Validity, reliability and sensitivity of a portable isometric dynamometer compared to a force plate to measure strength performance variables

Abstract

This study aims to investigate the test-retest reliability and validity of a portable isometric dynamometer compared to the force platform of the Isometric mid-thigh pull (IMTP) test. 20 national female and male athletes (age: 19.25 6.008 years) from various sporting and training backgrounds participated in 2 test sessions conducted a week apart from one another. They performed 3 IMTP efforts for each session, separated by a 2min rest between each trial. Peak force, rate of force development (RFD), and time-specific point values (100, 200, 300ms) were compared across devices and within devices. The isometric dynamometer reported high reliability (intraclass correlation coefficient (ICC) = 0.879, 90% confidence interval (CI): 0.755 - 0.942; Pearson’s product correlation r = 0.876; p = - 0.650) and acceptable validity (ICC = 0.973, 90% CI: 0.942 - 0.987, r = -0.284; p = 0.204) for peak force values. However, significant differences (p<0.001) were found between the dynamometer and force platform for RFD and force at the time-specific points. The dynamometer also did not show acceptable reliability for these measures of the IMTP assessment. Thus, the isometric dynamometer is a valid and reliable instrument to measure peak force, but not RFD and force at time-specific points. This supports the usage of the device as a portable and more affordable option compared to a traditional force platform to measure peak force. However, coaches and practitioners should understand that values from these 2 devices are not interchangeable.