Case A and case b are different in the lever arm of upper body weight (a) making
the different back muscle force. In case A the lever arm of upper body weight (a) is farther
than case B, but the lever arm of back muscle force (b) and upper body weight force (Fg)
are equivalent. It makes the larger back muscle force (Fs) in case A than in case B by the
equation which is Fs • b = Fg • a.
The reaction force of back muscle (Frs) combine with reaction force of the weight
(Frg) are the disc load of vertebral. If we are in the case A posture, it creates greater muscle
force and disc load than case B.