View Full Version : Response to Hatze and Baca's Letter of 5/17/94

Zvi Ladin
05-22-1994, 06:12 PM
In a recent long letter to the BIOMCH-L subscribers H. Hatze and A. Baca
take issue with the recent announcement of the Inertiator[TM] system by the
OsteoKinetics Corporation (Newton, MA, USA), and with a recent publication
in the Journal of Biomechanics coauthored by Ori Sarfaty and myself (J. of
Biomechanics, 26[8]:1011-1016, 1993. Hatze and Baca's letter suffers from
a combination of omissions and inaccuracies, not to mention questionable
scientific practices, as detailed in the follwing paragraphs:

1. Hatze and Baca advertise their upcoming letter to the editor of the
J. of Biomech. where they contend that the Sarfaty and Ladin paper uses
"essentially the same technique" as that published in Hatze and Baca in 1992.
I find it curious (to say the least) that rather than actually waiting for
the publication of their letter in a refereed journal, coupled with the
response of the original authors, Hatze and Baca chose to take a "shortcut",
bypassing the rigorous examination of a respectable peer-reviewed journal.
In order to set the record straight I would like to point out that our paper
was based on the MS thesis of Mr. Ori Sarfaty, that was published in
May, 1991, MORE THAN A YEAR BEFORE Hatze and Baca first presented their work,
and almost a year and a half before their published CONFERENCE ABSTRACT
(refereed???) finally made its way to the Boston area. Had Hatze and
Baca tried to carefully read our publication, they would have discovered that
the two approaches are similar to the extent that they both use a video
camera and a frame grabber. The actual procedures for data collection,
calibration and processing are quite different. However, the timing sequence
of the events listed above does put an iteresting perspective on the
"similarity" of the two approaches and the venue chosen by Hatze and Baca
to publish their system.

2. In carefully reviewing the 1992 paper cited by Hatze and Baca, I found
accuracy statements in the determination of inertial properties only
in comparison to their own anthropometric model. Comparing two approaches
that use the same model can hardly be considered adequate for the purpose
of documenting the accuracy of any system. The data given in our paper, on
the other hand, tested the accuracy by comparing the results to known solid

3. The 1991 paper cited by Hatze and Baca describes the use of a video
system to calculate kinematic variables such as joint angles. I fail to
see the relevance of this paper to the argument concerning the determination
of inertial parameters.

4. The e-mail service to the OsteoKinetics Corporation has been disrupted
due to some hardware problems over the past week. I was therefore asked
to respond to the above letter in the following manner:

I. Hatze and Baca suggest in their e-mail message that their letter
to the editor of the J. of Biomech. was somehow connected to the
release of the Inertiator[TM] system and the subscription of
OsteoKinetics to the BIOMCH-L list. The fact is that the
Inertiator[TM] system was announced during the North-American Gait
Lab Conference in the beginning of April in Portland, Oregon.
Unless it is suggested that the conference was scheduled to meet
the mailing date of Hatze and Baca's letter, one would have to
conclude that this is indeed a coincidence.

II. Hatze and Baca dispute the accuracy statement given in the
description of the Inertiator[TM]. Such a statement without
observing the performance of the system is quite puzzling. The
90-100 people who actually tested the system in Portland during
the Gait Conference were able to observe the system's accuracy
using a test body. The errors were indeed close to 2%.

III. Contrary to Hatze and Baca's suggestion, the Inertiator[TM]
does not need any "...special and hence expensive hardware
components...". It uses a standard video-camera and a frame grabber
that are quite common in a large number of laboratories.

I regret having to subject the subscribers of this list to such a long
response, however I had to factually rectify gross misstatements in the above


Zvi Ladin
Biomedical Engineering Department
Boston University
Boston, MA 02215