Relative movements between the tibia and femur induced by external plantar shocks are controlled by muscle forces in vivo

The purpose of this study was to investigate the role of muscle activation on the relative motion between tibia and femur. Impacts were initiated under the heels of four volunteers in three different activation levels of muscles crossing the extended knee joint: 0%, 30% and 60% of previously performed maximal voluntary isometric contractions. Impact forces were measured and tibial and femoral accelerations and displacements were determined by means of accelerometry. The accelerometers were mounted on the protruding ends of intracortical pins, inserted into the distal aspect of the femur and proximal aspect of the tibia. Under the 0%-condition the impact force (475±64 N) led to 2.3±1.2 mm knee compression and to 2.4±1.9 mm medio-lateral and 4.4±1.1 mm antero-posterior shear. The impact forces increased significantly with higher activation levels (619±33 N (30%), 643±147 N (60%)), while the knee compression (1.5±1.2, 1.4±1.3 mm) and both medio-lateral shear (1.8±1.4, 1.5±1.1 mm) and antero-posterior shear (2.6±1.3, 1.5±1.1 mm) were significantly reduced. This study indicated that muscles are effective in controlling the relative motion between tibia and femur when the knee is subjected to external forces.