Michael Sena, co-owner of Energy Training Center, a personal training facility in Chicago, says children can strengthen their back and stomach muscles for wearing backpacks by doing the following exercises twice a week:

Back extensions. Lie on the floor face down with arms at the side, keeping the face slightly off the floor. Contract the back muscles by lifting both legs while lifting the upper body off the floor, creating a banana shape. Hold the position for 3 to 5 seconds. Repeat 10 to 12 times.

Superman lifts. Lie down on the floor face down with both arms above the head, with palms facing down and knuckles up. Lift arms, chest and shoulders while also lifting legs. Hold the position for 3 to 5 seconds. Repeat 10 to 12 times.

These are effective exercises on their own and additional weight shouldnt be added, Sena says.