The objective of this study is to investigate the effect of the tensile force ratio between the two extensor muscles for the hip joint on the forces acting on the knee joint. We have created a 2D model of lower limbs and have performed some simulations to introduce the forces acting on the knee joint for various daily activities. Our simulation results demonstrated good agreements with the in vivo data available in the literature except that the tangential components of knee joint force became pretty larger than those from the in vivo data at high knee flexion angle. We considered that the discrepancy was attributed to the fact that we had assumed the hamstrings force is equal to the gluteus maximus force in order to solve an indeterminate problem, thereby introducing too large values of the hamstrings force. Then we expected that the above discrepancy could be eliminated if we change the tensile force ratio between the hamstrings and the gluteus maximus basing upon a certain biomechanical criterion. Thus we changed our model so that we could introduce the knee joint forces as a function of the above mentioned tensile force ratio. Simulation was performed for the various tensile ratio values and it was found that the knee joint force was sensitively affected by the tensile ratio and the above discrepancy could be eliminated if the ratio value was appropriately chosen. The subject of our future study is to find out the way how to determine the value of the synergetic muscles' force ratio to best fit the calculated joint force to the in vivo data with reflecting a biological rationality.
2012 The Japan Society of Mechanical Engineers