a laborer lifts a bag of cement from a bent position. Why is there more chance of back injury, ......?

Question

.....(specifically on the lumbosacral disk), if the bag is lifted briskly than if lifted slowly?

Answer 1

this is probably since when a person is in a bent position, the person's center of mass is moved toward the front of the person. Therefore when the person is trying to lift up an object in the position, they'll have to deal with an extra force from the center of mass.
As for lifting it fast or slow, it's due to the inertia of the bag of cement. lifting it fast doesn't give time for the resistant (cement) to change, so there is alot of pressure on the person. in contrast, slowly lifting decreases the amount of force the person needs to handle.

Answer 2

I think it has to do with the sudden jarring instead of a nice steady motion. The preferred lifting method would be with the back straight and knees bent.

Answer 3

For the same reason that it's easier to break a thin glass rod by bending it than by compressing it, or that you can exert a lot of pressure on an egg without breaking it by applying parallel pressure to both ends. A good argument against intelligent design is that if there really was a designer, the bastard wasn't very intelligent. Most living things, including us, are poor examples of engineering. One of our many fault-intolerant design features is our weak, vulnerable backbone. You can kill, cause lifelong pain to, or paralyse somebody very easily by breaking it.

Answer 4

Because human backs are poorly engineered for doing "crane" work. It's not designed for that. There's too little support to prevent vertebrae from slipping out of where they're supposed to be. The backbone functions best if the vertebrae are "stacked" normally upright.

Answer 5

cuz he is in stright posture he is only having one force on his spinal cord

when in bent it is having two forces which can produce torqu means double force

second force from the joints