BIOL 1103 Week 9: Joints & Biomechanics

Joint

A union point between two bones. In the skeletal system, joints exist between all bones

Articulation (of a joint)

The movement (from none to free) of a joint

Joint classification by function

Degree of allowed movement

1. Synarthotic: not movable

2. Amphiarthotic: slightly movable

3. Diarthotic: freely movable

Synarthrotic joints

Unmovable joints. Ex. tooth socket joint and skull sutures

Amphiarthrotic joints

Slightly movable joints. Ex. intervertebral discs

Diarthrotic joints

Freely movable joints. Ex. shoulder and elbow

Joint classification by structure

1. Fibrous joints

2. Cartilaginous joints

3. Synovial joints

Fibrous joints

Joints held by dense regular connective tissue. No cavity between bones, so therefore little or not movement. 

Three types of fibrous joints

1. Suture

2. Syndesmosis

3. Gomphosis

Suture (fibrous joint)

Interlocking edges of skull plates held together by a thin layer of dense fibrous connective tissue. Synarthotic (immovable) in adults, amphiarthrotic (slightly movable) in infants

Syndesmoses (fibrous joint)

Amphiarthrotic joint where bones are held tightly together by dense fibrous tissue. Ex. interosseous membrane between ulna and radius

Gomphosis (fibrous joint)

Synarthrotic, special type of syndesmosis where teeth in sockets of mandible and maxilla are held together by a ligament

Cartilaginous joints

Joints holding together bone, little or no movement between bones

Types of cartilaginous joints

1. Synchondrosis

2. Symphysis

Synchondrosis (cartilaginous joint)

Synarthrotic joint connecting structures with hyaline cartilage. Ex. epiphyseal plate

Symphysis (cartilaginous joint)

Amphiarthrotic joint connecting structures with fibrocartilage. Ex. intervertebral discs

Synovial joint

Type of joint that is freely movable (diarthrotic), containing a space between bones. Found in most joints in the appendicular skeleton

Parts of a synovial joint

1, Articular capsule 

2. Synovial membrane

2. Joint (synovial cavity)

3. Synovial fluid

4. Articular cartilage

Articular capsule (synovial joint)

Sleeve-like layer of fibrous connective tissue; attached to the periosteum of articulating bones

Synovial membrane (synovial joint)

Lines the inner surface of the joint capsule, but not the articular cartilage; secretes synovial fluid

Joint (synovial) cavity

Space between the two articulating bones

Synovial fluid (synovial joint)

Slippery fluid that lubricates joints and supplies nutrients to articular cartilage

Articular cartilage (synovial joint)

The ends of articulating bones in synovial joints; reduces friction and absorbs shock

Ligaments (synovial joint)

Bands of fibres typically external to joint, that provide strength and hold articulating bones together

Meniscus (synovial joint)

Cartilage pad cushioning between two bones to prevent bones from smashing into each other

Bursa (synovial joint)

Strategically located sac of synovial fluid that reduces friction during joint movement

What determines the movement of synovial joints?

The type of joint and muscles surrounding the bones determine the movements joints can make

Gliding movement between bones

Side to side and back and forth movement between two bones

Typical shape of joint surfaces

Flat or slightly curved

Angular movement

Decrease (flexion) or increase (extension) the angle between articulating bones

Flexion

Decreasing the angle between articualating bone

Extension

Increasing the angle between articulating bones

Rotation

Movement around tis own longitudinal axis. Ex. Atlas rotates on the axis to shake head “no”

Circumduction

Distal end of bone moves in circle, proximal end stays constant. Ex. making circle with finger

Types of synovial joints

1. Plane

2. Hinge

3. Pivot

4. Ellipsoidal, condyloid, condylar

5. Saddle

6. Ball and socket

Plane joint

Synovial joint where relatively flat/slightly curved bones meet. AKA the planar/gliding joint

Hinge joint

Synovial joint that bends and straightens. Ex. elbow and knee

Pivot joint

Synovial joint that twists or rotates, Ex, atlanto-axial joint in the neck

Ellipsoidal/condyloid/condylar joint

Synovial joint allowing movement in two planes. Ex. knee

Saddle joint

Synovial joint where bones are saddle-shaped, allowing movement in two planes, but restricts rotation (not as much as condyloid joints). Ex. joint at base of thumb

Ball and socket joint

Synovial joint where the ball of one bone fits into the socket of another. Allows for movement in many directions. Ex. shoulder and hip

Which structure holds muscles to bones?

Tendons

How do tendons help move joints?

Tendons transfer the force produced by muscles to move the joint

Where are the two ends of a muscle attached?

To different bones

What happens when a muscle contracts?

Muscle contraction causes one bone to move while the other stays more stable

What is the origin of a muscle?

The relatively stationary bone during muscle contraction.

What is the insertion of a muscle?

The more moveable bone during muscle contraction.

Do muscles push or pull?

Muscles only pull, never push

What happens as a muscle shortens during contraction?

The insertion generally moves toward the origin

What is muscular antagonism?

When one muscle (or muscle group) performs an action and another muscle (or group) reverses it

Lever

A rigid rod that moves on a fixed point → Ex. bones

Fulcrum

The fixed point upon which a lever moves → Ex. joints

When a muscle contracts and a bone moves? which two forces act upon the lever?

Effort and load

Which structure in the body typically forms the levers?

Bones typically form the levers

Which structure in the body typically forms the fulcrum?

Joints typically form the fulcrum

Which structure in the body typically supplies the force?

Muscles typically supply the force

Resistance

The weight of the body or part of the body & anything we lift/carry

First class lever

Fulcrum between effort and resistance → Ex. lifting up your head

Second class lever

Power lever. Resistance is between effort & fulcrum → Ex. standing on your toes

Third class lever

Speed lever. Effort is between fulcrum & resistance → Ex. flexing biceps

Mechanical advantage (MA)

Measure of the force amplification achieved by using a lever.

Mechanical advantage (MA) FORMULA

Effort Arm/Load Arm

Load arm

Distance between load and fulcrum

Effort arm

Distance between effort and fulcrum

When MA > 1,

You have to apply less force to the lever than the weight of the load you are trying to move

When MA < 1

You have to apply greater force to the lever than the weight of the load you are trying to move

What determines whether a lever is a power or speed lever?

A muscle’s (effort) position relative to the joint (fulcrum) and resistance (load)

The further away the insertion is from the fulcrum, ______________________

the more powerful the lever

The closer the insertion to the fulcrum, _________________________

the less powerful the lever

If a muscle’s insertion is closer to the fulcrum, what changes?

Range of movement is greater, and the lever’s speed of movement increases

Force formula

(Load)(Load arm) = (Effort)(Effort arm)

Force (effort) (F) formula

F = Load x (Load Arm/Effort Arm)