Bone Form and Function

Forces Constrained by Newton’s Laws of Motion 1.“Law of inertia” – Body in motion (or at rest) tends to stay that way. 2.“F = Ma” – a force gives a body acceleration in the direction of the force (Bemis et al. 2004). That acceleration increases with the force and decreases with the Mass of the object. 3.“Equal and opposite” – an object receiving a force from another object, in turn, delivers an equal and opposite force.

Center of Mass CoM = “center of gravity” – point about which a solid body is evenly balanced. To find the Center of Mass:

Vectors and components of force Scalar quantities – values with magnitude and no direction –Examples: length, mass, temperature Vector quantities* - magnitude and direction –Examples: moving objects.

Leg supporting the body Fb = downward and backward force Ground reaction force pushing back Fg = upward and forward force Fv = Fg sine(theta) Fh = Fg cosine(theta) Properties of the arrow drawings: direction and magnitude

Corbin and Reilly, 1998 Reilly, 2000

Amount of time foot on the ground decreases with increasing speed

Bone juxtapositions constantly changing Hence, bones (and other materials) have to be able to respond to changing forces

Stress and Strain Gravity acting on center of mass –Strong downward force: Feet Supporting skeletal elements Joint surfaces Contraction of muscles –Strong forces on attachment sites Biting or chewing –Muscular attachment sites Bones (jaws and skull) –Teeth (surrounding flesh and bone)

Stress and Strain Stress – measured as pressure over cross-sectional area Strain – deformation in material caused by stress

Compare elastic rubber band and rigid bone Stress-strain curve:

Plastic region Stress (force/c.s. area) Strain (deformation) Elastic region Yield point fracture

Connective tissues (revisited) Extracellular matrix / producing cells = High ECM –HOH, Proteins, Carbs. –Ex. “loose fibrous CT”, dense CT, Tendon, Ligament, Bone, etc. Morphology of CT: –Collagen/Elastin ratio –Arrangement of fibers dependent upon forces acting on the material.

A – major components of Loose, fibrous CT B – Molecular structure of the material

Arrangement of collagen fibers in dense CT A – dense irregular CT (dermis of skin) B – layers of a ligament C – cable-like arrangement of a tendon