Biomechanics Exam

moment

magnitude of force applied to a rotational system at a distance from the axis of rotation

M = (F)(d)

- force (N) x moment arm (in metres)

moment arm

the perpendicular distance from the force to the axis of rotation

moment arm = d x sinθ

ground reaction force (GRF)

the reaction force provided by the horizontal support surface

centre of pressure (COP)

the point on a plane surface at which the resultant pressure on the surface may be taken to act

how do we measure kinetic forces

1. biological signal (ex. stepping on ground)

2. measures analog signal

3. converts to digital signal

force platform

instrument used to measure the reaction forces in 3 dimensions and moments about 3 axes (rotation around x, y, z)

- more reliable (gives accurate measurements)

limitations to force platforms

- mounting

- altered walking patterns

- conscious walking

pressure sensors (insoles)

generate electrical charge in response to stress

- not as reliable or practical

limitations to pressure sensors

- vertical loading (only active with vertical loading - only gives y, z values)

- temperature

- slipping

vertical force analysis - walking

- dotted line

- 2 peaks (double foot support - both feet in contact with the ground)

vertical force analysis - running

- thick solid line

- 1 peak (flight)

4 phases of vertical jump

1. negative acceleration - if Fz force is less than than body weight (preparatory phase)

2. positive acceleration - GRF is greater than body weight (push off phase)

3. flight phase - force is zero (not touching ground) around 400-500ms

4. impact phase - force spikes, greater than initial force

normal force (N)

force exerted between surfaces that are in contact with each other; acts perpendicular to surface

linear kinematics - position

on objects location in space relative to some reference

linear kinematics - motion

motion occurs when an object or body changes position

linear kinematics - displacement

measured in a straight line from one point to the next point

velocity

a vector quantity defined as the rate of change of position (over time)

how to calculate velocity from displacement

V = displacement/time

V = P2 - P1/ t2- t1

first central difference method

- better estimate of velocity

- looks at larger scale/frame

- illustrated on instantaneous velocity graph

acceleration

The rate at which velocity changes

- acceleration is independent of the direction of motion

angular kinematics

all parts of the body move through the same angle but do not undergo the same linear displacement

- occurs about an axis of rotation

relative joint angles

the angle between two longitudinal axes of two segments (ex. elbow - shoulder to elbow, elbow to wrist)

- does not describe the position of segment in space

- all relative angles are calculated using cosnie law

what 3 points of interest are needed for relative joint angles

- proximal (ex. shoulder)

-vertex (ex. elbow)

- distal (ex. wrist)

absolute angles

the angle of inclination of a body segment relative to some fixed reference in the environment

- two points of interest (proximal and distal)

calculating absolute angles

- The angle calculated relative to the right horizontal is called the segment angle

anatomical angles

measured relative angles is compared to angle of joint in the anatomical positions

hip angular motion

- marker locations: torso, hip, knee

- formula = 180 - relative angle of hip

knee angular motion

- marker locations: hip, knee, ankle

- formula = 180 - relative angle of knee

ankle angular motion

- marker locations: knee, ankle, toe

- formula: angle = 90 - relative angle of ankle

purpose of gait

to get from on location to the next

gait - major motor functions (1)

1. maintenance of support of the upper body (do not collapse the lower limb)

2. maintenance of an upright posture and balance of the HAT segment

3. control of the foot trajectory for ground clearance and gentle heel/toe landing (not tripping)

4. generation of mechanical energy to maintain velocity

5. absorption of mechanical energy for shock absorption and stability or to decrease velocity

gait parameters - stride length

distance between successive points of initial contact of the same foot

gait parameters - step length

distance between the point of initial contact of one foot and the point of initial contact of the opposite foot

gait parameters - step width

lateral distance between successive points of opposite feet

gait parameters - foot angle

describes an angle between the line of progression and a line drawn between the midpoints of the calcaneus and the second metatarsal head

cadence

number of steps per minute (increases as we age)

double support time

period of time in gait cycle when both feet are simultaneously in contact with the ground

stance/support time

period of time in gait cycle when foot is in contact with the ground

swing time

period of time in gait cycle when foot is in the air

gait cycle timing

demonstrates gait disorders (ex. ALS or Parkinsons) through the observations of:

- asymmetry

- abnormal cadence

- improper time in double or single stance

challenges during gait

- COM is made to fall forward outside of the feet to maintain progression

- stabilization occurs during double support phase (but feet are not flat during this)

- during single support COM is passing medially so you are naturally unstable

- heel velocity must be reduced at initial contact to avoid skidding

centre of mass (COM)

COM is considered the point about which the body is evenly distributed

COM calculated by using segments

requires knowledge of the masses and location of each of the body's segments

- need to assume each segment is rigid

Free Body Diagram (FBD)

the essential feature of a FBD is the isolation of the system or body of part of the body under consideration and the inclusion of all forces which act on the body

assumptions for showing an FBD

- static

- all forces are acting in the same 2D plane (x or y direction)

- joints are considered hinge joints (even if not)

- muscles under consideration are represented as single muslce equivalent

- unknown forces are acting in positive direction

forces to consider in FBD

- force due to the weight of the body segment (Fwt)

- reaction forces at the joint (Rx and Ry)

- muscle force (Fm)

- ground force reaction (Fgrf)

- applied loads (Fa)