## The origin of ground reaction forces

Considerable confusion sometimes exists about the origin of the forces measured by a force platform. The best way to understand these is to think of the force platform as a whole-body accelerometer. Since Force = mass x acceleration (Newton's 2nd Law of Motion), any acceleration of the body will be reflected in a reaction when at least one foot is on the ground. An upwards acceleration (as occurs at push-off) will be reflected in an increase in the vertical load (weight) recorded, while a downwards acceleration (as occurs during mid-stance) will reduce the effective body weight. Note that a downwards deceleration (such as occurs at initial contact) is equivalent to an upwards acceleration, and vice versa.

It is clear from the above that the ground reaction forces reflect accelerations of the body's centre of mass. They are not, as many wrongly people believe, influenced by changes in footwear. This is illustrated in the following set of curves taken from a subject walking three times barefoot and shod, then walking like Groucho Marx (minimal vertical accelerations) and as if on the moon (large vertical accelerations).

It can be seen that there are no significant differences in the curves according to footwear, but considerable differences when trunk accelerations are modified.

### Feedback

Hello Kirtley,

I came across your theory of the "Origin of ground reaction forces" while
searching for somewhat similar data.
I am an engineer at Blue Giant.  We manufacture material handling equipment
used in factories and warehouses.  One of our products is a mechanical dock
leveler that bridges the gap between a tractor-trailer and the factory
floor.  The ramp is mechanically raised by the use of springs under the
dock.  A lip then extends to bridge the gap.  Personnel must walk up the
ramp in order to bring the lip down to the truck bed.  When doing this they
must overcome the strength of extension springs under the dock.

Sometimes a lighter person has trouble forcing the deck downwards because
they are not heavy enough.  They must walk further along the dock and
sometimes bounce downward to get it moving downward.  They should never be
jumping, but what they do is bend their knees quickly, producing a slight
acceleration of their upper body, and hence more force on the dock.  I am
having trouble determining the force somebody can produce by performing this
motion.  We have a scale that I have bounced on but the response time is not
article, it seems as though you might have or be able to direct me to, info
on producing a force greater than your body mass by quickly bending at the
knees.

You can call by phone or e-mail me.
Andy Erjavec

Blue Giant Limited
1-800-668-7078
ext 291

Dr. Kirtley

I have been trying to interpret the GRF tracings during the stance phase of gait and have found
what appears to be conflicting information from the CGA website.

I will do my best to present this information as clearly and completely as possible.

In your "The origins of ground reaction forces" /faq/grfs.html, it states the following:
An upwards acceleration (as occurs at push-off) will be reflected in an
increase in the vertical load (weight) recorded, while a downwards acceleration (as occurs
during mid-stance) will reduce the effective body weight.
I am not clear regarding the downward acceleration occurring at midstance, that would evidently
be the source of the reduced bodyweight and the midstance valley.

In an additional CGA posting, from Kinematic Definitions, it states and there is an
illustration that represents the fact that the COM actually moves upwards during midstance and
is higher in midstance than anywhere else during the normal gait cycle.  (
/teach-in/kinematics.html)

I seem to have difficulty trying to reason this out, (it probably should not be that
difficult), yet I continue to struggle.  Any information that would help me clear this up and
assist in my understanding of these GRF's would be greatly appreciated.  In addition, I have
looked for textbooks that explain the use and interpretation of force plates but have not been
satisfied with what I have found.  A few that I have looked through are Perry, Whittle,
Durward, and several general Biomechanics textbooks.  Are there any texts that you would
recommend?

Dear Kurt,

No discrepancy - you're just getting mixed up between position and
acceleration. Displacement, velocity and accelerarion are always 90
degrees out of phase. Velocity is the slope of the displacement, and
acceleration the slope of the velocity. Think of it in stages (see
attached fig.):

1. When displacement is rising

•  Slope is positive, so velocity is positive
•  Slope of velocity (acceleration) is zero because velocity is fairly
constant

2. When displacement is highest

•  Slope is zero, so velocity is zero
•  Acceleration is now negative, because velocity has gone from positive
to zero

3. When displacementis falling

•  Slope is negative, so velocity is negative
•  Slope of velocity is zero because velocity is fairly constant

4. When displacement is lowest

•  Slope is zero, so velocity is zero
•  Acceleration is now positive, because velocity has gone from negative to zero
Does that help?

Chris

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#### Plantar pressure distribution

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#### Foot structure and function

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#### Control of GRFs

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#### Friction

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#### Energetics

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Hammill, J., Freedson, P.S., Boda, W., and Reichsman, F. (1987).
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Hayes, J., Smith, L., and Sanpietro, F. (1983). The effect of
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