In case A, there is the bending of trunk, which increase the lever arm of upper body weight
(a). It affects back muscle (Erector spinae) to produce force by the equation which is Fs • b = Fg • a.
When that force is produced, the intervertebral disc is compressed and there occurs workload. The force
which compress the disc are combination of the reaction force of back muscle force and reaction
force of the weight force.