Rigid body dynamics and soft tissue loads are solved simultaneously in a cosimulation framework to couple musculoskeletal dynamics and tissue mechanics. The goal of this work was to implement a validated, open-source cosimulation framework of the knee to determine how this coupling affects computed cartilage loads. The kinematic knee joint of a generic whole body model in the open-source software OpenSim was replaced by a previously developed discrete element knee model that consisted of a six degree of freedom (dof) tibiofemoral joint and one dof patellofemoral joint. A serial approach was initially used to estimate muscle forces and cartilage contact loads for a simple flexion movement. Then, a cosimulation framework was implemented for a simple knee flexion movement in which neuromusculoskeletal dynamics and knee mechanics were simultaneously solved using a computed muscle control (CMC) algorithm. This work highlights that the choice of computation method and the precise acquisition of all the dofs of the knee are important factors to consider when estimating soft tissue loads.