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Mark D. Grabiner
04-01-1993, 08:32 AM
Some number of weeks ago I initiated a dialog on the topic of targeting
force plates during gait analysis. I am not certain as to how many of the
replies were directed only to me vs. those which were posted for the
entire network users group. Presently, I would like to thank everyone
for their insights and observations and to share with you all a summary of
the responses to my inquiry. I had been particularly interested as to
whether there were explanations, supported by quantitative research, for
the two disparate philosophies related to asking subjects to target force
plates. What follows is a rather approximate categorization of responses
that I hope portrays the contributor intent. In many cases, several
readings of a message were required to arrive at what I believe to be the
actual opinion on targeting vs. nontargeting.

1. Animals, goats in particular, do not seem to know or care about
force plates.
2. One respondant estimated a 10% miss rate using a nontargeting
3. A second repondant estimated an 80% miss rate using a nontargeting
4. One respondant opined that the expected effects of targeting a force
plate may be random rather than systematic.
5. There were two reposnses favoring a targeting protocol.
6. There were four responses arguing against a targeting protocol.
7. Three respondants do not tell the subjects about the force plate.
8. One respondant asks the subject to target a location some distance
prior to the location of the force plate.
9. One respondant does not ask the subjects to target the force plate
but the subject initiates gait one step or so before the force plate
(this is a specialized application and patient group)
10. Six respondants focus attention on adjusting the intial distance to
the plate over repeated gait trials.

In my opinion, numbers 7, 8, and 10 represent variations on the "do not
target" philosophy. Thus, of the limited number of samples collected in
this exchange, one could conclude that there may be more responses against
targeting than those supporting targeting. There were few respondants who
could provide quantitative information regarding differences in GRF between
the two conditions. However, a paper by Martin and Marsh (J Biomechanics,
25:1237-39, 1992) used a targeting protocol and supported this method by
citing a paper by Patla et al (J Exp Psych: Hum Perception Perform, 15:603-
17, 1989) in which it was reported that the differences in impulse
measures between targeting and nontargeting methods were not significant.
We have recently completed an experiment using 15 subjects who performed a
targeting-nontargeting protocol. The results revealed that the
differences between the two methods for x,y, and z-components of the
GRF were not significant using a dependent variable quite different from
Patla et al. This may provide support for the previously mentioned comment
that the effect of targeting may be random rather than systematic. It is
our conclusion that asking a subject/patient to specifically target a force
plate will not significantly affect GRF data and thus should not
influence those calculations derived from the GRF data. The primary
advantage of a targeting protocol may be one of expediency. For a given
period of time, even with a conservative estimate for the "miss rate", the
targeting protocol can allow more trials to be collected. Nevertheless, it
is also our opinion that the researcher must still remain vigilent for
those trials during which a subject obviously alters his/her gait to hit
the plate. Thus, an element of subjectivity remains. Thanks again for an
interesting exchange on this topic. I hope you find this summary helpful.