To prevent injury when lifting a 50-lb box, the body part where the most strength should be focused is your:

Focusing on the legs when lifting a heavy object, such as a 50-lb box, is essential for proper lifting technique and injury prevention. The legs are the strongest muscles in the human body and are designed to handle heavy loads. By bending at the knees and using the strength of the legs to lift, rather than relying on the back, you significantly reduce the risk of strain or injury to the spine and back muscles.

Additionally, when using your legs to lift, you create a more stable base, which enhances your balance and control of the load. This approach also encourages good posture, keeping the spine in a neutral position, which is critical for safe lifting practices. Utilizing the lower body rather than higher body parts such as the back or arms helps distribute the weight more efficiently, thereby minimizing the risk of injury while maximizing the effectiveness of the lift.