Answer
See solution.
Work Step by Step
Stress$=\frac{F}{A}$
Because area is constant, as force increases, stress increases. To calculate the force on the lower spinal column, we can write the net torque equation by including the force of the weight of the arms, upper body, and head into consideration.
Even though the weight and distance from pivot point (hip)
are equal, the lever arm for each force when standing upright and bending down is larger, creating a greater torque about pivot point. Therefore, greater force, and therefore greater stress, is required by the lower spinal column to keep the body in equilibrium position.
See image for a stick figure of a person in upright and sitting positions.