Criterion-related Validity and Reliability of Surae Muscle Tendon Unit Stiffness Measured Using a Custom Made Passive Torque Indicator

[Purpose] To confirm the criterion-related validity and reliability of surae muscle tendon unit stiffness measured using a custom-made passive torque indicator (CPTI), a portable device composed of an angle-adjustable foot plate and strain gauge. The data were compared to readings obtained using an isokinetic dynamometer. [Subjects and Methods] Fourteen healthy young adults participated in this study. Torques at four angles of dorsiflexion of the ankle joint of the subjects were recorded using the CPTI and a dynamometer. The slope of the portion of the passive torque-angle curve was defined as stiffness. Three measurements were performed with each device. [Results] The intra-class correlation coefficients (ICC) (1, 3) were 0.81 and 0.86 for the CPTI and dynamometer, respectively. The degree of stiffness determined using the CPTI was significantly less than that determined using the dynamometer. A Bland-Altman plot revealed there was a fixed error between the CPTI and dynamometer. [Conclusion] The results of this study indicate that three measurements with the CPTI are as reliable as those made with an isokinetic dynamometer. However, the degree of stiffness determined using the CPTI tended to be less than that determined using the dynamometer.