A person bending forward to lift a load “with his back”
(Fig. P8.23a) 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 P8.23b of a person bending
forward to lift a 200.-N object. The spine and upper body
are represented as a uniform horizontal rod of weight 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) the tension in the back muscle
and (b) the compressional force in the spine.
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