if a person lifts a box in a bent position,why is there more chance of back injury if it is lifted quickly?

Answer 1

well the spine is at it's strongest when it's in a straight position. just like a pencil if you try to bend a pencil it will bend so far then snap. Thats why they say bend at the knee's when picking some thing up off the floor that way you are keeping your spine as straight as possible. Your back has no natural joints in it so if you look at your self in a mirror while bending over you'll notice your actually bending at the waist and just barely arching your back hence the pencil test.

Answer 2

If you lift a box in a bent position you are risking back injury whether you do it fast or slow. What you want to do is bend your knees and lift with your legs holding the object close to your body. I work in a job that requires a lot of lifting and our safety department has safety meetings on this all the time. I cant remember what the exact figure is but an example is like you lift a box weighing 30 pounds with your back bent it puts hundreds of pounds of strain on your back

Answer 3

If you lift it quickly, you not only have the force of gravity to contend with, but also the force needed to accelerate the box upwards.
Force = Mass * acceleration

The faster you try and lift, the more this second force