In response to the discussion concerning inexpensive force platforms
and contact time detecting mats, I would like to sound a serious word
of warning regarding the accuracy of both, the methods used and the
contact time detecting mats (force mats) involved.

My laboratory has recently conducted an extensive investigation into the
usefulness for practical purposes of a "jumping ergometer" allegedly developed
by the Czechs Hamar and Tkac but actually based on a proposal by Bosco.
This "jumping ergometer" is, in fact, a mat that measures with a certain
accuracy the contact and non-contact times of an athlete performing repeated
vertical jumps. According to an advertisement, the mat and the associate
microprossesor system permit the precise determination of the jumping
height, the work done per kg body mass, the (average ?) power per kg body
mass, ans the propulsion period during take-off.

We compared the results obtained by using this method with the results
obtained by measuring force histories with a highly accurate Kistler force
plate and by employing the relevent equations derived from basic principles.

It was found that the average errors of the results obtained with the
mat as compared to our reference method were approximately 24 % for the
jumping height (lifting height of center of mass), 12 % for the (average)
work done per kg body mass, and 31 % for the (average) power per kg body
mass. It is, therefore, advisable to exercise extreme caution when using
the results obtained by using such mats.

H. Hatze