basic anatomy

Two reasons why we care about the musculoskeletal system

Provide guidelines for design; minimize cost of injuries

Two functions of connective tissues

Force transmission and movement; metabolism (blood cell manufacture)

Two fiber types associated with connective tissues

Collagen and elastin

Collagen – effect on mechanical properties

Prevent connective tissues from tearing and stretching too easily when subjected to high loads

Elastin – effect on mechanical properties

Once the stretching force is removed, they allow the tissue to snap back to its original shape and resting length

Tendon

Connects muscle to bone; allows body to move joints and maintain posture

Ligament

Connects bone to bone; holds articulating bones in proper alignment and limits abnormal ranges of motion (preventing dislocations)

Stress-strain curve: Y-axis

Stress (force/unit area that results from an applied load-tension), within the tissue

Stress-strain curve: X-axis

Strain (physical deformation response of a material)

Elastic region: Linear & Elastic Phase

Stress is directly proportional to strain; if the applied load is removed anywhere in this region, the tissue completely returns to its original size and shape

Elastic region: Limits of Proportionality

The point where the curve bends slightly; the tissue remains elastic, but the linear relationship between stress and strain ends

Elastic region: Yield Strength

The critical threshold marking the boundary between temporary elastic behavior and permanent structural change

Plastic region: Permanent Deformation

Tissue is permanently altered by stress and cannot fully recover

Stress-strain curve: Point B

Unloading

Stress-strain curve: Point A

The maximum stress level the tissue can withstand before structural breakdown accelerates

Stress-strain curve: Fracture Point

The final point on the curve where the tissue undergoes structural failure, tearing or rupturing completely

Wolff's Law

Bone is deposited where needed and reabsorbed where not needed, based on the mechanical stresses on it

How age affects bone density

Gradual loss of minerals and structural growth

One function of joints

Hold bones together, giving stability, but also gives skeleton mobility

Three types of joints

Synovial, cartilaginous, fibrous

Synovial joint – example location

Elbow

Cartilaginous joint – example location

intervertabrae disc

Fibrous joint – example location

Skull

Three types of cartilage

Hyaline cartilage, fibrocartilage, elastic cartilage

Hyaline cartilage – location

On bones, articular surfaces

Fibrocartilage – location

On disc, intervertebral discs

Elastic cartilage – location

Ear and nose

Why it is difficult to recover from cartilage injuries

Because cartilage lacks a direct blood supply

Two variables that affect spine compressive strength

Age and gender

Three factors that increase the risk of joint injury

Aging, inflammation, previous injuries

Two functions of muscles

Skeletal motion and skeletal stability

Three types of muscles

Skeletal, cardiac, smooth

Skeletal muscle – example

Bicep

Cardiac muscle – example

Walls of heart

Smooth muscle – example

Stomach

How muscles are organized

Organized in agonist-antagonist pairs

Agonist

Muscle causing movement

Antagonist

Muscle opposing agonist

How muscles contract

From the binding/unbinding of two protein molecules: actin and myosin

Two important mechanical properties of muscles

Length-tension relationship and velocity-tension relationship

Length-tension relationship

Describes hohow much force a muscle can produce depends on its length before it contracts

Sliding Filament Theory

The foundational mechanism explaining how muscles contract at a cellular level

When peak muscle force occurs

Peak muscle force (strength) depends on muscle length (posture) and occurs at the optimal resting length

Velocity-tension relationship

how much force a muscle can produce depends on how fast it’s moving. The faster it shortens, the less force it can produce

Two muscle fiber types

Slow (type I) twitch fibers and fast (type II) twitch fibers

Slow (type I) twitch fibers

Fatigue resistant, low force

Fast (type II) twitch fibers

Quick to fatigue, high force

Rohmert curve

Relationship of static endurance time vs. percent of max muscle force; more effort a static hold takes, the faster endurance drops

Rohmert curve – implication on design

Minimize static postures and holds; use mechanical aids and fixtures; keep exertions below the 15% threshold

Two effects aging has on muscles

Loss of muscle mass; general decrease in muscle capacity/strength

Name 1-3 and 7

articular cartilage, joint capsule, synovial fluid, synovial membrane