A person bending forward to lift a load "with his back" (Figure a) 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 involved, and to understand why back problems are common among humans, consider the model shown in Figure b, of a person bending forward to lift a Wo = 195–N object. The spine and upper body are represented as a uniform horizontal rod of weight Wb = 290 N pivoted at the base of the spine. The erector spinalis muscle, attached at a point two-thirds of the way up the spine, maintains the position of the back. The angle between the spine and this muscle is 12.0°.
In figure (a), a man bends his back forward to lift a set of weights. The hips are labeled as the pivot and the back muscle is also labeled to the right of the pivot. In figure (b), a horizontal rod is shown. There are two points on the rod. One of the points is on the left end of the rod and an arrow labeled Rx extends rightward from the point while an arrow labeled Ry extends upward from the point. The second point is slightly to the right of the center of the rod. An arrow labeled vector T originates from here and points up and to the left making and angle 12.0° with the horizontal. An arrow labeled Wb originates from the center of the rod and points down, it is shorter than vector T. An arrow labeled Wo originates from the right end of the rob and points down. It is shorter than Wb.
(a) Find the tension in the back muscle. kN
(b) Find the compressional force in the spine. (Enter the magnitude.) kN
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