Question

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 W_o = 200–N object. The spine and upper body are represented as a uniform horizontal rod of weight W_b = 350 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°.

Find:

A)Tension T in the back muscle

B)The compressional force in the spine

C)Is this method a good way to lift a load? Explain using results from part A and B

D) Can you suggest a better method to lift a load?

Answer 1

(a) the sum of the torques about the left end must be zero.

T sin 12 * 2L / 3 = 350 * L/2 + 200 * L

cancel 'L' on both sides, we have

T sin 12 * 2/3 = 175 + 200

T = 2705.47 N

or

T = 2.71 kN

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(b)

To find the compression force

F = T cos 12

F = 2646.35 N

or

F = 2.65 kN

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(c) No, this is not a good method to lift heavy loads, it may lead to spine injury

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(d)

The best way to lift is bend the knees in such a way that spine stays perpendicular to the ground and then lift the load and go back to standing positing while keeping the spine straight,