A 68 {rm kg} skier is on flat snow waiting in line for a ski lift. While goofing around, he bends forward stiffly, as shown in the figure. To hold this position, tension in his back muscles must support the weight of his upper body. His upper body has 55 % of his mass, and the center of gravity of his upper body is 43 {rm cm} from his hips, in the middle of his trunk. What torque do his back muscles exert about an axis through his hips?