Reliability and Utility of Load-Cell Derived Force–Time Variables Collected During a Constrained and Unconstrained Isometric Knee Extension Task on a Plinth

Rate of force development (RFD) and impulse (IMP) are important mechanical measures of muscular performance but are relatively unused within the rehabilitation and performance community. Due principally to access to low-cost testing devices and understanding the utility of these measures. The aim of this study therefore was to quantify the reliability of various force–time variables using load-cell technology collected via isometric knee extension whilst constrained in an isokinetic device (CON90) or unconstrained on a physiotherapy plinth at 60 and 90 degree angles (UNCON60 and UNCON90). Thirty-two volunteers had their peak force (PF), RFD, peak RFD (PRFD), and IMP assessed across three protocols. For all variables, UNCON60 had the largest variability across all measures. PF and PRFD were found to have small variability (ICC > 0.67 and CV < 10%). With regards to RFD 2080 all three protocols were found to have moderate variability all ICCs above 0.75, however, all CVs were greater than 10% ranging from ~ 11%–22%. Finally, IMP 2080 was found to have moderate variability for both CON90 and UNCON90, the absolute consistency once more greater than 10% (~ 11%–25%). Using the constrained and unconstrained protocols, PF and PRFD can be measured reliably between trials with 90 degree knee position.