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

force platform.


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

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.

Loading rate-

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

as well.

Loading rate is positively related to running speed.

Impact Peak-

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.

Free Moment-

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.