one or more fragility fractures.

• Epidemiology and Mortality:

  • Affects over $10\text{ million}$ Americans.

  • Causes $\sim 300,000$ hip fractures per year in the U.S.

  • Hip fractures require surgery and carry a $15\text{--}20\%$ post-surgical mortality rate.

  • Vertebral crush fractures cause loss of height, kyphosis, and chronic pain. Wrist fractures are also common.

• Risk Factor Matrix:

  • Genetic Factors:

  • Race: Whites and Asians are at higher risk than African Americans and Hispanic Americans.

  • Sex: Females are $4\times$ more likely than males (lighter bones, post-menopausal estrogen loss, longer lifespan).

  • Heredity: Family history of osteoporosis.

  • Body Build: Petite, small-framed individuals (less peak bone mass to lose).

  • $tt$ Genotype: Vitamin D receptor genotype impairing calcium absorption (spine loss).

  • Nutritional Factors:

  • Chronic low calcium or Vitamin D intake.

  • High caffeine intake ($>3\text{--}4\text{ cups/day}$; $1$ cup coffee = $100\,mg$, tea = $40\,mg$, soda = $24\,mg$ caffeine) promoting calcium loss.

  • Diet soft drinks (interferes with calcium uptake).

  • Excessive alcohol use (damages bone tissue, leads to poor diet).

  • Consistently high protein intake.

  • Lifestyle Factors:

  • Lack of physical activity / sedentary lifestyle.

  • Cigarette smoking (lowers serum estrogen, causes early menopause).

  • Medical Conditions & Pharmacotherapy:

  • Conditions: Bariatric surgery, chronic kidney failure, inflammatory bowel disease (IBD), rheumatoid arthritis, liver disease, eating disorders.

  • Medications: Corticosteroids, anticonvulsants, thyroid hormone replacement, immunosuppressants, warfarin.

• Pharmacological & Clinical Therapies:

  • Estrogen Replacement Therapy (ERT), Selective Estrogen Receptor Modulators (SERMs), statins, bisphosphonates, and progressive resistance exercise.

Reproductive Endocrinology, Exercise, and Bone Density

• Categorization of Amenorrhea:

  • Primary Amenorrhea: Delayed age of menarche.

  • Secondary Amenorrhea: Cessation of menses for greater than $90$ days after established periodicity.

• Menarche Dynamics in Female Athletes:

  • U.S. mean menarcheal age: $12.3\text{--}12.8$ years (range $9\text{--}17$ years).

  • Competitive athletes show delayed menarche (swimmers: minimal delay; dancers and gymnasts: delayed $\sim 2$ years).

  • Delay correlates with level of competition. Mother's age of menarche is the best predictor.

• Hypoestrogenic Impact on Bone Integrity:

  • Secondary amenorrhea creates an estrogen deficiency (hypoestrogenic state).

  • Uncouples bone remodeling: Resorption exceeds deposition.

  • Bone loss rate is highest ($4\text{--}5\%/year$) immediately following menses cessation.

  • Trabecular bone in the spinal vertebrae is highly vulnerable (e.g., a $25\text{-year-old}$ amenorrheic athlete demonstrated BMD values equal to a $51\text{-year-old}$ woman).

  • Hard-training athletes who maintain normal menstrual function show normal or enhanced BMD.

• Reversibility and Intervention:

  • Resumption of menses restores bone density, though recovery may be incomplete.

  • Interventions: Reduced training volume, increased caloric intake, calcium supplementation, and physician-guided estrogen therapy.

  • Other causes of amenorrhea: Pregnancy, contraceptives, breast-feeding, stress, medication, thyroid malfunction, pituitary tumor, uterine scarring, primary ovarian insufficiency.

Skeletal Traumatology: Macrotrauma and Microtrauma

• Macrotrauma (Acute Injuries):

  • Sudden, high-force mechanical impacts (e.g., broken leg, clavicle fracture, growth plate fracture).

  • Closed Fracture: Skin remains intact.

  • Open (Compound) Fracture: Bone penetrates skin; high risk of infection and severe bleeding.

  • Morphological Variants: Comminuted (shattered into multiple pieces), spiral (twisting fracture line), greenstick (one side broken, one side bent), compression (vertebral collapse), epiphyseal (growth plate separation).

• Microtrauma (Overuse Stress Injuries):

  • Chronic repetitive loading leading to uncoupled remodeling where resorption outpaces deposition (stress reaction/response).

  • Asymptomatic Stage: Early hyperactivity shows no clinical pain, swelling, or tenderness.

  • Symptomatic Stage: Unchecked overuse leads to bone inflammation, micro-cracks, cartilage damage, bone spurs, epiphyseal damage, avulsion fractures, or stress fractures.

  • Stress Fracture: Hairline break occurring without acute trauma.

  • Diagnostic Lag: Stress fractures may be undetectable on X-rays or bone scans for $3\text{--}4$ weeks after clinical pain onset.

• Risk Factors in Adolescent Athletes:

  • Peak Incidence Age: $10\text{--}15$ years of age (during adolescent growth spurt).

  • Etiology: Rapid longitudinal growth outpaces matrix mineralization. Tight muscles/tendons across elongating joints reduce shock absorption, transferring stress directly to bone.

  • Primary Cause: Training errors—specifically abrupt workload increases exceeding $10\%/week$ (safe progression rate is $\sim 5\%/week$).

  • Secondary Causes: Muscle imbalances, improper footwear, bad technique, anatomic malalignment, and hard running surfaces (concrete/asphalt).