Torques and Levers

internal force

- produced from structures within the body

active force= muscle contraction

passive force= joint capsule or ligament limiting movement

external forces

produced by something outside the body

ex: gravity

stress strain curve measures

the ability of connective tissue to tolerate a load

what does the stress-strain graph depict?

Changes in a connective tissues length as strain is applied (ligament)

Y axis (stress) is the internal resistance from the ligament as it resists deformation (strain, lengthening)

X axis (strain) is the change is ligament length relative to its resting length

toe/non-linear region

where the ligament is designed to function optimally; when the ligament is becoming taught before excessive tension is applied to the ligament (use TB as an example here)

part of elastic region

linear region

load beyond what the ligament is designed to resist, deformation is occurring, ligament is still intact; Linear relationship between stress and strain

part of elastic region

stress-strain curve: plastic region

increased strain does not = increased stress to same degree or rate ; tissue has been overstretched and is beginning to fail - grade 1 and 2 injury occurs here before a complete failure of tissue at the ultimate failure point

Yield point = microscopic failure is occurring

Ultimate failure point = tissue loses its ability to hold shape - partial or complete tearing occurs

creep

describe this progressive slow strain of a tissue exposed to a constant load over time.

gravity

acts on all points of a body

Point of application of gravity = center of gravity (COG)

Hypothetical point at which all the mass appears to be concentrated

Line of gravity (LOG) acts from the COG

vectors

arrow that depicts a forces magnitude and direction

ex: magnitude, spatial orientation, direction, point of application

magnitude

length of arrow

spatial orientation

position of the shaft of the arrows

direction

indicated by the arrowhead

point of application

where the force acts on the body (or segment)

angle of insertion

- angle formed between a tendon of a muscle and the long axis of the bone into which it inserts

- this changes as the joint moves through ROM, or muscle shortens or lengthens

- when it increases or decreases on a muscle, it changes the angle which then impacts how much force the muscle can produce

joint reaction force

- force generated within a joint in response to forces acting on a point

rotation

- when forces are applied some distance perpendicular to the axis of rotation

what is moment arm (lever arm)

- perpendicular distance between the axis of rotation of the joint and the force

force x moment arm = torque

- torque (motion around a joint in a plane perpendicular to the axis of rotation)

internal torque

- internal force (muscles) x internal moment arm

external torque

- external force (gravity) x external moment arm

isometric muscle contraction

- static rotary equilibrium during isometric contractions

--> internal= external torque= no change in muscle length of joint angle

concentric muscle contraction

--> internal > external torque --> muscle shortens; muscle accelerates body or limb

eccentric muscle contraction

--> external > internal torque --> muscle lengthens; muscle decelerates body/limb

what are the 4 components of a lever

- a rigid beam (moving bone)

- a pivot or fulcrum (a joint)

- effort force (muscle work)

- resistance (the mass of the body part)

what is a 1st class lever

- fulcrum is in the middle

- mechanical advantage: = 1, >1 or <1

- mechanical advantage depends on how far the load and effort is from the fulcrum --> not most ideal for human movement

(ex. seesaw) (skull)

what is a 2nd class lever

- least common

- load is in the middle

- mechanical advantage: greater than 1

- (ex. wheelbarrow) (foot/calf)

what is a 3rd class lever

- most common

- effort is in the middle (the application of effort force is the insertion point)

- mechanical advantage: less than 1 (our body will overpower the load)

- (ex. fishing rod) (elbow curl)

what is a positive (uncompensated) trendelenburg sign

- if weak, load will overpower effort, hip will fall into adduction

- internal force < external force (pelvis will not be level, origin and insertion will be farther apart)

what is a (compensated) trendelenburg sign

- weak hip abductors can also cause a shift in the center a mass

- upper trunk leans toward weight- bearing side

example of tredelenburg sign (type 1 lever) (sls right side)

- rigid beam (line from hip bone to other hip bone)

- fulcrum (femoracetabular jt)

- effort (abductors)

- load (body weight/gravity)

example of type 1 lever

OA joint

seesaw

fulcrum in center

example of type 2 lever

calf raise

wheelbarrow

load in center

example of type 3 lever

bicep curl

fishing rod

effort in center

the nonlinear/toe region is where the ligament is designed to work

best

where will internal force be?

at point of muscle insertion

where does joint reaction force point?

into the joint creating compression

external force will change depending on

point of application

1st class lever image

2nd class lever image

3rd class lever image