LETTERS TO THE EDITOR

The appropriateness of these performance expectation limits, as well as their cross-validity remain questionable. In focusing upon the R”s associated with regressions of this type. we are simply highlighting one obvious source ofpredictive error. Extensive analysis beyond R’ should be done on their models using such techniques as split-sample prediction to discover how well they function as predictors. R’ is but a first step because it can be greatly inlluenced by the number of noninformative parameters used in the model. 4s predictive models continue to gain acceptance and undergo retinement, their application to a prospective payment system remains intuitively appealing. However, serious questions remain concerning the validity. consistency. and general applicability of these models. They deserve careful and critical scrutiny. Robert Rondinelli, MD lyniversity of Kansas Medical Center Kansas City, KS 66103 James R. Murphy, PhD Llniversity of Colorado School of Medicine Denver, CO 80220

Falls in the Elderly Fleming and associates, in their article “A Portable, Easily Performed Muscle Power Test and its Association With Falls by EIderly Persons” (Arch Phys Med Rehabil I99 1:72:886-9). conclude that “Falters had a lower dF/dT than non-failers. 17 of 22 failers were identified by a reduced dF/DT and reduced overshoot force.” It is apparent from figure 2 that al/ of the failers, but only 2 of the 23 non-falters. were more than 60 years of age. Therefore. using their data. if a cut-off of 60 years of age was set. one could predict with IOO? sensitivity that those people would fall. The speciiicity would be 9O”i. A cut-off at 60 years of age to predict falls would be more practical and statistically more powerful than the dF/dT.

Leg-Length Discrepancy and Postural Sway In their recent, interesting study. Murrell’ and associates concluded that “true” leg-length discrepancy (LLD) does not affect postural sway.. This conclusion differs from the findings of our study on subjects with simulated LLD.’ Murrell attributes this difference in tindings to ‘I. long-term adaptation by the neuromuscular system in those individuals with an actual LLD.” This is a plausible explanation, but there may be others. First. our simulated LLDs were of greater magnitude (1. 3. 3 and 4cm) than the mean (SD) of I. I3 (0.32 I cm of Murrell’s subjects. Assuming a normal distribution of LLDs among their subjects, there was a sufficient variability (SD of 0.22cm) to indicate that some of their subjects had LLDs of less than I .Ocm. Although the changes we found in mediolateral postural sway were present with as little as Icm LLD. the sway increased in proportion to the magnitude of the discrepancy. Second. their subjects stood with their feet together; ours stood with each foot angulated outward I? degrees and the mid-heels IOcm apart. Foot position affects standing balance.’ Third. although the travel (postural sway) was derived in both studies. we also reported differences ion the mean position of the center of pressure, which moved significantly toward the longer leg with as little as Icm LLD. Fourth, we used the standing height of the iliac crests to determine leg length:“ Murrell’s group used the slightly less valid distance from the anterior superior iliac spine to the medial malleolus. Fifth. our study had slightly greater “power”’ because we evaluated I4 subjects using within-subject comparisons (matchedpairs t-tests). whereas Murrell compared 9 subjects with LLD with I 1 controls. We suggest that these methodological differences are as likely to account for the differences in the findings of the two studies as any real effect due to “true” v’j “simulated” LLD. R. L. Kirby, MD R. K. Mahar, MD 1). A. MacLeod, MSc J. B. Garner, PhD Nova Scotia Rehabilitation Centre Halifax, Nova Scotia, Canada

Todd ‘I‘. Best, MD Mt. Clemens. MI 48044

The

first author replies:

Dr. Best’s comments are well taken. Our retrospective study was designed to provide proof ofthe concept of utilizing a simple measurement system as a predictor of fall risk. Our 13 falters were selected from a much larger total nursing home population and constituted less than 25% ofthe whole. Therefore, the estimates of sensitivity and specificity are grossly exaggerated. We have subsequently made additional measurements on 35 well and independently living older individuals as part of a prospective study. Ten men, ages 65 to 70 years, produced values (X + SD) of change in force arising from a chair of 3.10 t 0.30kg. sec. ’ - kg-‘. Similar values for II women in the same age range were 3.0 I :t 0.36kg. set-’ ekg-‘. Another group of subjects, ages 71 to 76 years, produced similar values of 3.57 t 0.67kg. set ’ . kg-’ and 3.23 k 0.34kg.sec-‘. kg-’ for men (n ~=9) and women (n = 5). respectively. These values fall within the range predicted

for non-fallers

shown

in figure 2 of our paper. Beth E. Fleming, PhD SUNY at Buffalo Buffalo, NY 142 14

401

References

Murrell P. Cornwall MW, Doucet SK Leg-length discrepancy: effect on the amplitude of postural sway. .r\rch Ph\ s Med Rehabil 199 1:72:646-S. Mahar RK. Kirby RL, MacLeod DA. Simulated leg-length discrepancy: its effect on mean center-of-pressure position and postural sway. Arch Phys Med Rehabil 1985:66:821-4. Kirby RL, Price N. MacLeod DA. The influence of foot position on standing balance. J Biomechanics 1987:X):423-7. Woerman AL. Binder-MacLeod SA. Leg length discrepancy assessment: accuracy and precision in five clinical methods of evaluation. J Orthop Sports Phys Ther 1984:5:330-9.

The authors reply: We welcome the opportunity to respond to Dr. Kirby and his associates. and we will address each of their concerns. First. concerning the use of subjects with less than I .Ocm LLD. Although the mean LLD tested in our study was 1.I3 (022)cm. the distribution was slightly skewed toward the positive end. The range of LLD for our subjects was between I Ocm and I .58cm-a narrow range of values in comparison with Kirby’s study. However. our goal was not to duplicate their study, but to sample the distribution of LLD in a normal population. We believe this goal Arch Phys Med Rehabil Vol73, April 1992

Leg-length discrepancy and postural sway.

LETTERS TO THE EDITOR The appropriateness of these performance expectation limits, as well as their cross-validity remain questionable. In focusing u...
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