Over the last 15 years, our team has been developing a modeling approach that utilizes biological signals to assess muscle use and passive tissue loading."? The actual anatomical model is a detailed, 3-dimensional representation with over 90 laminae of muscle, approx- imately 13 representative ligaments at each lumbarjoint, and nonlinear elastic disks, all of which deform under loading and under motion as the model mimics the motion of a real instrumented person. Three- dimensional spine kinematics coupled with individual passive tissue deformation relationships are used to quantify passive tissue loading throughout the full range of motion in flexion and extension, lateral bending, and axial twisting. During data collection from an individual, many channels of electromyographic (EMG) signals (usually 14) are placed over the torso musculature to obtain individual muscle load-time histories.