Correction.

In a research report published in Physical Therapy in 1993, Oatis (1) calculated stiffness and damping coefficients of the knee joint, based on the moment of inertia of the leg and foot with respect to the center of mass of the leg-foot segment. She alerts Journal readers to an error in those calculations leading to the results presented in Table 3 of the article. Equations 1 and 2 should have used the moment of inertia with respect to the knee.

Oatis reanalyzed and recalculated the stiffness and damping coefficients (shown in Tabs. 1 and 2), using the moments of inertia with respect to the knee joint. As in the 1993 study, assessment of stiffness and damping coefficients demonstrated moderate to high reliability, with most intraclass correlation coefficients (ICCs) greater than .7 and 2 ICCs greater than .59. (2) Oatis found that the revised stiffness and damping coefficients were larger than those reported in the 1993 study.

The original study reported that both stiffness and damping coefficients were increased in the 45-degree test. In the reanalysis, damping coefficients were significantly increased in the test at 45 degrees compared with the test at 75 degrees, whereas the increase in the stiffness coefficients at the 45-degree test position was statistically insignificant. Effects of gender appeared to be similar to those reported in the original study, with increased stiffness and damping coefficients in male subjects at both test positions. Effects of age remain unclear. The 1993 study reported that stiffness at the 75-degree test position was increased in the 30- to 39-year-old group compared with the 70+ age group. The reanalysis also showed a statistically significant increased stiffness in the 30- to 39-year-old subjects compared with the 20- to 29-year-old group. These differences may reflect the significant structural differences in these joints or the differences in data collection, or both. No attempt was made in the reanalysis to identify any structural or functional differences among the different age groups.

References

(1) Oatis CA. The use of a mechanical model to describe the stiffness and damping characteristics of the knee joint in healthy adults. Phys Ther. 1993;73:740-749.

(2) Portney LG, Watkins MP. Foundations of Clinical Research: Applications to Practice. 2nd ed. Upper Saddle River, NJ: Prentice Hall Health; 2000:565.

Table 1.

Stiffness Coefficients (N x m/rad) Calculated from 2 Different Moments
of Inertia: Stiffness Coefficients Calculated Using the Moment of
Inertia With Respect to the Knee Joint (Corrected Values) Are Larger
Than When Calculated Using the Moment of Inertia With Respect to the
Center of Mass (Original Values)

Age Group   Original                      Corrected

(y)         45 [degrees]   75 [degrees]   45 [degrees]   75 [degrees]

20-29
  [bar.X]   3.27           3.14           10.92          9.770 (a)
  SD        1.27           1.12           2.828          2.541
  Range     1.29-5.68      1.03-5.71      8.919-12.92    7.973-11.57
30-39
  [bar.X]   4.30           4.06 (a)       13.13          12.38 (a)
  SD        1.47           1.29           4.793          4.505
  Range     1.70-7.24      2.26-6.33      9.745-16.52    9.196-15.57
40-49
  [bar.X]   3.69           3.49           11.49          10.87
  SD        0.93           0.79           2.885          2.330
  Range     2.13-5.50      1.73-5.21      9.452-13.53    9.222-12.52
50-59
  [bar.X]   3.44           3.21           11.27          10.29
  SD        1.33           0.86           4.607          2.806
  Range     0.62-5.71      1.97-5.04      8.012-14.53    8.308-12.28
60-69
  [bar.X]   3.46           3.22           10.77          10.12
  SD        1.23           1.03           1.916          3.577
  Range     1.39-5.34      1.84-4.91      9.415-12.12    7.587-12.65
70+
  [bar.X]   3.25           2.75 (a)       10.74          9.037 (a)
  SD        1.08           1.13           4.460          3.323
  Range     1.95-4.74      1.23-4.93      7.586-13.89    6.687-11.39

(a) Post hoc Bonferroni method of multiple correlations to examine
effects of age, significant at P<.05.

Table 2.

Damping Coefficients (N x m/rad) Calculated from 2 Different Moments of
Inertia: Damping Coefficients Calculated Using the Moment of Inertia
With Respect to the Knee Joint (Corrected Values) Are Larger Than When
Calculated Using the Moment of Inertia With Respect to the Center of
Mass (Original Values)

Age Group   Original                      Corrected

(Y)         45 [degrees]   75 [degrees]   45 [degrees]    75 [degrees]

20-29
  [bar.X]   0.10           0.07           0.2910          0.2223
  SD        0.03           0.02           0.0834          0.0728
  Range     0.03-0.16      0.03-0.10      0.2320-0.3500   0.1708-0.2738
30-39
  [bar.X]   0.13           0.10           0.3930          0.2996
  SD        0.06           0.06           0.1146          0.1661
  Range     0.04-0.25      0.03-0.26      0.3120-0.4740   0.1821-0.4170
40-49
  [bar.X]   0.11           0.08           0.3388          0.2505
  SD        0.04           0.03           0.0915          0.0861
  Range     0.05-0.17      0.04-0.15      02741-0.4035    0.1897-0.3114
50-59
  [bar.X]   0.11           0.08           0.3807          0.2789
  SD        0.08           0.06           0.2241          0.1369
  Range     0.02-0.34      0.04-0.27      0.2222-0.5392   0.1821-0.3757
60-69
  [bar.X]   0.10           0.09           0.3086          0.2703
  SD        0.05           0.03           0.0133          0.0784
  Range     0.04-0.26      0.04-0.15      0.2992-0.3180   0.2149-0.3258
70+
  [bar.X]   0.08           0.09           0.2566          0.3043
  SD        0.02           0.05           0.0795          0.1659
  Range     0.05-0.11      0.04-0.19      0.2004-0.3128   0.1869-0.4216