Data-Driven Recommendations for Assessing the Early-Phase Rate of Torque Development: An Intersession Reliability Study in Physically Active Men and Women

The purpose of this study was to provide data-driven recommendations for assessing early-phase rate of torque development (RTD) in isometric knee extension and flexion, focusing on the optimal number of repetitions performed and trial selection criteria to ensure robust intersession reliability. Twenty subjects (10 males, 10 females) completed two test sessions with nine repetitions of unilateral “fast and strong” isometric contractions for the knee extensors and flexors. RTD was calculated from torque onset to 50 ms (RTD50) and 100 ms (RTD100), and peak RTD was assessed in a moving 50 ms interval within the first 100 ms (Peak50). Intersession reliability was assessed using intraclass correlation coefficient (ICC) for three to nine repetitions and four different criteria for trial selection. A mixed-effect linear regression model was performed with measured RTD as dependent variable with number of repetitions, selection criteria, and test session as independent variables. The results revealed high intersession reliability for RTD100 in knee extension (ICC 0.849–0.961) and flexion (ICC 0.835–0.944) across repetitions and selection criteria. Three repetitions were sufficient to optimize intersession reliability and measured values of knee extensor RTD. Assessment of knee flexor RTD required five repetitions to optimize reliability for RTD50 (ICC ≥ 0.700) and six repetitions to optimize absolute measured values of RTD100. Selection criteria based on peak RTD100 ensured optimal reliability and absolute RTD measurements for both peak50, RTD50 and RTD100. The assessment of knee flexion requires more repetitions than knee extension to ensure reproducible early-phase RTD outcomes.