Construction of Constant-Load (Isotonic) and Constant-Velocity (Isokinetic) Torque-Velocity-Power Profiles <em>In vivo</em> for the Rat Plantar Flexors

Understanding the torque-velocity-power relationship of muscle is critical for assessing dynamic muscle performance, which has implications for health and disease. While dynamometry studies in humans use isokinetic (constant velocity) or isotonic (constant load) contractions to assess this relationship, rodent models have almost exclusively relied on isokinetic contractions due to the complexity of running isotonic load clamp experiments on commercially available systems. Isotonic contractions better align with the constant-load conditions of movements that occur outside the laboratory. Here, we present a novel and minimally invasive transcutaneous electrical stimulation protocol and a step-by-step workflow for assessing the isotonic torque-angular velocity-power relationship in vivo for the rat plantar flexors. Isotonic and isokinetic torque-angular velocity-power curves were compared in 10 Sprague-Dawley rats. Isotonic contractions yielded a higher maximum shortening velocity (Vmax) and peak power compared to isokinetic contractions that used average torque. However, isokinetic peak power better matched isotonic peak power when measuring peak torque from isokinetic contractions due to a reduction in the curvature of the torque-angular velocity curve. Furthermore, torque and velocity at peak power differed depending on which protocol type was used. All datasets fit Hill's equation strongly (R² > 0.98), though isotonic data had the 'weakest' fits, likely due to the more variable nature of constant-load contractions. Based on these findings, using isotonic contractions is recommended when possible, especially if aiming to assess Vmax or peak power. If isokinetic tests must instead be used, a valuable guide is provided herein to understand the limitations and maximize the translatability to constant-load contractions.