Describe the principle of antagonism.
two or more muscles acting in opposite directions
task: stop and control movements
What is necessary to enable movement?
all muscles have to have a lever arm with respect to the joint they are acting on
What is the force relation between agonist and antagonist?
example upper arm: Ft = 0.4*Fb
Why are joint forces higher than the original mass?
- lever arms
- principle of antagonism
Difference between normal and shear stresses?
normal: perpendicular direction of the plane
shear: acting in x-dir. on y-plane
Bone growth characteristic?
in direction of the normal stresses
-> prevention of shear stress
What is a stress state?
consists of normal and shear stresses. it depends on the outer forces and the cutting plane (like rotation)
How does the bone try to prevent shear/tensile stresses?
by growing according to the direction of the principle stresses
Describe the principle stress state.
only normal stresses are acting, shear is zero.
stress state and direction is different in each point
What do the material laws describe?
how materials deform und forces:
- relation between force and displacement (stress+strain)
- relation between kinetic and kinematic behavior
Origin of plastic bone behavior?
results from breakdown (fracture) of microscopical structures. bone plasticity may heal, may lead to permanent deformations
What behavoir do ligaments, cartilage, ligaments and natural materials have?
liga: hypoelastic
tendons: hypoelastic
carti: visco-elastic
natural: displacement-dependant
the quicker a tendon/ligament is pulled, the stiffer it gets.
-> velocity dependant material behavior
Which material properties describe elastic behavior?
- youngs modulus
- poissons ratio
- shear modulus
What joint principles exist?
- fibrous (synarthrodial)
- cartilargiuous (amphiarthrodial)
- synovial (diarthrodial)
biomechanical tasks of joints?
- damping (spongious bone & cartilage)
- movement (incongruency, capsule, ligaments)
- peak stress/force reduction (large proximal, thin cortical bone)
Describe the features of joints.
- incongruency (surfaces do not fit together)
- > allows for other movements
- larger proximal bone parts compared to distal
- spongious in proximal part, no in distal
- cartilage in the bearing surface
- ligaments for the stabilization and movement restriction
- capsule
Name the types of joints.
- ball and socket (3dof): hip, shoulder
- egg (2dof): wrist
- hinge (1dof): elbow
- pivot (1dof): radio-ulnar
- saddle (2dof): thumb
- rotational (5dof): knee
Describe the movement patterns.
- swing: flex/exten, adduc/abduc
- spin: rotation
- diagonal: circumflexion
Why are muscle movements mostly diagonal?
- several muscles involved (agonist, antagonist)
- > increased velocity, force, stability, control