In this study, theoretical analyses are performed to investigate the characteristics of the static and dynamic stiffness of a nonlinear vibration isolator system. The vibration isolator system is modeled as an equivalent nonlinear oscillator. Based on the model, the static equilibrium and frequency response solutions are obtained with the variations of external static load and/or system parameters. It is shown that the static stiffness of the nonlinear vibration isolator tends to be hardened with the increase of external static load, which prevents the occurrence of excessively large deflection. This static stiffness-hardening effect is more remarkable with a larger spring constant ratio. The dynamic stiffness is also strengthened when the spring constant ratio increases, which enlarges the force transmissibility and reduces the isolation frequency bandwidth. Thus, the static stiffness- hardening improves the robustness of the nonlinear vibration isolator, whereas the dynamic stiffness-hardening rather degrades its performance. Thus, the opposite tendency of the static and dynamic stiffness-hardening effects should be considered in the design process of the nonlinear vibration isolator.

The purpose of this study was to compare the intra-rater and inter-rater reliability of three methods for measuring iliotibial tract length in 40 knees of 20 subjects. The measurements were performed by two examiners and one assistant. The length of the iliotibial tract was measured using three methods: the first used a modified Ober test with an inclinometer, the second used a modified Ober test with a marker, and the third used active adduction test with a marker in the supine position. Each examiner used the three methods in two sessions. For the first method, the inclinometer value was recorded manually. For the second and third methods, Image J analysis software was used to analyze the photographs taken. The intra-rater and inter-rater reliability of the measured value was assessed by means of intra-class correlation coefficients [ICC (3,1)] and the standard error of measurement. The intra-rater reliabilities of the three methods were excellent (ICC=.97, .95, and .94, respectively), and the inter-rater reliabilities of the first and second methods were fair to good (ICC=.64 and .65, respectively). The inter-rater reliability of the third method was excellent (ICC=.88). Our results suggest that the active adduction test is an objective and qualitative measurement method for measuring iliotibial tract length. We recommend that the active adduction test be used for measuring iliotibial tract length in the clinical setting.