Correction.In a research report published in Physical Therapy in 1993, Oatis (1) calculated stiffness and damping damping /damp·ing/ (damp“ing) steady diminution of the amplitude of successive vibrations of a specific form of energy, as of electricity. coefficients of the knee joint, based on the moment of inertia of the leg and foot with respect to the center of mass center of mass, the point at which all the mass of a body may be considered to be concentrated in analyzing its motion. The center of mass of a sphere of uniform density coincides with the center of the sphere. The center of mass of a body need not be within the body itself; the center of mass of a ring or a hollow cylinder is located in the enclosed space, not in the object itself. 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 Correlation Coefficient A measure that determines the degree to which two variable's movements are associated.The correlation coefficient is calculated as: ![]() Notes: The correlation coefficient will vary from -1.0 to 1.0. -1.0 indicates perfect negative correlation, and 1.0 indicates perfect positive correlation. (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 |
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