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Updated: Oct 15, 2025

Muscle Imbalances: Testing and Training Functional Eccentric Hamstring Strength in Athletic Populations
Published on: May 1, 2018
Effects of Voluntary Quadriceps-Hamstring Cocontraction on Tibiofemoral Force During Isometric Knee Extension and
Andrea Biscarini1, Roberto Panichi1, Cristina V Dieni2
1University of Perugia.
Abstract:
A biomechanical model has been developed to assess the effects of a voluntary effort of quadriceps-hamstring cocontraction on tibiofemoral force during isometric knee flexion and knee extension exercises with constant external resistance. The model establishes the analytic condition in the moment arms and traction angles of the quadriceps and hamstring muscles that determines the direction (anterior/posterior) of the tibiofemoral shear force developed by the cocontraction. This model also establishes the mechanical effect (loading/unloading) on the anterior cruciate ligament (ACL). At about 15° of knee flexion (where the ACL experiences its maximum quadriceps-induced strain) a voluntary quadriceps-hamstring cocontraction effort yields: (1) nearly the same enhancement in hamstring and quadriceps activation, (2) an increase in hamstring force about 1.5 times higher than that of the quadriceps, and (3) posterior (ACL unloading) tibial pull and compressive tibiofemoral force that increase linearly with the level of quadriceps and hamstring activation. The sensitivity of the results to intersubject variability in the posterior slope of the tibial plateau and muscle moment arms has been estimated with the use of anatomic data available in the literature. An anterior (ACL loading) tibial pull is actually developed at 15° of knee flexion by a voluntary effort of quadriceps-hamstring cocontraction as the posterior tibial slope exceeds 14°.
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