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Most of us ( untrained persons) have tight gluts and hamstrings, that prevents pelvic tilt on lifting objects, trained persons have stretchable muscles and so can tilt their pelvis more efficiently.

The stiff pelvic tilt adds stress on lower vertebrae and long back muscles.

While bending forward or to one side, twisting your back, the intra-discal pressure (before failure) is much less than vertical
compression.

That pushes disc material towards the spinal nerves causing its compression or irritation, that translates to back muscle spasm or leg pains (sciatica) .