Kin 550 GRF chapter and Article:
Force plates-natural frequencies excited by a runner’s footfall.
GRF - is not just a function of lower extremity kinematics- it results from
the total acceleration of the body- With the point of impact at the foot.
Forces- have magnitude, direction and point of application.
GRF’s can be in three perpendicular (orthogonal directions)
These three represent the components of 1 resultant force exerted on the
GRF represents the accleration of the total body
GRF- Force = mass * acceleration
GRF is the sum of the total body accelerations of the body segments.
Contributions of body segments represent individual masses *
(see scale- use arms)
The way in which a force platform is calibrated useful, but not relevant for
this class. sampling rate is usually between 500 and 1000 Hz.
Most GRF characteristics are running speed dependent.
Want to view the runners strides to make sure the stride doesn’t change
distance or cadence in order to hit the force platform.
force insoles- that’s what we have here- there is a calibration issue.
GRF’s from X,Y coordinate data.
displacement--> velocity--> accel --> force get error at each point in the
Center of Pressure- ---
Pressure - force / area
this means the center of all the force divided by the area-
There are many creative ways to measure center of pressure. Actually the
force insoles do a relatively good job of that.
Center of Pressure can be used to determine initial contact ( lateral versus
medial) for different shoe stiffnesses. more lateral for harder midsoles
than softer midsoles.
The larger the differences between points the greater the accelerations ( this
is true for all points or graphs that are graphed over a fixed time scale)
See figure 8.2 for different Center of Pressure pathways.
stance time and running speed are negatively related.
In general, the faster the running speed, the shorter the stance time.
Vertical Ground Reaction Component
Vertical component is the largest one by far.
For stiffer running shoes - the stiffer the shoe the faster the loading rate.
One would think the harder the running surface, the faster the loading rate
Loading rate is positively related to running speed.
Shock waves through the musculoskeletal system to cause injuries.
There may be a positive relationship between the magnitude of the impact
peak , overuse injuries, and lower back pain.
rear foot landings may result in a faster impact peak.
how to reduce this peak.
softer shoes ( of course more pronation is another possible problem)
softer running surfaces - grass rather than cement
running uphill- lower impact peaks rather than level or downhill
Decay rate- pushoff phase.
Average vertical GRF-
They use this as an average measure of GRF over a stride. Actually it’s not
used very often.
It does increase linearly for faster running speeds.
Braking propulsion GRF component- Posterior / Anterior forces
In figure 8.4 it is the lower curve. It roughly corresponds with the peak
portion of the vertical GRF curve.
Maximal Braking and propulsion curves
If a person is braking- the first component would be larger, if the second
component is larger the person might be accelerating.
It switches from braking to propulsion approximately halfway throught the
stride as might be expected for smooth running.
Medial- lateral GRF component
reaction is generally lateral and later reaction is medial
This is the most variable component and the least used component. It is
highly dependent on running mechanics and probably foot shape as well.
Shear forces ( twisting moment around the vertical (or longitudinal axis))
This is also not often measured
They exist. But most analyses assume that they do not.