Lecture 5 Flashcards
How is the sum of forces determined? (when in motion)
Force = GRF + mass x (g + a)
How is the sum of forces determined? (when stationary/standing still)
GRF + mass x g (accel. due to gravity) = 0
GRF = - mass x g (accel. due to gravity)
(the same)
How can acceleration of centre of mass be determined (during walking)?
GRF + mass x g (accel. due to gravity) = mass x acceleration of CoM
GRF = mass x (accel. CoM - accel. gravity)
GRF / mass = accel. CoM - accel. gravity
How does the GRF appear during normal gait? (sagittal view)
At heel strike: GRF acts as a ‘brake’ to slow you down (acts on the posterior side). Maximum at transition to single support
As you approach toe off, it acts as an accelerator (on the anterior side) with the maximum reading being at toe off
When GRF is negative in posterior-anterior direction, a deceleration is occuring
Which direction does the GRF act in the stance phase (single leg)(frontal plane)?
Medially (would point in the direction of the CoM)
What is interesting to note about the GRF % of bodyweight in the vertical direction?
The summation of the vertical force during the single support phase is more than 100% BW. This is because every support phase cause the body to “fall” but this acceleration peak (more than 100% BW) helps to compensate
What is inverse dynamics?
Process by which forces and moments are indirectly determined from the known external/resultant forces
What is inertia?
Inertia is the resistance to movement
- Resistance to tarnslation => mass => scalar (e.g. 100kg)
- Resistance to rotation => moment of inertia
Why is the moment of inertia a matrix?
Because the resistance to movement depends on the axis you are trying to rotate around
What are the limitation of inverse dynamics? (4 answers)
- Net moments only
- Dependent on the accuracy of the kinematic data
- Depends on the accuracy of GRF data
- Depends on estimation of anthropometrics
