Final answer:
The safest way to move a heavy object is to keep the back straight, bend the knees, and lift with the legs, reducing strain on the spine. The safest option when attempting to move a 250 lb. object is represented by: C) Keep the back straight, bend at the knees, and lift with the legs.
Step-by-step explanation:
When attempting to move a 250 lb. object, the safest option according to the rules of body mechanics is to keep the back straight, bend at the knees, and lift with the legs (Option C). This technique minimizes the forces exerted on the spine, which is relatively weak compared to our hips, knees, and ankles. Lifting with the back straight and using the legs to raise the body and load creates significantly smaller forces on the back – approximately 5.6 times smaller than if the person were bending at the waist with their back. Improper lifting by bending at the waist with straight legs, keeping the back straight and bending at the waist, or twisting the body while lifting can all result in substantially larger forces on the back muscles and spine, leading to an increased risk of injury. This technique emphasizes maintaining a straight back to protect the spine, bending at the knees to distribute the load on the powerful leg muscles, and lifting with the legs rather than the back. This method minimizes the risk of back injuries and ensures that the force is distributed more evenly throughout the body, reducing the strain on the spine. Options A and B both involve unsafe practices that can lead to back injuries. Option D, twisting the body while lifting, is also unsafe and can result in muscle strain or disc damage.