Week 10 Flashcards
what is inverse dynamics
when we solve backwards using kinematics (acceleration) and external forces to determine internal body forces
why can’t we measure internal forces directly
super invasive ( need to have direct access to the muscle or tendon) so instead we predict
what are the two main inverse dynamics equations
Force = ma
Moment = Ialpha
what is the step by step process of solving for inverse dynamics
- determine what you want to solve
- draw a free body diagram
- write equations of motion
-determine all of the ‘known’ parameters (measurements, external forces, kinematics and anthropometrics) - solve for the ‘unknown’ parameters
how to draw a free body diagram for the rigid link model
- define coordinate system
- isolate at point of interest
- draw all known/unknown forces and moments
how does your FBD change for multiple linked segments
the forces in the y and x of the previously found distal joint gets changed to the opposite direction (same for acceleration)
what is the kinematics part of the equation implying
track the displacement of the positions over time
what information do we get from the rigid linked segment models
net joint reaction forces
net joint moments of forces
what do the two points of information really tell us in the rigid linked segment models
net joint F = have very little physiological information or meaning and is more of a stepping stone
net joint M of F = direction and magnitude of muscle effort (which group of muscles active and how hard they are working)
why/how do the reaction forces and moments ignore anatomy
- act in a global coordinate system
- doesn’t care about anatomical relevance or orientation
- ignores anatomical difference between all of the joints
what do the reaction forces represent
the stuff happening externally (GRF, segment mass, loads in hands)
what does the net joint moments represent
accounts for things going on externally however they represent the action of the muscles that generate these moments
why are muscle forces often greater than the external force
- due to muscle attachments being closer to the joint they have smaller moment arms so they must produce more force to get the same moment the weight or object is producing at the segment.
- moment arms of external objects, forces, and masses are much longer than the moment arms of the muscle
what is an example of how muscles overcome having shorter moment arms
the patellar ligament uses the patella to increase the moment arm of the quadriceps in order to decrease the work of the muscle group
how do we represent muscle forces (3 ways)
single muscles equivalent
optimization
EMG driven
what does the challenge of redundancy mean
there are more muscles than needed to perform a specific movement which is a good thing but mathematically is very difficult as then there are too many unknown muscle forces to solve for
what is the single muscle equivalent
represents all muscles as a single line of action (from origin to insertion) and is a weighted average of all the muscles
what are the common objective functions for optimization
- minimize joint compression
- minimize the sum of muscle forces
- minimize the sum of muscle stresses
- minimize the sum of cubed muscle stresses
what are the common constraints of optimization
- muscle forces must be greater than 0
- sum of muscle moments = net joint moment
- maximum force for each muscle is dependent on its size
what is the equation for Fshear
F rxn shear + F muscle shear (always 0)
