Abstract: A method of calculating in vivo force on an anterior cruciate ligament (ACL) by measuring one or more biomechanical properties during a biomechanical screening task to obtain one or more biomechanical datum from the measured one or more biomechanical properties, and calculating a total load on an anterior cruciate ligament from an ACL force model using the one or more biomechanical datum as inputs to the ACL force model. The ACL force model is defined by FACL=FACLsag+FACLfront+FACLtrans+?jCTj, wherein FACL is the total force on the ACL, FACLsag is the force on the ACL in a sagittal plane, FACLfront is the force on the ACL in the frontal plane, FACLtrans is the force on the ACL in the transverse plane, and CTj is the ACL force interaction relationships among the sagittal-frontal (SF), sagittal-transverse (ST), and frontal-transverse (FT) planes, where j=SF, ST, FT.