Question

51. A person bending forward to lift a load "with his back" (Fig. P12.51a) rather than with his knees" can be injured by large forces exerted on the muscles and vertebrae. The spine pivots mainly at the fifth lumbar vertebra, with the principal supporting force provided by the erector spinalis muscle in the back. To see the magnitude of the forces in- volved, and to understand why back problems are com- mon among humans, consider the model shown in Figure P12.51b for a person bending forward to lift a 200-N ob ject. The spine and upper body are represented as a uni- form horizontal rod of weight 350 N, pivoted at the base of the spine. The erector spinalis muscle, attached at a Back muscle Pirot R T 200N 350N Figure P12.51 FOCUS

Answer 1