Calisthenics Instructor Certification Chapter 5: Kinesiology

Chapter 5: Kinesiology

Kinesiology is the study of human movement. It draws on principles from physics (mechanics), anatomy, and physiology to analyze how the body moves. In rehabilitation and allied health professions, kinesiology helps practitioners ensure that patients perform movements with:

  • Adequacy (full, correct range)
  • Efficiency (minimal wasted effort)
  • Safety (protecting tissues and joints)

5.1 Gravity

  • Definition: The force by which the Earth attracts all masses toward its center.
  • Role in Movement:
    • With gravity: e.g. controlled lowering of the body (eccentric muscle work).
    • Against gravity: e.g. lifting the body (concentric muscle work).
    • Neutralized gravity: e.g. in water or with mechanical support, which changes movement demands entirely.
  • Developmental Impact: From infancy onward, gravity shapes motor milestones (rolling, sitting, standing, walking).

5.2 Weight & Center of Gravity

  • Weight (W): The force of gravity on a body:W=m×g W = m \times gwhere m is mass and g is gravitational acceleration.
  • Center of Gravity (CoG):
    • The point at which weight is considered to act.
    • In a uniform object (e.g., a sphere), CoG coincides with its geometric center.
    • In the human body, CoG shifts as posture and limb positions change.
    • Standing posture: In the adult, the CoG lies just anterior to the second sacral vertebra (S2).
    • Dynamic changes: Raising the arms or flexing the trunk moves the CoG anteriorly and superiorly.
  • Equilibrium Point: Often used interchangeably with CoG, since balance about this point is critical for postural control.

5.3 External Forces

A force is any influence that can change an object’s motion. As a vector, it is defined by:

  1. Point of application (where it acts on the body)
  2. Magnitude (how strong it is)
  3. Direction (the line along which it acts)
  4. Sense (the “arrowhead,” indicating push vs. pull)

Although invisible, external forces are sensed constantly—examples include:

  • Gravity acting on body segments
  • Ground reaction forces under the feet
  • Resistance (weights, bands, water) applied by the environment or equipment
  • Contact forces from other bodies (e.g. pushing a door)

5.4 Newton’s Laws of Motion

1. Law of Inertia
A body at rest stays at rest, and a body in uniform motion stays in that motion, unless acted on by an external force.

2. Law of Acceleration
The net force (F) applied to a body equals its mass (m) multiplied by its acceleration (a):

F=m×a

3. Law of Action–Reaction
For every force that body A exerts on body B, body B exerts an equal and opposite force on body A.

Understanding these laws allows clinicians to:

  • Predict movement outcomes when applying forces (e.g., how much resistance to give)
  • Design safe rehabilitation exercises (e.g., controlling acceleration to protect healing tissues)
  • Analyze compensatory strategies when one muscle group is weak or overloaded

5.5 Newton’s First Law: Law of Inertia

A body at rest tends to remain at rest, and a body in motion tends to remain in motion, unless acted upon by an external force. This property—inertia—is the resistance of an object to any change in its state of motion.

Clinical Example:
A passenger in a car that accelerates suddenly will be thrown backward, often hyperextending the neck, because their body “wants” to stay at rest. Conversely, if the car stops abruptly, the passenger’s body continues moving forward, which can cause whiplash injuries to the cervical spine.

5.6 Newton’s Second Law: Law of Acceleration

The acceleration of an object depends on the net force applied and its mass. Mathematically:

F=m×aF = m \times a

  • F = net external force
  • m = mass of the body segment
  • a = acceleration produced

Implications in Human Movement:
Heavier body segments require greater muscular force to achieve the same acceleration. During rehabilitation, therapists must account for segmental mass when prescribing strengthening or mobility exercises—lighter distal segments accelerate more easily under muscle contraction.

5.7 Newton’s Third Law: Action–Reaction

For every action force, there is an equal and opposite reaction force. When body A exerts a force on body B, body B simultaneously exerts a force of the same magnitude back on body A.

Clinical Example:
On a trampoline, the jumper pushes down (action) and the trampoline pushes up with equal force (reaction), propelling the jumper into the air. The higher the push, the greater the reaction force and subsequent rebound.

5.8 Internal Forces

Internal forces originate within the body’s tissues, primarily from muscle contractions. These forces are:

  • Vectorial: They have a point of application (the tendon’s insertion), a direction (along the muscle fibers), and a magnitude (proportional to the number of activated fibers).
  • Transmitted via tendons: The tendon may redirect the line of pull if it attaches far from the muscle’s origin, altering joint mechanics.

When muscles contract, they generate linear forces that, acting across joints, produce rotational or circular motions of limbs. Understanding internal force vectors is essential for:

  • Joint torque analysis
  • Designing safe and effective strengthening programs
  • Preventing overload or improper loading of healing tissues

Planes, Axes, and Levers in Human Movement

5.9 Planes of Motion

To describe human movement, we assume the anatomical position: standing upright, arms at the sides, palms facing forward, feet hip-width apart. Three orthogonal planes partition the body:

  • Sagittal (Anteroposterior) Plane
    Divides the body into left and right halves. Movements in this plane move forward or backward (e.g., flexion, extension).
  • Frontal (Coronal) Plane
    Divides the body into anterior (front) and posterior (back) halves. Movements in this plane move sideways (e.g., abduction, adduction).
  • Transverse (Horizontal) Plane
    Divides the body into superior (upper) and inferior (lower) halves. Movements in this plane rotate around a vertical axis (e.g., axial rotation).

All three planes intersect at the body’s center of gravity when in anatomical position.

5.10 Axes of Rotation

Each plane of motion has an associated axis of rotation, perpendicular to that plane:

  • Frontal Axis (Mediolateral)
    — Perpendicular to the sagittal plane
    — Movement: flexion/extension
  • Sagittal Axis (Anteroposterior)
    — Perpendicular to the frontal plane
    — Movement: abduction/adduction; lateral flexion
  • Vertical Axis (Longitudinal)
    — Perpendicular to the transverse plane
    — Movement: internal/external rotation; pronation/supination

5.11 Fundamental Movements

When all joint angles are at 0° (anatomical reference with palms facing each other), the following primary movements occur:

Sagittal Plane (around Frontal Axis):

  • Flexion: Decrease in joint angle (e.g., elbow flexion).
  • Extension: Increase in joint angle back to 0°.
  • Hyperextension: Extension beyond anatomical zero (e.g., trunk hyperextension).

Frontal Plane (around Sagittal Axis):

  • Abduction: Movement away from the midline (e.g., shoulder abduction).
  • Adduction: Movement toward the midline.
  • Lateral flexion: Side-bending of the trunk or neck.

Transverse Plane (around Vertical Axis):

  • Rotation: Turning a body part right or left (e.g., head rotation).
  • Internal (medial) rotation: Toward the midline (e.g., thigh internal rotation).
  • External (lateral) rotation: Away from the midline.
  • Pronation/Supination: Forearm rotation; palm down/up.

Oblique (Diagonal) Movements:
Many functional tasks combine planes (e.g., reaching across the body), producing diagonal or multiplanar motions called circumduction.

5.12 Torque (Moment of Force)

  • Torque (τ): The rotational equivalent of force.
  • Definition:τ=F×r\tau = F \times rwhere F is the force magnitude and r is the perpendicular distance (moment arm) from the axis of rotation to the line of action of the force.
  • Implication: Increasing the moment arm (lever arm) increases torque for the same force.

5.13 Levers in the Human Body

A lever is a rigid bar that pivots about a fixed point (fulcrum). In the body:

  • Fulcrum: Joint
  • Effort (Force): Muscle contraction
  • Resistance: Weight of limb segment or external load
  • Lever arms: Distances from fulcrum to force and resistance applications

Three classes of levers depend on the relative positions of fulcrum (F), effort (E), and resistance (R):

  1. First-Class Lever (F between E and R):
    • Example: Neck extension, with cervical vertebrae as fulcrum, neck extensors applying effort posteriorly, and head weight anteriorly.
  2. Second-Class Lever (R between F and E):
    • Example: Standing on tiptoes—fulcrum at the metatarsal heads, resistance is body weight at the ankle, effort from the calf muscles via the Achilles tendon.
  3. Third-Class Lever (E between F and R):
    • Most common in the body.
    • Example: Elbow flexion—fulcrum at the elbow joint, effort from the biceps inserted on the radius, resistance is the weight in the hand distal to the joint.

5.14 Mechanical Advantage

  • Mechanical Advantage (MA): Ratio of the effort lever arm (rE) to the resistance lever arm (rR):MA=rErR MA = \frac{r_E}{r_R}
  • Equilibrium Condition:F×rE=R×rR F \times r_E = R \times r_Rwhere F is effort force and R is resistance force.
  • Interpretation:
    • MA > 1: Effort arm longer—less force needed (e.g., second-class levers).
    • MA < 1: Effort arm shorter—more force needed but greater speed and range of motion (third-class levers).

5.15 The Musculoskeletal (Kinetic) System

The musculoskeletal system (also called the motor or kinetic system) is responsible for all voluntary movements. It comprises:

  1. Bones
  2. Joints
  3. Skeletal (striated) muscles
  4. Tendons and ligaments
  5. Neural control

Muscles attach to bones via tendons; when they contract under neural command, they pull on bones, producing movement at the joints they span.

5.15.1 The Human Skeleton

  • Total bones: Approximately 206 in the adult.
  • Connective framework:
    • Bones are joined by joints, forming the rigid scaffold of the body.
    • Muscles insert on specific bony landmarks; their pull on one bone moves it relative to another.
  • Neural integration: Motor commands from the nervous system coordinate muscle activations to produce smooth, efficient, and safe movement.

5.15.2 Roles of the Skeleton

  1. Support & Shape
    • Provides the framework that supports body weight and maintains form.
  2. Protection
    • Encases vital organs in bony cavities:
      • Cranial vault: Protects the brain
      • Thoracic cage: Protects heart and lungs
      • Pelvic basin: Houses pelvic viscera
  3. Movement
    • Bones act as levers, muscles generate the forces, and joints serve as fulcra.
  4. Mineral reservoir
    • Stores calcium and phosphorus, releasing them under hormonal control.
  5. Blood cell production
    • Bone marrow in select bones produces red and white blood cells and platelets.

5.15.3 Major Bony Cavities

Cavity Bony Borders Contents
Cranial Frontal, parietal, temporal, occipital, sphenoid, and ethmoid bones Brain, meninges
Thoracic (rib cage) Thoracic vertebrae, ribs, sternum Lungs, heart, great vessels
Pelvic Hip bones (ilium, ischium, pubis) and sacrum Urinary bladder, reproductive organs, distal intestine

These cavities both protect their contents and serve as anchors for muscles involved in posture and movement.

5.16 Bones: Definition and Functions

Bone is a hard, white organ composed of osseous (bone) tissue. Its external surface is irregular, featuring ridges, tubercles, and processes that serve to:

  • Anchor muscles and ligaments
  • Transmit nerves and blood vessels
  • Increase mechanical leverage

Despite great variation in shape, size, density, and internal architecture, all bones share the same basic composition and fulfill these roles:

  1. Support of body structures
  2. Protection of internal organs
  3. Movement via muscle attachments
  4. Mineral storage (calcium, phosphorus, magnesium)
  5. Blood cell formation in bone marrow

5.17 Bone Classification by Shape

Type Characteristics Examples
Long One dimension significantly longer; contains a marrow (medullary) cavity; two ends (epiphyses) and a shaft (diaphysis) separated by growth plates in youth. Femur, tibia, humerus, radius, ulna
Short All dimensions roughly equal; mostly spongy bone with a thin cortical shell. Carpals (wrist), tarsals (ankle), vertebrae
Flat Two large, parallel cortical plates with spongy bone between; broad surface for muscle attachment and protection of organs. Scapula, ribs, pelvis bones, cranial bones
Irregular Complex shapes that don’t fit other categories. Certain skull bones (e.g., sphenoid), vertebrae
Sesamoid Small, round bones embedded within tendons to modify pressure and reduce friction. Patella (largest), pisiform

5.18 Bone Composition

All bone tissue consists of two main components:

  1. Organic matrix
    • Cells: Osteocytes (maintain bone), osteoblasts (build bone), osteoclasts (resorb bone)
    • Ground substance: Collagen fibers and proteoglycans, providing tensile strength and flexibility
  2. Inorganic mineral phase
    • Hydroxyapatite crystals: Calcium phosphate salts that confer compressive strength and rigidity

5.19 Microscopic and Gross Structure

1. Osseous (Bone) Tissue

  • Compact (cortical) bone:
    • Dense outer layer of all bones and the shafts of long bones
    • Organized into osteons (Haversian systems) aligned along stress lines
  • Spongy (trabecular) bone:
    • Internal network of trabeculae forming a “honeycomb”
    • Found in epiphyses of long bones, short bones, and the interior of flat bones

2. Periosteum

  • A fibrous membrane covering all non-articular bone surfaces
  • Rich in blood vessels and nerves
  • Serves as the site of muscle and ligament attachments and contains osteogenic cells for growth and repair

3. Medullary (Marrow) Cavity

  • Central canal within the diaphysis of long bones
  • Houses bone marrow:
    • Red marrow (hematopoietic) in children and in adult flat bones and epiphyses
    • Yellow marrow (fatty) in adult diaphyses

5.17 Joints (Articulations)

A joint (articulation) is the anatomical connection between two or more bones, providing both stability and mobility. Each joint’s structure determines the specific movements it allows while maintaining sufficient stability to protect the connected bones.

5.17.1 Classification by Structure & Mobility

Type Mobility Structural Features Examples
Synarthrosis Immobile No joint space; bones united by tissue Sutures of skull; tibiofibular syndesmosis; pubic symphysis (cartilaginous)
Amphiarthrosis Slightly mobile Bones connected by cartilage (intervertebral discs) Intervertebral joints; pubic symphysis (fibrocartilage)
Diarthrosis Freely mobile Synovial cavity; articular cartilage; capsule; synovial fluid Shoulder, hip, knee, wrist, ankle
  • Synarthroses
    • Fibrous (syndesmosis): Bones joined by dense connective tissue (e.g., distal tibiofibular joint).
    • Cartilaginous (synchondrosis or symphysis): Bones united by cartilage (e.g., growth plate—temporary synchondrosis; pubic symphysis—symphysis).
    • Bony fusion (synostosis): Ossification of a prior suture or synchondrosis (e.g., skull sutures in adulthood).
  • Amphiarthroses
    • Permit limited movement, cushioning and distributing loads (e.g., vertebral bodies separated by intervertebral discs).
  • Diarthroses (Synovial joints)
    • Characterized by a fluid-filled joint cavity, hyaline cartilage covering the bone ends, a fibrous capsule reinforced by ligaments, and often accessory structures (menisci, labra).
    • Allow various movement types: hinge (elbow), ball-and-socket (hip), pivot (atlanto-axial), saddle (thumb), condyloid (wrist), plane (carpals).

5.18 Ligaments

Ligaments are bands of dense, regular connective tissue that:

  • Connect bone to bone around or within a joint
  • Provide passive stability by resisting excessive or undesired motions
  • Are tailored in number, orientation, and strength to each joint’s functional demands

Examples:

  • Knee ligaments: Medial and lateral collateral, anterior and posterior cruciate ligaments
  • Hip: Ligamentum teres, iliofemoral, pubofemoral, ischiofemoral ligaments

5.19 Joint Stability

Joint stability arises from multiple factors working in concert:

  1. Ligaments & capsule
    • Primary passive stabilizers; their tension limits joint excursion.
  2. Articular geometry
    • Bony congruence (e.g., deep acetabulum of hip versus shallow glenoid of shoulder).
  3. Cartilaginous structures
    • Labra (shoulder, hip) and menisci (knee) deepen sockets, absorb shock, and guide movement.
  4. Muscles & tendons
    • Dynamic stabilizers; their tone and coordinated contraction reinforce ligamentous support.
    • Example: Quadriceps tendon and patellar alignment contribute to anterior–posterior knee stability alongside the cruciate ligaments.

A well-balanced combination of these elements ensures each joint can move through its intended range safely and efficiently.

5.18 Skeleton of the Trunk

The trunk skeleton comprises two major components:

  1. Vertebral Column
  2. Thoracic Cage (ribs and sternum)

5.18.1 Vertebral Column

  • Composition: 33–34 vertebrae arranged in five regions:
    1. Cervical (C1–C7): 7 vertebrae
    2. Thoracic (T1–T12): 12 vertebrae
    3. Lumbar (L1–L5): 5 vertebrae
    4. Sacrum: fusion of 5 sacral vertebrae into one bone
    5. Coccyx: fusion of 4–5 coccygeal vertebrae
  • Intervertebral Discs:
    • Fibrocartilaginous pads between adjacent vertebral bodies
    • Provide shock absorption and allow slight movement between vertebrae
  • Vertebral Anatomy:
    • Body: Weight-bearing anterior portion
    • Vertebral Arch: Posterior ring of bone that, together with the body, encloses the vertebral foramen (passage for the spinal cord)
    • Processes: Spinous, transverse, and articular processes for muscle attachment and intervertebral articulation
  • Natural Curvatures: When viewed laterally, the spine forms four gentle curves:
    1. Cervical Lordosis (inward curve of the neck)
    2. Thoracic Kyphosis (outward curve of the upper back)
    3. Lumbar Lordosis (inward curve of the lower back)
    4. Sacrococcygeal Kyphosis (outward curve of the sacrum and coccyx)
  • Pathological Deviations:
    • Scoliosis: Lateral curvature and rotation of the spine on frontal view
    • Hyperkyphosis (Kyphosis): Excessive outward curvature of the thoracic spine (“hunchback”)
    • Hyperlordosis (Lordosis): Excessive inward curvature of the lumbar spine

5.18.2 Thoracic Cage

The thoracic cage protects the heart, lungs, and great vessels while providing attachment sites for respiratory muscles. It consists of:

  • Sternum
  • Twelve pairs of ribs
  • Twelve thoracic vertebrae

1. Sternum

  • Manubrium: Superior “handle” with jugular and clavicular notches
  • Body (Gladiolus): Central segment articulating with ribs 2–7 via costal cartilages
  • Xiphoid process: Inferior tip; cartilaginous in youth, ossifies in adulthood

2. Ribs (12 pairs)

  • True (Vertebrosternal) ribs (1–7): Directly attach to the sternum by their own costal cartilages
  • False (Vertebrochondral) ribs (8–10): Connect indirectly to the sternum via the cartilage of the rib above
  • Floating (Vertebral) ribs (11–12): No anterior attachment; terminate in the abdominal musculature

Each rib features a head (articulates with vertebral bodies), a tubercle (articulates with a transverse process), and a shaft (provides a costal groove for intercostal neurovascular bundles).

3. Thoracic Vertebrae (T1–T12)

  • Possess costal facets on the body and transverse processes for rib articulation
  • Long, downward-sloping spinous processes overlap adjacent vertebrae

Function of the Thoracic Cage

  • Protection: Encloses the thoracic organs within a semi-rigid, yet flexible, vault
  • Respiration: Ribs elevate and depress about the costovertebral joints during breathing, changing thoracic volume
  • Support & Attachment: Serves as the origin for muscles of the neck, back, chest, and upper limb

This integrated bony framework combines stability and mobility to safeguard vital structures and facilitate the mechanics of ventilation.

5.19 Skeleton of the Upper Limb

The upper limb skeleton comprises the pectoral (shoulder) girdle, the arm, the forearm, and the hand, forming a continuous chain from the trunk to the fingertips.

5.19.1 Pectoral (Shoulder) Girdle

Connects the upper limb to the trunk and provides sites for muscle attachment.

  • Scapula (Shoulder Blade)
    • Flat, triangular bone on the posterior thorax (ribs 2–7).
    • Key landmark: Glenoid cavity—shallow socket for the humeral head.
  • Clavicle (Collarbone)
    • S-shaped long bone lying horizontally above the first rib.
    • Medial articulation: Sternoclavicular joint with the manubrium.
    • Lateral articulation: Acromioclavicular joint with the scapular acromion.

5.19.2 Arm (Brachium)

  • Humerus
    • Long bone spanning shoulder to elbow.
    • Proximal end: Head (into glenoid), anatomical and surgical necks, greater/lesser tubercles.
    • Shaft (Diaphysis): Deltoid tuberosity for muscle attachment.
    • Distal end: Trochlea (medial) for the ulna; capitulum (lateral) for the radius.

5.19.3 Forearm (Antebrachium)

Two parallel long bones that transmit forces from the hand to the arm.

  • Radius
    • Lateral bone (thumb side).
    • Proximal head: Articulates with humeral capitulum and radial notch of the ulna.
    • Distal end: Broad; articulates with carpal bones.
  • Ulna
    • Medial bone (pinky side).
    • Proximal end: Olecranon and coronoid processes form the trochlear notch for the humerus.
    • Distal end: Styloid process articulates with the wrist.

5.19.4 Hand (Manus)

1. Carpal Bones (Wrist)

Eight small bones arranged in two rows:

  • Proximal row (lateral → medial): Scaphoid, Lunate, Triquetrum, Pisiform
  • Distal row (lateral → medial): Trapezium, Trapezoid, Capitate, Hamate
2. Metacarpal Bones (Palm)

Five long bones (I–V), one per digit:

  • Base: Articulates with distal carpals
  • Head: Forms the knuckle
3. Phalanges (Fingers)
  • Digits II–V: Three phalanges each—proximal, middle, distal
  • Thumb (Digit I): Two phalanges—proximal, distal

5.20 Skeleton of the Lower Limb

The lower limb skeleton comprises the pelvic girdle, the thigh, the leg, and the foot, forming a continuous chain from the trunk to the toes.

5.20.1 Pelvic Girdle

Forms a ring (pelvic ring) that connects the spine to the lower limbs and supports abdominal and pelvic organs.

  • Hip Bones (Os Coxae, 2)
    Each hip bone is formed by the fusion of three elements (ilium, ischium, pubis).

    • Anteriorly: The two hip bones meet at the pubic symphysis (fibrocartilaginous joint).
    • Posteriorly: Each hip bone articulates with the sacrum at a sacroiliac joint.

5.20.2 Thigh

  • Femur
    • Proximal end:
      • Head—articulates with the acetabulum of the pelvis (hip joint).
      • Greater and lesser trochanters—muscle attachment sites.
    • Shaft (Diaphysis): Slightly bowed for weight distribution.
    • Distal end:
      • Medial and lateral condyles—articulate with the tibia (knee joint).
      • Intercondylar notch/fossa—site of cruciate ligament attachment.

5.20.3 Leg

Two parallel long bones transmit body weight from the knee to the ankle.

  • Tibia (Shinbone)
    • Proximal end: Medial and lateral condyles with the tibial plateau; tibial tuberosity for patellar ligament.
    • Shaft: Triangular cross-section; anterior border forms the “shin.”
    • Distal end: Medial malleolus—prominent inner ankle.
  • Fibula (Calf Bone)
    • Proximal end: Head articulates with the tibia (not the knee joint).
    • Shaft: Slender, lateral.
    • Distal end: Lateral malleolus—prominent outer ankle, stabilizing the ankle joint.

5.20.4 Foot

Tarsal Bones (7)

Arranged in proximal, intermediate, and distal rows to form the ankle and heel:

  • Proximal row: Talus (ankle bone) and Calcaneus (heel)
  • Intermediate: Navicular
  • Distal row: Cuboid and three cuneiforms (medial, intermediate, lateral)
Metatarsal Bones (5)

Long bones (I–V) forming the sole and arches of the foot:

  • Base: Articulates with distal tarsals
  • Head: Articulates with proximal phalanges
Phalanges (14 total)
  • Digits II–V: Three phalanges each (proximal, middle, distal)
  • Great Toe (Hallux, Digit I): Two phalanges (proximal, distal)

5.21 Diarthroses (Synovial Joints)

A diarthrosis is a joint in which the articulating bone ends are separated by a fluid-filled cavity, permitting controlled, voluntary movement. Most of the body’s large and functionally important joints—such as the shoulder, hip, knee, and elbow—are diarthroses.

5.21.1 Key Structural Features

  1. Articular Cartilage
    • Hyaline cartilage covers the bone surfaces, reducing friction and distributing load.
  2. Joint (Synovial) Cavity
    • A potential space between the bones, filled with synovial fluid.
  3. Articular Capsule
    • Fibrous outer layer: Dense connective tissue that envelops the joint.
    • Synovial membrane (inner layer): Secretes synovial fluid and lines the cavity except over cartilage.
  4. Synovial Fluid
    • Viscous lubricant that nourishes cartilage and further reduces friction.
  5. Ligaments
    • Bands of dense connective tissue—either intrinsic (thickened parts of the capsule) or extrinsic—providing passive stability.
  6. Accessory Structures (when present)
    • Menisci or articular discs: Fibrocartilage structures that improve fit and absorb shock (e.g., knee menisci, TMJ articular disc).
    • Labrum: Fibrocartilaginous rim that deepens a socket (e.g., glenoid labrum in shoulder, acetabular labrum in hip).
    • Bursae and tendon sheaths: Fluid-filled sacs that reduce friction where tendons or skin glide over bone.

5.21.2 Classification by Shape and Movement

Joint Type Movements Allowed Example
Hinge Flexion and extension Elbow, interphalangeal joints
Pivot Rotation around a single axis Atlanto-axial joint (C1–C2), proximal radioulnar
Condyloid (Ellipsoid) Flexion/extension, abduction/adduction, circumduction Radiocarpal (wrist), metacarpophalangeal (knuckles)
Saddle Flexion/extension, abduction/adduction, opposition Carpometacarpal joint of the thumb
Ball-and-Socket Flexion/extension, abduction/adduction, rotation in all planes Shoulder (glenohumeral), hip
Plane (Gliding) Sliding or gliding in multiple directions Intercarpal, intertarsal joints

5.21.3 Functional Significance

  • Mobility vs. Stability Trade-off:
    • Joints requiring large ranges of motion (e.g., shoulder) have shallower sockets and rely more on muscles and ligaments for stability.
    • Joints requiring load bearing and stability (e.g., hip) have deeper sockets and stronger capsular and ligamentous support.
  • Clinical Relevance:
    • Understanding diarthrosis anatomy guides rehabilitation strategies—e.g., which movements to limit after injury, and which muscles to strengthen to support a hypermobile joint.
    • Therapeutic mobilizations utilize joint capsules and accessory structures to restore range when motion is restricted by injury, inflammation, or postural adaptations.

5.22 Anatomical Features of a Synovial Joint (Diarthrosis)

A synovial joint is characterized by the following five key structures:

1. Articular Surfaces

  • Definition: The regions of the opposing bones that come into contact.
  • Covering: Hyaline (articular) cartilage—smooth, glass-like tissue that:
    • Reduces friction
    • Distributes load
    • Protects underlying bone from wear
  • Note: Articular cartilage is avascular and aneural; it relies on diffusion from synovial fluid for nutrition, making it vulnerable to degeneration.

2. Joint Capsule (Articular Capsule)

  • Fibrous Outer Layer: Dense connective tissue that attaches to peri-articular bone margins, encapsulating the joint and isolating it from surrounding tissues.
  • Function: Provides passive stability, resisting excessive distraction and torsion.

3. Joint (Synovial) Cavity

  • Definition: A potential space between the articular surfaces, enclosed by the capsule.
  • Contents: Synovial fluid and sometimes accessory structures (menisci, discs, fat pads).

4. Synovial Membrane

  • Location: Lines the inner surface of the fibrous capsule (except where cartilage covers bone).
  • Function: Secretes synovial fluid and clears debris via resident macrophage-like cells.

5. Synovial Fluid

  • Appearance: Pale yellow, highly viscous.
  • Functions:
    • Lubrication: Minimizes friction during movement.
    • Nutrition: Transports oxygen and nutrients to the avascular cartilage.
    • Shock Absorption: Distributes compressive forces across the joint.

Clinical Correlation:

  • Osteoarthritis: Degeneration of articular cartilage leads to pain and reduced joint space.
  • Capsulitis: Inflammation of the joint capsule restricts motion (e.g., “frozen shoulder”).
  • Synovitis: Excess synovial fluid production causes swelling and pain (e.g., rheumatoid arthritis).

Understanding these components is essential for diagnosing joint pathologies and designing targeted rehabilitation—whether restoring cartilage health, preserving capsule elasticity, or optimizing synovial fluid dynamics.

5.23 Peri-articular Stabilizing Structures

Although the synovial joint itself (capsule, cartilage, fluid) provides the basic framework for movement, true joint stability is a product of several auxiliary (peri-articular) elements working together:

  1. Musculature
    • Surrounding each joint is a “cuff” or group of muscles whose tone and active contraction cocontract to compress the joint surfaces and resist unwanted motions.
    • Example: The rotator cuff muscles (supraspinatus, infraspinatus, teres minor, subscapularis) dynamically center the humeral head in the shallow glenoid fossa.
  2. Ligaments
    • Dense bands of fibrous connective tissue that reinforce the capsule, limiting extreme excursions.
    • In the knee:
      • Anterior and posterior cruciate ligaments resist anterior–posterior tibial translation.
      • Medial and lateral collateral ligaments resist varus–valgus stresses.
  3. Labra and Menisci
    • Labrum: A fibrocartilaginous rim that deepens a socket, increasing congruency and stability.
      • Hip (acetabular) labrum encircles the acetabulum.
      • Glenoid labrum compensates for the shallow glenoid fossa to stabilize the shoulder.
    • Menisci (knee): Crescent-shaped fibrocartilage pads interposed between femur and tibia that:
      • Distribute load over a larger surface area
      • Improve congruency for smooth motion
      • Absorb shock and provide proprioceptive feedback

By integrating muscular tone, ligamentous constraint, and fibrocartilaginous buffering, the human joint achieves a sophisticated balance of mobility and stability—allowing controlled, powerful movements without sacrificing protection of the articular surfaces.

5.24 Types of Synovial Joints by Axes of Movement

Synovial joints are classified by the number of axes (degrees of freedom) around which they can move:

1. Uniaxial Joints (1 DOF)

  • Movement: Occurs in a single plane around a single axis.
  • Examples:
    • Hinge joints (ginglymus): Elbow (humeroulnar), interphalangeal joints – flexion/extension in the sagittal plane around a mediolateral (frontal) axis.
    • Pivot joints (trochoid): Proximal radioulnar, atlanto-axial – rotation in the transverse plane around a vertical axis.

2. Biaxial Joints (2 DOF)

  • Movement: Occurs in two planes around two perpendicular axes.
  • Examples:
    • Condyloid (ellipsoid) joints: Wrist (radiocarpal), metacarpophalangeal – flexion/extension in the sagittal plane about a frontal axis, and abduction/adduction in the frontal plane about a sagittal axis.
    • Saddle joints: Thumb carpometacarpal – similar biaxial movements plus limited circumduction.
    • Knee joint: Primarily flexion/extension (sagittal), with a small amount of internal/external rotation (transverse) when flexed.

3. Multiaxial (Triaxial) Joints (3 DOF)

  • Movement: Occurs in three planes around three mutually perpendicular axes.
  • Examples:
    • Ball-and-socket joints: Shoulder (glenohumeral) and hip – allow flexion/extension (sagittal/frontal axis), abduction/adduction (frontal/sagittal axis), internal/external rotation (transverse/vertical axis), plus circumduction (combining all three).

Degrees of Freedom (DOF):
Each independent axis of rotation counts as one degree of freedom. Thus, uniaxial = 1 DOF, biaxial = 2 DOF, multiaxial = 3 DOF.

Understanding joint DOF guides clinical assessment and rehabilitation exercise selection, ensuring movements respect each joint’s natural constraints and capacities.

5.25 Joints of the Lower Limb

The lower limb is divided into the thigh, leg, and foot, each linked by specific diarthroses that permit characteristic movements.

1. Pelvic Girdle Joints

  • Sacroiliac Joints (paired)
    • Articulating surfaces: Auricular surfaces of the sacrum and ilium.
    • Movements: Very limited “nutation” and “counter-nutation” during trunk flexion/extension and gait—provides stability to transfer weight from spine to lower limbs.
  • Pubic Symphysis
    • Type: Fibrocartilaginous amphiarthrosis.
    • Movements: Slight compression/expansion during walking and childbirth.

2. Hip Joint (Coxal Joint)

  • Articulating surfaces: Femoral head in the acetabulum (deepened by the acetabular labrum).
  • Movements:
    • Flexion/extension (sagittal plane)
    • Abduction/adduction (frontal plane)
    • Internal/external rotation (transverse plane)
    • Circumduction (combination of all three)

3. Knee Joint

  • Components:
    • Tibiofemoral joint: Femoral condyles on the tibial plateau (menisci interposed).
    • Patellofemoral joint: Patella on the femoral trochlea.
  • Movements:
    • Flexion/extension (primary, uniaxial)
    • Screw-home mechanism: slight internal rotation of the tibia on the femur in terminal extension, and external rotation during flexion.

4. Tibiofibular Joints

  • Proximal (Superior) Tibiofibular – Plane synovial joint; gliding movements accompany ankle dorsiflexion/plantarflexion.
  • Distal (Inferior) Tibiofibular – Syndesmosis; slight widening/closing of the mortise during dorsiflexion.

5. Ankle (Talocrural) Joint

  • Articulating surfaces: Trochlea of the talus between the medial and lateral malleoli.
  • Movements:
    • Dorsiflexion (up) and plantarflexion (down) in the sagittal plane around a mediolateral axis.

6. Subtalar and Transverse Tarsal Joints

  • Subtalar (Talocalcaneal) joint: Inversion and eversion of the hindfoot in the frontal plane around an oblique axis.
  • Talonavicular and Calcaneocuboid (Transverse tarsal) joints: Coupled with subtalar motion to adjust midfoot and forefoot orientation.

7. Tarsometatarsal Joints

  • Plane synovial joints between distal tarsals and bases of metatarsals; allow slight gliding to accommodate uneven terrain.

8. Metatarsophalangeal Joints

  • Articulating surfaces: Heads of metatarsals and proximal phalanges.
  • Movements:
    • Flexion/extension (sagittal)
    • Abduction/adduction (frontal)
    • Circumduction in digits II–V

9. Interphalangeal Joints

  • Proximal and distal interphalangeal joints: Uniaxial hinge joints; flexion/extension of toes.

Each of these joints is optimized—by its bony architecture, ligaments, fibrocartilaginous structures (labra, menisci), and muscular support—to balance the demands of mobility, weight bearing, and stability necessary for upright posture, locomotion, and fine control of the foot.

5.26 Joints of the Lower Limb

The lower limb comprises the thigh, leg, and foot, each linked by key diarthroses that enable locomotion and support body weight.

1. Hip Joint (Coxal Joint)

  • Articulating Surfaces:
    • Acetabulum (pelvic side): Deep, cup-shaped cavity, deepened by the acetabular labrum.
    • Femoral head (thigh side): Spherical.
  • Movements (3 DOF):
    1. Flexion / Extension (sagittal plane)
    2. Abduction / Adduction (frontal plane)
    3. Internal (medial) / External (lateral) Rotation (transverse plane)
    4. Circumduction (sequential combination of the above about all three axes)

2. Knee Joint (Tibiofemoral & Patellofemoral)

  • Articulating Bones: Femur, tibia, and patella.
  • Articular Surfaces:
    • Tibiofemoral: Medial and lateral femoral condyles on corresponding tibial condyles, separated by medial and lateral menisci (fibrocartilage).
    • Patellofemoral: Posterior patellar surface on the femoral trochlea.
  • Movements (primarily 1 DOF plus screw-home):
    • Flexion / Extension in the sagittal plane about a mediolateral axis.
    • Accessory Rotation: Small internal/external tibial rotation when the knee is flexed (screw-home mechanism during the last degrees of extension).

3. Ankle Joint (Talocrural Joint)

  • Articulating Surfaces:
    • Tibia and fibula form a mortise that grips the trochlea of the talus.
  • Movements (1 DOF):
    • Plantarflexion (pointing the foot down)
    • Dorsiflexion (lifting the foot up)
    • Both occur in the sagittal plane around a mediolateral axis.

4. Subtalar & Transverse Tarsal Joints

  • Subtalar (Talocalcaneal): Inversion / eversion of the hindfoot in the frontal plane about an oblique axis.
  • Talonavicular & Calcaneocuboid (Transverse tarsal): Work with the subtalar joint to allow complex adjustments of the midfoot and forefoot (e.g., pronation/supination of the foot).

5. Intertarsal & Tarsometatarsal Joints

  • Intertarsal: Plane joints between adjacent tarsal bones—allow small gliding motions.
  • Tarsometatarsal (Lisfranc): Plane joints between distal tarsals and metatarsal bases—permit slight gliding to conform to varied terrain.

6. Metatarsophalangeal Joints

  • Articulating Surfaces: Heads of metatarsals with bases of proximal phalanges.
  • Movements (2 DOF):
    • Flexion / Extension (sagittal plane)
    • Abduction / Adduction (frontal plane)
    • Circumduction in digits II–V

7. Interphalangeal Joints (Toes)

  • Articulations: Proximal interphalangeal (PIP) and distal interphalangeal (DIP) joints.
  • Movements (1 DOF): Flexion / Extension in the sagittal plane.

Together, these articulations provide the mobility, stability, and shock-absorption necessary for standing, walking, running, and fine control of the foot during balance and propulsion.

5.27 Articulations of the Vertebral Column

The vertebral column comprises multiple small synovial and cartilaginous joints between adjacent vertebrae and interposed intervertebral discs. Its divisions and their mobility are:

A. Intervertebral Joints (Cartilaginous, Symphyses)

  • Location: Between vertebral bodies
  • Structure:
    • Intervertebral disc: Annulus fibrosus (fibrocartilage ring) + nucleus pulposus (gelatinous core)
  • Movement: Small degree of flexion, extension, lateral flexion, and rotation; summed across all levels provides the column’s overall mobility.

B. Zygapophyseal (Facet) Joints (Synovial, Plane)

  • Location: Between superior and inferior articular processes of adjacent vertebrae
  • Structure: Hyaline cartilage–lined facets within a fibrous capsule and synovial cavity
  • Movement:
    • Cervical region: Greatest multiaxial freedom—flexion/extension, lateral flexion, rotation
    • Thoracic region: Limited by ribs—small rotation and lateral flexion
    • Lumbar region: Predominantly flexion/extension, limited rotation
    • Sacral/coccygeal segments: Fused—no motion

C. Regional Mobility Summary

Region Vertebrae Disc + Facet Motion Primary Movements
Cervical C1–C7 Greatest total range via 7 discs + 14 facet joints Flexion/Extension, Lateral Flexion, Rotation
Thoracic T1–T12 Disc contributions moderate; facets restrict motion Rotation > Lateral Flexion > Flexion/Extension
Lumbar L1–L5 Facets oriented sagittally allow flex/ext Flexion/Extension mainly; minimal rotation
Sacral S1–S5 Fused None
Coccygeal Co1–Co4/5 Fused None

D. Functional Implications

  • Combined segmental motions allow complex spinal movements:
    • Sagittal plane: Flexion, extension, hyperextension
    • Frontal plane: Right and left lateral flexion
    • Transverse plane: Right and left rotation
  • Stability vs. mobility trade-offs:
    • High mobility in cervical region for head movement
    • Stability in thoracic region to protect thoracic organs and provide rib cage support
    • Lumbar region balances load-bearing stability with necessary flexion/extension for lifting and posture

Understanding each vertebral articulation’s structure and motion is essential for diagnosing spinal pathologies (e.g., herniated discs, facet syndrome) and prescribing targeted therapeutic interventions.

5.28 Joints of the Thorax

The thoracic cage consists of 12 pairs of ribs, 12 thoracic vertebrae, and the sternum. These bones are linked by both cartilaginous and synovial joints that allow the rib cage to expand and contract during breathing, while otherwise maintaining a rigid protective vault.

1. Costosternal Joints

  • True ribs (1–7):
    • Articulate anteriorly with the sternum via their own costal cartilages.
    • Joint type: Secondary cartilaginous (synchondrosis).
    • Movement: Virtually none at rest; during deep inspiration the costal cartilages bend to allow upward/outward rib motion.

2. Costochondral Junctions

  • Ribs to costal cartilage:
    • Joint type: Primary cartilaginous—not synovial.
    • Movement: Minimal, permits slight elasticity during respiration.

3. Sternocostal Joints 2–7

  • Between costal cartilages and sternum body:
    • Joint type: Plane synovial.
    • Movement: Small gliding to accommodate pump-handle motion of the upper ribs.

4. Costovertebral & Costotransverse Joints

  • Costovertebral: Head of each rib with the bodies of two adjacent thoracic vertebrae and the intervening disc.
  • Costotransverse: Tubercle of each rib with the transverse process of its same-numbered vertebra.
  • Joint type: Plane synovial.
  • Movement: Allows the ribs to rotate and glide, producing:
    • Pump-handle action (ribs 2–6): ↑ anterior–posterior diameter of the thorax
    • Bucket-handle action (ribs 7–10): ↑ transverse (lateral) diameter

5.29 Thoracic Movements

  1. Spinal Coupling: Rib cage follows the thoracic spine’s flexion, extension, and lateral flexion.
  2. Ventilation Mechanics: Contraction of respiratory muscles (diaphragm, intercostals, accessory muscles) elevates/depresses ribs:
    • Inspiration: Rib heads glide upward/backward, costal cartilages flex—thoracic volume ↑
    • Expiration: Elastic recoil lowers ribs—thoracic volume ↓

5.30 Pelvic Joints

The pelvic ring comprises two hip bones (innominate) joined anteriorly at the pubic symphysis and posteriorly to the sacrum at the sacroiliac joints.

1. Pubic Symphysis

  • Type: Secondary cartilaginous (symphysis).
  • Structure: Fibrocartilaginous disc between the pubic bodies.
  • Movement: Very slight during walking; widens minimally during childbirth under hormonal influence.

2. Sacroiliac Joints

  • Articulating surfaces: Auricular surfaces of the ilium and sacrum.
  • Type: Irregular plane synovial anteriorly; fibrous syndesmosis posteriorly.
  • Movement: Nutation and counternutation (small tilting motions) that accommodate trunk flexion/extension and transfer weight from the spine to the lower limbs.

End of Chapter 5’s coverage of trunk, thorax, and pelvic articulations.

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