Abstract: A trunk dynamometer measures isometric and concentric strength and muscular endurance of the extensors and flexors of the lumbar and thoracic spine. A stabilization seat assembly immobilizes the pelvis, thighs, knees, and feet of the subject and is movable relative to a mainframe to permit the subject's axis of spinal flexion and extension to be aligned with the rotation axis of a torque transducer shaft. A laser alignment arrangement cooperates with a sacral stabilizer pad to assure proper alignment of the axes. Anterior and posterior trunk pads are adjustable in height and are linked to the torque transducer shaft to transmit the subject's spinal flexion and extension efforts as torque to the transducer shaft. An accessory shaft, also linked to the trunk pads and aligned with the transducer shaft axis, has a counterweight rigidly suspended therefrom to compensate for torque applied to the transducer by the trunk pads alone.