Which of the following is a correct practice when lifting heavy materials?

Bending at the knees is a fundamental practice when lifting heavy materials because it utilizes the strength of your legs instead of straining your back. This technique allows for a safer and more controlled lift by lowering your center of gravity and distributing the weight evenly across your body. By maintaining a straight back and bending at the knees, you reduce the risk of injury to the spine and muscles, which can occur if the lift is performed incorrectly.

In addition, this technique encourages proper body mechanics, which are essential for preventing fatigue during the lifting process. Keeping arms extended and twisting at the waist, as suggested in some options, can lead to imbalance and increase the risk of muscle strain or injury. Lifting with a jerking motion is also detrimental as it can cause sudden stress on the joints and muscles, potentially resulting in acute injuries. Therefore, bending at the knees is the safest and most effective method for lifting heavy items.