Assessing the Knee Extensor's Rate of Force Development Using a Fixed Handheld Dynamometer: An Inter- and Intrasession Reliability Study.

Context: The rate of force development (RFD), defined as the ability to rapidly generate muscle force, is commonly tested using an electromechanical dynamometer in isometric mode. However, these devices are expensive and not readily available. Therefore, this study aims to evaluate the interrater reliability of a fixed handheld dynamometer to measure the knee extensors' RFD and provide reference values using the proposed method.

Design: This study used a cross-sectional study design.

Methods: Using a fixed handheld dynamometer (microFET2) and a 3-dimensional-printed adapter, we evaluated the knee extensor muscles  in participants seated at the edge of a treatment bed. Each participant performed a standardized warm-up, followed by 3 maximal isometric knee extension trials. The outcome measures were peak force and early and late phase RFD (0-100 and 0-200 ms, respectively). The study consisted of 3 sessions: Visit one comprised of an initial session (session 1A) followed by a second session (session 1B) after 30 minutes for intrasession reliability; and visit two, conducted on week later, comprised the third session (session 2) for intersession reliability.

Results: Fifty-one participants were enrolled in the study. The in-session intraclass correlation coefficient for the early phase RFD was .87 (95% CI, .74-.92) and .91 to .92 (95% CI, .87-.94) for the late phase. The between-session intraclass correlation coefficient for the early phase RFD was .83 to .86 (95% CI, .74-.91) and .87 to .90 (95% CI, .80-.94) for the late phase. Finally, the peak force's intraclass correlation coefficient was .95 (95% CI, .92-.97) for the in-session and .91 to .92 (95% CI, .86-.95) for the between-session reliability.

Conclusions: Our approach provides a reliable, cost-effective, and quick method to evaluate the knee extensor muscles' RFD and peak force.