Anatomy & Physiology: Joints, Limb Attachments, and the Appendicular Skeleton

Stability-Mobility Trade-off in Joints

Overview

Joint pain relief often requires reduced joint or muscle usage, as excessive movement can exacerbate discomfort. The relationship between joint stability and mobility is inversely proportional, meaning that as one increases, the other decreases.

• Sutures: Maximum stability, zero mobility

• Intervertebral joints: High stability, limited mobility

• Synovial joints: Variable stability/mobility balance

• Shoulder joint: Maximum mobility, minimum stability

"If you're going to be mobile, you can't be stable at the same time."

Three Factors for Synovial Joint Health

Factor	Description	Can Modify?	Impact
Muscle tone	Muscles crossing joints provide stability through tension	Yes	Primary stabilizer - toned muscles hold joints together
Ligament count	More ligaments = more stability	No	Hypermobile individuals have fewer ligaments
Articular surface shape	Bone shape determines stability/mobility balance	No	Hip vs. shoulder joints demonstrate this

Joint Motion Types & Classifications

Uniaxial Joints

• Pivot joints (e.g., C1-C2 vertebrae): Allow rotation around a single axis.

• Hinge joints (e.g., elbow, ankle): Permit flexion and extension.

  • Flexion: Bones come closer together

  • Extension: Bones move apart

  • Note: Knee orientation is opposite to elbow

Special Rotational Movements

Movement	Description	Memory Aid
Supination	Palm faces anterior/upward	"Hold soup"
Pronation	Palm faces posterior/downward	"Show ring/nails"

Critical: These motions occur at the elbow joint, not the wrist.

Biaxial Joints

• Condylar & saddle joints: Permit two planes of motion (flexion/extension + abduction/adduction).

• Abduction: Move away from midline (e.g., spreading fingers)

• Adduction: Move toward midline

Multiaxial Joints

• Ball-and-socket joints (shoulder, hip): Allow movement in multiple directions.

• All previous motions plus:

  • Protraction: Push forward

  • Retraction: Pull backward

  • Elevation: Lift up

  • Depression: Move down

Special Foot Movements

Dorsiflexion and Plantarflexion

Movement	Description	Example
Dorsiflexion	Point toes upward	Taking foot off gas pedal
Plantarflexion	Point toes downward	Putting foot on gas pedal, calf raises

The foot's orientation comes from embryonic development; the foot twists so the back becomes the front during growth.

Inversion and Eversion

Movement	Description	Soccer Application
Inversion	Pull sole inward	Controlling ball with inside of foot
Eversion	Pull sole outward	Controlling ball with outside of foot

Appendicular Skeleton Overview

Homologous Structures

Upper and lower limbs develop equivalently during embryonic development.

Upper Limb	Lower Limb	Notes
Pectoral girdle (scapula, clavicle)	Pelvic girdle (os coxae)	Os coxae are composite bones fused from 3 bones
Humerus	Femur	Very similar in structure
Radius	Tibia	Both on thumb/big toe side
Ulna	Fibula	Both on pinky side

The leg is "backwards"—your palm is equivalent to the sole of your foot, and your thumb aligns with your big toe.

Limb Attachment to Axial Skeleton

Lower Limb Attachment

• Direct attachment through sacrum (part of vertebral column)

• Os coxae articulates with sacrum, creating solid connection

• This direct attachment allows torso to move with limb propulsion

Upper Limb Attachment

• Indirect attachment—primarily through clavicle

• Scapula and clavicle articulate with each other

• Clavicle is the only true connection to axial skeleton

• Additional support comes from muscles on the back

Study Recommendations

• Attend lab sessions to study bones and structures

• Review week 1 lab coincides with lecture exam timing

• Use lab as the primary tool for both lecture exam and upcoming practical

• Focus on skeletal system, joints, appendicular skeleton, and axial skeleton

Scapular Anatomy & Evolutionary Adaptations

Overview

The scapula demonstrates remarkable evolutionary adaptations, especially for bipedal locomotion.

• No direct skeletal attachment to the axial skeleton

• Muscular attachment only via the clavicle at the sternum

• Dorsal positioning critical for bipedal balance

The scapula's dorsal position counterbalances the vertical weight distribution, preventing forward falls during bipedal locomotion. Quadrupeds have laterally positioned scapulae and do not require the same counterbalancing mechanism.

Os Coxa (Pelvic Girdle) Anatomy

Composite Structure

Bone	Position	Description
Ilium	Superior	Upper portion of acetabulum
Pubis	Anterior/Medial	Forward portion, medial side
Ischium	Posterior	C-shaped posterior portion

• Acetabulum: Socket where three bones fuse together

• Ischial tuberosity: Weight-bearing surface when sitting

• Obturator foramen: Hole for neurovascular structures

Pelvic Articulations

• Sacroiliac joint: Between sacrum and ilium (auricular surfaces)

• Pubic symphysis: Fibrocartilage disc between pubic bones

Note: The pubic symphysis refers specifically to the fibrocartilage, not the bone itself.

Iliac Spine Identification

Spine	Location	Mnemonic
Anterior Superior	Front, Top	ASIS - visible in low-rise jeans
Posterior Superior	Back, Top	-
Anterior Inferior	Front, Bottom	-
Posterior Inferior	Back, Bottom	-

Orientation tip: The greater sciatic notch (candy-cane shape) always indicates the posterior side.

Clinical Considerations

• Bony projections serve critical functions but pose risks:

  • Muscle and ligament attachment sites

  • Pressure points in immobile patients

  • Pressure ulcer formation when blood flow is compromised

• Healthcare providers must monitor these areas in patients with limited mobility to prevent tissue breakdown and pressure injuries.

• Bed sores (pressure ulcers) develop when blood flow is restricted due to prolonged pressure on bony projections.

Pelvic Anatomy: Male vs Female

Key Differences Table

Feature	Male Pelvis	Female Pelvis
Pubic inlet shape	Heart-shaped/circular	Oval-shaped
Coccyx position	Anterior (forward)	Posterior (backward)
Pubic arch	< 90° (pointed)	> 90° (rounded)
Overall width	Narrower	Wider
Acetabulum	Closer together, lateral	Farther apart, medial

The female pelvis is wider to accommodate the uterus and childbirth, requiring sufficient space for a baby's head to pass through the birth canal.

Pelvic Ligaments and Structures

Major Ligaments Table

Ligament	Attachments	Structures Passing Through
Obturator membrane	Covers obturator foramen	Obturator artery, vein, nerve
Inguinal ligament	ASIS to pubic tubercle	Femoral artery, vein, nerve, lymphatics, ductus deferens
Sacrospinous ligament	Sacrum to ischial spine	Creates greater sciatic foramen
Sacrotuberous ligament	Sacrum to ischial tuberosity	Creates lesser sciatic foramen

Sciatic Foramen Contents: The greater sciatic foramen contains the sciatic nerve—the largest, thickest, longest nerve in the body. Some structures pass through the greater sciatic foramen and return through the lesser sciatic foramen, including certain blood vessels and nerves.

Pectoral Girdle Anatomy

Overview

The pectoral girdle consists of the scapula and clavicle, equivalent to the pelvic girdle's os coxae.

• Acromioclavicular joint: Scapula's acromion to clavicle's acromial end

• Sternoclavicular joint: Clavicle's sternal end to sternum's clavicular notch

The sternoclavicular joint is the only direct attachment between the upper limb and axial skeleton, making the upper limb relatively easy to disarticulate compared to other joints.

Pectoral Girdle Joints

1. Sternoclavicular joint: Where the sternal end of the clavicle sits in the clavicular notch of the manubrium (upper portion of sternum)

2. Acromioclavicular joint: Where the acromial end of the clavicle articulates with the acromial end of the scapula

Both joints are syndesmosis—connected by ligaments with slight movement but not synovial joints.

Clavicle Anatomy

• The clavicle is an S-shaped bone with two curves, distinguishing it from ribs which are C-shaped with only one curvature.

• Key identification features:

  • Acromial end: More rounded shape

  • Sternal end: More flat edge

• Avoid using medial/lateral terminology—learn to identify acromial vs sternal ends by examining actual bones.

Scapula Structure

• The scapula is a triangular-shaped bone with three key features:

  • Supraspinous fossa

  • Spine of scapula

  • Acromion process