Rapid Torque Production of the Knee Extensors: An Integrative Analysis of Neuromuscular and Muscle-Tendon Determinants

The physiological determinants of explosive strength are usually investigated separately. We aimed to quantify the relative contributions of muscle excitation, structural, and mechanical determinants of rapid torque production in the knee extensors in a multi-domain framework. Twenty-four young adults performed ballistic isometric knee extensions. High-density surface electromyographic (HD-sEMG) signals were recorded from vasti lateralis and medialis muscles. Patellar tendon stiffness and muscle architecture were assessed via ultrasound. Impulse and RTDpeak were outcomes. The predictors were from four domains: muscle excitation, muscle thickness, fascicle shortening velocity, and tendon stiffness. Linear regressions and LMG were used to quantify the weight of each predictor. The models explained substantial variance in impulse: R2 = 0.62 (0-50 ms), 0.76 (0-75 ms), 0.74 (0-100 ms), 0.61 (0-125 ms), 0.70 (0-150 ms). Early impulse (50 and 75 ms) was mainly explained by tendon stiffness and muscle excitation; mid-phase impulse (100 ms) by fascicle velocity and tendon stiffness; late impulse (125 and 150 ms) by fascicle shortening velocity and muscle thickness. RTDpeak was explained (R2 = 0.70) by patellar tendon stiffness (~32% of impulse variance), fascicle velocity (~18%), muscle thickness (~12%), and muscle excitation (~8%). Four predictors across different neuromechanical domains explained up to 76% of the variance in rapid torque production. Interestingly, the determinants shift over time: tendon stiffness and muscle excitation dominate earlier, while fascicle dynamics and muscle size become more important later. RTDpeak was largely determined by tendon stiffness and a balanced contribution of muscular predictors.