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Clarification of Findings

In the June 2002 issue of Physical Therapy, Martin et al ("Gait Initiation in Community-Dwelling Adults With Parkinson Disease: Comparison With Older and Younger Adults Without the Disease") reported on a method that might be useful for detecting subtle signs of motor dysfunction in patients with Parkinson disease. The following 4 inconsistencies in their article, however, appear to be enough to prompt a request for clarification of their findings.

In both Table 1 and the text of the article, the authors indicate that 12 people with Parkinson disease participated in the study. When describing the Hoehn and Yahr stages of these people, the authors state, "1 participant was in stage 1.5, 2 participants were in stage 2, 7 participants were in stage 2.5, and 6 participants were in stage 3." How can these numbers be reconciled, unless a given participant can be in more than one stage simultaneously?

In Table 2, the mean values for event B are considerably smaller than the mean values for event A, even though the 2 events happened at almost the same time, as implied in the corresponding values for % gait initiation cycle in Table 3. No units are specified for the values in Table 2, although the reader can guess that the units might be centimeters. The sudden change in distance between event A and event B is difficult to imagine and is certainly not borne out by the nature of the graphic information in Figures 1 and 2.

In the "Results" section, the authors state that lateral displacement of the center of pressure at event A was nearly twice as large for the younger subjects as for the other 2 groups, but the information in Table 2 indicates that the displacement by the older subjects without Parkinson disease was 94% of the magnitude of that of the younger subjects.

In Tables 3 and 4, the lateral displacements of the center of mass reach values too large to consider physically feasible. If the units were millimeters instead of centimeters, the magnitudes would be not only reasonable but also remarkably consistent with the graphic information in Figures 1 and 2. In addition, the reader has to guess whether a given magnitude is displacement toward the stance side or toward the swing side.

Any one of these observations by itself could easily be passed over as a minor oversight by both authors and referees, but the constellation raises enough doubt about the data that the authors or the editor might wish to offer an explanation.

Paul D Andrew, PT, PhD, Professor

Ibaraki Prefectural University of Health Sciences
Ami-machi, Inashiki-gun, Ibaraki-ken, Japan

Author Response:

1. There is a discrepancy between the number of study subjects with Parkinson disease (PD) in Table 1 and the "Subjects" section of the text. The numbers listed in Table 1 are correct. There were 1 subject in stage 1.5, 2 subjects in stage 2, 4 subjects in stage 2.5, and 5 subjects in stage 3.
   
2. The mean values for point B as listed in Table 2 are substantially smaller than those in point A, even though the % gait initiation cycle values in Table 3 are virtually identical. This discrepancy exists primarily because all the point B values as printed are too small by a factor of 10. This error occurred during our transcription of the data to the published table from the original working tables. It is an error of presentation and as such does not affect the statistics because the error was applied uniformly across the comparison groups. Nor does this error affect conclusions of the work, although it certainly makes them much more difficult to understand. Professor Andrew is correct that the units for all the remaining variables in Table 2 are centimeters and should have been labeled as such.
   It should be pointed out that even though the point A and B values occur very closely temporally, spatially they can be quite far apart. About half of the trajectories studied closely match the trajectory paths as illustrated in Figure 1, where the 2 points are almost identical. The other trajectory paths show point B much closer to the midline than point A. This can occur because once the center of pressure (COP) begins to move toward the stance limb, it moves very fast as the stance foot begins toe-off. This is easily seen in Figure 1, where the COP points are further apart during this period than any other portion of the gait initiation cycle. Because both the COP and the center of mass (COM) are moving in the same direction at this time, that is, laterally toward the stance foot, the COP actually moves closer to the COM spatially, resulting in a mean COM-COP distance that is slightly smaller at point B than at point A.
   
3. There is a discrepancy between the fourth paragraph of the "Results" section and the data it describes in Table 2. The data in Table 2 are correct. We incorrectly stated in the "Results" section: "The maximum motion of the COP...was nearly twice as large for the younger subjects without PD as for the other 2 groups of subjects." The statement should have read: "The maximum motion of the COP...was nearly twice as large for both the younger and the elderly subjects without PD as it was for the subjects with PD."
   
4. The lateral displacements of the COM in Tables 3 and 4 are too large to be physically feasible unless the tables are mislabeled with centimeters instead of millimeters. In Tables 3 and 4, we inadvertently reported all values for "COM along the x-axis" in centimeters and thus these values are larger than the actual measurements by a factor of 10. Note, however, that we have confirmed that all COM-COP distances were correctly computed and reported in millimeters.
   

We are deeply chagrined that these errors of presentation found their way into the published version of this article and are even more deeply grateful to Professor Andrew and the Journal for bringing them to our attention and allowing us the opportunity to publish corrections. We would like to emphasize that despite the use of inconsistent units when reporting our data, the analyses and overall interpretations of the data are accurate.

For those readers who request reprints, we will include a corrected copy of the table to facilitate reading and interpreting the data.

Matthew Martin, PT, MS

Staff Physical Therapist
64th Combat Support Hospital
US Army
Wurzburg, Germany

James J Carollo, PhD, PE, Director

Center for Gait and Movement Analysis
The Childrens Hospital
Denver, Colo

University of Colorado Health Sciences Center

Denver, Colo

Margaret L Schenkman, PT, PhD, Professor

Physical Therapy Program
University of Colorado Health Sciences
Center, C-244
4200 E Ninth Ave
Denver, CO 80262-0244
(margaret.schenkman{at}UCHSC.edu)

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