Week 3 Flashcards
Human motion study begins with the
classifying of the types of movement.
Standing analysis starts with
discussion of overall and local stability and friction.
Kinematics -
branch of mechanics dealing with the motion of bodies, without considering mass or force.
Kinematics involves (2):
- tracking the motion of the center of mass of the body
- tracking angles (θknee,θthigh, θankle) versus time and versus each other during a walking or running cycle
criterion for overall stability during standing -
for the center of mass to be over the area spanned by the feet
When are we most stable?
when the line of gravity is near the center of the support base
stance phase -
from foot strike to toe off
swing phase -
from toe off to foot strike
stance phase in walking:
longer than the swing, 60-65%
stance phase in running:
shorter than swing, 40%
swing phase in walking:
shorter, 35-40% of the cycle
swing phase in running
longer, 60% of the cycle
difference b/w running & walking (2):
- leg becomes almost straight during walking, but it never does during running
- during walking each foot is on the ground for more than half the time, Sometimes both are on the ground at the same time. during running, each foot is on the ground for less than half the time, Sometimes neither foot is on the ground
How high you can jump, depends on
how fast you can take off
kinetic energy at takeoff is all converted
into potential energy at the top of free flight
How are the jump height and takeoff time related?
you can decrease the extension time τ, you can jump higher
Momentum -
vector quantity defined as the product of mass and velocity
SI units of momentum are kg m/s
Elastic collision -
collision of bodies in which the total kinetic energy is the same before and after the collision
in totally elastic or partially elastic collisions, the objects bounce back
Inelastic collision -
collision of bodies in which total kinetic energy is not conserved
Sticky inelastic collision -
inelastic collision where the two objects are stuck together after the collision. Sometimes known as a totally (or perfectly) inelastic collision
what happens to linear momentum and E in a collision?
IN A COLLISION, LINEAR MOMENTUM AND ENERGY ARE CONSERVED. However, if the collision is not elastic, kinetic energy is not conserved and heat and/or object deformation is produced
UPON COMPRESSION BONES TYPICAL BREAK WHEN SUBJECTED TO A FORCE PER AREA ABOVE - ?
What is this damage threshold called?
170 MPa
This damage threshold is called the ultimate compressive stress
height from which jumping is dangerous?
stresses on the tibia exceed UCS (ultimate compressive stress), so jumping from 1 m and landing on one stiff leg will break your leg
how to reduce the effect of the impact on the bones (2)?
- increasing time of collision
and/or - increasing the area of contact A
The collision time can be increased by (3):
- using better padding at the site of the collision
- increasing the duration of the impact by adjusting your body during actual impact
- rolling over
Collisions are survivable depending on:
- what part of the body makes contact
- what type of surface it hits
- the deceleration time
HUMAN COLLISIONS ARE USUALLY NOT SURVIVABLE FOR DECELERATIONS OF
175– 200 G OR GREATER
During a very slow step the forces on the foot
slowly build up from 0 to mbg
During running the forces on the foot
reach a peak of approximately 2.0–3.6 mbg
Fracture due to direct blow (2):
- low E => 2 pieces (transverse)
- high E => many pieces
Fracture due to indirect blow (4):
- traction => transverse fracture perpendicular to F
- bending => transverse w/ butterfly fragment
- torsion => spiral fracture
- bending & torsion => oblique fracture
