Week 10: Movement & Stability 2

Learning Objectives

Students will be able to describe the six major functions of the skeletal system and explain their physiological importance
Students will be able to distinguish between the axial and appendicular skeleton and identify major bones in each division
Students will be able to explain the process of bone remodeling involving osteoclasts and osteoblasts
Students will be able to describe key developmental and age-related changes in bone, including growth plates, fontanelles, and fracture risk with aging
Students will be able to describe the structure of skeletal muscle at macroscopic and microscopic levels
Students will be able to explain how muscles act as agonists, antagonists, synergists, and fixators during movement
Students will be able to identify the different types of joints and explain synovial joint structure and function
Students will be able to discuss pelvic support, foot arches, and age-related musculoskeletal changes that affect mobility and fall risk

Your Skeleton and Muscles: The Framework of Movement

Your skeleton is like the framework of a building, providing support and structure for your entire body. Made up of 206 bones in adults, the skeleton serves multiple purposes: it supports your body against gravity, protects vital organs like your heart and lungs inside the ribcage, helps you move by providing attachment points for muscles, stores important minerals like calcium for later use, and produces blood cells in the bone marrow.

Bones are not solid and lifeless - they're living, growing tissues that are constantly being broken down and rebuilt through a process called remodeling. This process ensures bones stay strong and can repair themselves when injured. The skeleton divides into two main parts: the axial skeleton (your skull, spine, and ribcage) forms the central axis of your body, while the appendicular skeleton (arms, legs, shoulders, and hips) attaches to this center and allows movement.

Your muscles work with bones to create movement. Skeletal muscles attach to bones via tendons and pull when they contract, causing bones to move at joints. Even when you're resting, muscles maintain slight tension called muscle tone. However, as we age, muscle tissue gradually gets replaced by connective tissue and fat, starting as early as age 30-50. This explains why older adults may have decreased strength and slower reflexes.

Joints are where two or more bones meet, and they come in different types. Some joints, like those in your skull, don't move at all to protect your brain. Others, like your knee or elbow, are synovial joints that allow smooth movement thanks to a special fluid-filled capsule. Balance depends on the interaction between your sensory systems (eyes, inner ear), muscles, and nerves. As people age, deterioration in these systems leads to increased risk of falls and injuries.

Musculoskeletal System: Structure, Function, and Aging

The skeletal system comprises 206 bones in adults organized into the axial skeleton (skull, vertebral column, thoracic cage) and appendicular skeleton (pectoral and pelvic girdles, upper and lower limbs). The skeleton serves six primary functions: structural support for the body against gravitational forces; protection of vital organs (neurocranium protecting the brain, thoracic cage sheltering the heart and lungs); facilitation of movement through muscle attachment sites; mineral storage (calcium and phosphorus) for metabolic homeostasis; hematopoiesis (blood cell production) within the red bone marrow; and energy storage as triglycerides within yellow marrow.

Bone Tissue: Exists in two forms: compact (dense) bone forming the outer cortex providing strength, and spongy (cancellous) bone containing trabeculae that house marrow and reduce weight while maintaining structural integrity. Long bones (humerus, femur) exhibit characteristic features including diaphysis (shaft), epiphyses (ends), articular cartilage, periosteum (outer vascular covering), endosteum (inner lining), and medullary cavity containing marrow.

Bone Remodeling: Occurs continuously through coupled resorption by osteoclasts and formation by osteoblasts, maintaining calcium homeostasis and adapting to mechanical stresses via Wolff's Law. Osteoclasts are large multinucleated cells that break down bone tissue, while osteoblasts are bone-forming cells that synthesize collagen and matrix components.

Skeletal Muscle: Constitutes the muscular system attached to the skeleton via tendons (muscle-to-bone) and aponeuroses (broad, sheet-like tendons). Muscle contraction generates force transmitted through tendons to produce movement at joints. The contractile apparatus consists of sarcomeres containing actin (thin filaments) and myosin (thick filaments) organized into myofibrils within muscle fibers (cells). During contraction, calcium ions trigger cross-bridge cycling between actin and myosin, powered by ATP hydrolysis.

Joints and Articulations: Synovial joints are freely movable joints characterized by a fluid-filled cavity, articular cartilage, and synovial membrane. Synovial fluid lubricates joints, nourishes cartilage, and absorbs shock. Tendons attach muscles to bones, while ligaments connect bone to bone, providing joint stability.

Aging Effects: Sarcopenia (age-related loss of muscle mass) begins around age 30-50, with muscle tissue gradually replaced by connective tissue and fat. Bone density decreases with age, particularly in postmenopausal women, increasing fracture risk. Balance and postural control deteriorate due to changes in sensory systems, muscle strength, and neural processing.

Additional Bone and Joint Clinical Anatomy

Osteoprogenitor cells: These are bone stem-like cells found in the periosteum and endosteum. They can develop into osteoblasts, especially during growth and fracture repair.

Bone blood supply: Bone is highly vascular. Nutrient arteries, periosteal vessels, and vessels to the epiphyses all help keep bone tissue alive, support remodeling, and allow healing after injury.

Origin and insertion: Muscles usually have an origin on the less mobile attachment and an insertion on the more mobile attachment. When the muscle contracts, the insertion is usually pulled toward the origin.

Clinical joints: The TMJ combines hinge and gliding movement, the shoulder is very mobile but easy to dislocate, the elbow is a stable hinge joint, the hip is a strong weight-bearing ball-and-socket joint, and the knee depends heavily on ligaments and menisci for stability.

Development, Support, and Movement Control

Growing and aging bone: In infants, the skull has fontanelles (soft spots) that allow moulding during birth and brain growth. Long bones grow in length at epiphyseal plates, which close after puberty. With aging, bone density falls and intervertebral discs lose hydration, increasing stiffness and fracture risk.

Pelvis and foot support: The false pelvis is the broader upper part above the pelvic brim, while the true pelvis is the lower part surrounding pelvic organs. The foot's longitudinal and transverse arches help support body weight, absorb shock, and improve walking efficiency.

Muscle teamwork: Muscles usually act in groups. The agonist is the main mover, the antagonist opposes it, synergists assist the action, and fixators stabilise the origin so movement stays controlled.

🎥 Video Lectures

Overview

Introduction to the skeletal system and movement.

Topic Title

Select a topic from the list to view detailed information.

📄 Lecture Notes

Key Terms

Axial Skeleton

The central portion of the skeleton consisting of the skull, vertebral column, and thoracic cage

Appendicular Skeleton

The portion of the skeleton comprising the pectoral girdle, pelvic girdle, and upper and lower limbs

Compact Bone

Dense, hard outer layer of bone providing strength and protection; composed of osteons (Haversian systems)

Spongy Bone

Porous, trabecular inner bone tissue that reduces weight while providing structural support; contains red marrow

Osteoclasts

Large multinucleated cells that break down and resorb bone tissue during remodeling

Osteoblasts

Bone-forming cells that synthesize and secrete collagen and other matrix components during bone formation

Hematopoiesis

The production of blood cells (red blood cells, white blood cells, platelets) within red bone marrow

Tendon

Dense connective tissue attaching muscle to bone, transmitting contractile force to produce movement

Ligament

Dense connective tissue connecting bone to bone, providing joint stability while allowing controlled movement

Synovial Joint

Freely movable joint characterized by a fluid-filled cavity, articular cartilage, and synovial membrane; includes knee, shoulder, hip

Articular Cartilage

Smooth, gliding hyaline cartilage covering bone ends at synovial joints, reducing friction and absorbing shock

Synovial Fluid

Viscous fluid secreted by synovial membrane that lubricates joints, nourishes cartilage, and absorbs shock

Sarcomere

The basic contractile unit of skeletal muscle, composed of overlapping actin and myosin filaments

Actin

Thin contractile protein filaments in muscle that interact with myosin to produce contraction

Myosin

Thick contractile protein filaments in muscle that use ATP to pull actin filaments, generating force

Muscle Tone

Sustained, weak, involuntary contraction of motor units that maintains muscle readiness and posture

Sarcopenia

Age-related loss of muscle mass, strength, and function; begins around age 30-50

Periosteum

Dense fibrous membrane covering bone surface containing blood vessels, nerves, and osteogenic cells

Diaphysis

The shaft of a long bone, consisting primarily of compact bone surrounding the medullary cavity

Epiphysis

The end of a long bone, containing spongy bone and articular cartilage for joint formation

Fontanelle

A soft fibrous gap between infant skull bones that allows moulding during birth and brain growth after birth

False Pelvis

The broader superior part of the pelvis above the pelvic brim that supports abdominal contents

True Pelvis

The inferior part of the pelvis below the pelvic brim that encloses the pelvic cavity and pelvic organs

Agonist

The prime mover muscle mainly responsible for producing a specific movement

Antagonist

The muscle that opposes or controls the action of the agonist during movement

Longitudinal Arch

The lengthwise arch of the foot that helps distribute body weight, absorb shock, and support gait

Interactive Activity: Bone Remodeling Sequence

Drag and drop the steps of the bone remodeling process into the correct order. Learn how osteoclasts and osteoblasts work together to maintain healthy bones.

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End of Week Test

Test your knowledge of skeletal functions, development and aging, bone remodeling, muscle coordination, and joint anatomy with the full end-of-week assessment.

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Clinical Case Study

Apply your knowledge of Musculoskeletal System to a clinical scenario.

Open Case: The Hip Fracture →