Strengthen your back to carry your backpack
1: Strengthen your abdominals by performing crunches, 3 sets of 10 repetitions. If it seems that you need a challenge, perform them on an exercise ball or do bicycle crunches.
To do a bicycle crunch lay on your back with your hands behind your head. Bring one knee over to the opposite elbow alternating between sides.
2: Perform leg raises while laying on your belly. With a towel roll under your chin, lift you legs up towards the sky and back down to strengthen your lower back. If it is difficult to do 2 legs at the same time do one at a time.
3: Do 2 sets of 10 push ups to strengthen you upper back and shoulders.
4: Do 10 wall sits. Slide down the wall with your knees bent to 90 degrees without your knees over your toes hold for 5 seconds each rep.
5: Lay on your side and keeping your legs straight lift the top leg up, and back down while staying on your side. Perform 2 sets of 10 on each side to help strengthen your hips.