exam 1 biomechanics

Definition of Biomechanics

study of the sturcture and function of biological systems by means of mechanics

father of biomechanics

Giovanni Alphonsi Borelli

Which branch or type of mechanics is most important/relevant in biomechanics?

Classical/newton

Why are cheetahs so fast?

They have a long stride length (speed= stride lengthX stride frequency)

Which gait is spring mass model intended to represent?

Running

What is key parameter of spring-mass model and what does it represent?

stiffness

can the spring mass model work well hypothetically with other gaits

yes, walking and running

What are the three central kinematic variables and how are they related mechanically?

position (P): where the body in the space

Velocity (V): rate position is changing - v=dx/dt

acceleration (a): rate velocity changing - a=dv/dt

Why these three

Position: goal of movement

velocity : the rate at which goal pursued

acceleration : link to kinetics (F=ma)

all vectors

derivatives and/or integrals of time

What are some examples of how the primary mechanical variables from physics (force, work, power, energy) are relevant to biomechanics and kinesiology?

Force (unit: N) : physical interaction tends to change the motion of a body

Work (unit: J): force times displacement, tends to affect energy

Power (unit: W or J/s): P=dw/dt

Energy (unit: joules or calories):

(1) the capacity of a system to do work eg human food energy

(2) different types

chemical, mechanical or thermal

- total energy is constant

- can be transformed

Where is COM located?

Weighted average of each body segment's centre of mass.

Not in a fixed position

Not necessary inside body

Uses of center of mass

Describe whole-body motion (direction)

Examine quality of whole body motion

Speed

Height

Efficiency - wasteful motion and extra movement (move COM inefficiently)

why humans move

Why humans move: translate the center of mass

What factors affect the airborne distance traveled, e.g. in a long-jump?

Release height

Angle (sacrifice angle for faster speed)

Velocity*

**Range is more sensitive to small change in take-off speed

When are drag and lift important forces to consider?

fast speeds!

What is the relationship between impulse and momentum?

impulse : a force that changes the momentum of an object

math definition: J=f x t (average force x time）

Momentum : the amount of motion an object has

p= mv (mass x velocity)

relationship : impulse equals change in momentum

F⊿t =m⊿v

Important during contact and collisions

Source: Newton's 2nd law

What ground-based variables influence vertical jump height?

⊿h= (v2)²/2g

impulse equals momentum

f⊿t=m⊿v

mg(t2-t1)=m(v2-v1)

v2=(t2-t1)*g

h???? How

⊿h= (v2)²/2g

Take-off velocity dictates jump height

How do "rotation" athletes regulate their speed?

torque= r * f=0 when r = 0

Changing distribution of mass

Moving arms and legs in/out from their body

What is the first law of thermodynamics as it relates to biomechanics, e.g. at the human body- or muscle-level?

When energy is used on a body to do work, some is lost as heat energy to the environment

What are the different types of total mechanical energy?

Kinetic energy (KE) : energy of motion KE= ½(mv²+Iw²)

Potential energy (PE) ： energy could become KE due to gravity PE=mgh

Strain energy (SE) : energy could become KE due to elastic deformation(x) SE=½ (kx²)

K is the stiffness of demormed material

X is distance of demormation (stretched or compressed )

How does the total mechanical energy of a closed system change?

Total Mechanical Energy (TME)= KE+PE+SE

For a closed system, TME is conserved, cannot change TME without doing work on the system W=F*d (⊿TME)

What is the fundamental difference in mechanical energetics of the center of mass between walking and running?

Walking: Use gravity and momentum for efficiency

KE goes up, PE goes down

Running: Use elastic energy storage/return for efficiency

Both KE and PE go up/down

What is missing and what's the other type: SE

What simple mechanical models explain global motor behavior in walking and running?

Walking : Inverted pendulum model

Key parameter: leg length

Running: Spring-mass model

Key parameter: spring stiffness

.Humans appear to choose our preferred gait parameters based largely on what quantity?

conservation of energy/reduced caloric expenditure

What does the inverted pendulum model assume about the body and about how gait is powered and controlled?

Inverted pendulum assumptions:

Body is represented by a point mass

Legs are massless

Legs are rigid/inextensible

TME=KE+PE

PE=mgh↓ KE=½ mv² ↑

During mid-stance

-Translate CoM with "passive energy exchange"

-Powered by gravity

-Substantial muscle activity unnecessary

Foot-ground "impact" (heel-strike)

-Source of energy loss; replace with muscle work

How accurate is inerted pendulum model?

Not completely accurate because it doesn't consider strain energy

Assumptions also are inaccurate in terms of their outlook on the human body

What is a free body diagram and what does it tell me about a biomechanical system?

A model to show all forces acting on an object using newtons second law at degrees of freedoms to describe and predict its behavior by deriving an equation of motion

What factors trigger the gait transition between walking and running?

Trigger： speed (stride length and stride frequency)

Metabolic cost, cheaper to run when walking at high speed to save energy

Metabolic cost= metabolic rate / speed

which gait is spring-mass model intended to represent?

running

key parameter of spring-mass model and what does it represent?

k--> stiffness

Why do even the best long-jumpers in the world use take-off angles that are much flatter than the "optimal" angle predicted by physics?

The flatter angles help them maintain take-off speed, which is more important for world-class performance than angle

Where does your center of mass move to if you abduct your right shoulder from the anatomical position to an angle of about 90°?

up and to right

How fast would a 75-kg person have to run into a headwind of 2 m/s in order to "fly" (i.e. generate a lift force equal to their bodyweight)? Assume the density of the air is 1.15 kg/m3, the flow area is 0.75 m2, and the lift coefficient is 1.0.

lift= 1/2 pv^2CliftAflow

= 1/2 (1.15)(2^2)(1)(.75)