The core is probably the most under-trained part of youth and high school football players.  If you are unfamiliar with the core, your core includes your abdominal, back, pelvic and diaphragm muscles.  All of these muscles are required to be developed in order to maximize your balance, stability, explosion or ability to shed near-by opponents.  Too many players are spending hours doing crunches and sit-ups to train only one part of their core muscles.  It is important to do a full core workout in order to ensure that you are getting maximum results.  A football player should train his core twice a week just like any other muscle group and its suggested that it is done on the same day that you are strength training your lower body.  Remember, just like your other muscles, your core does need rest as well so be sure not to over train.  Below are a 3 exercises that will help you strengthen your core, and increase your balance and stability, while becoming more explosive on the football field. 

Plank to Push-Up

Place your body in the standard plank position while resting your body weight on your forearms.  Be sure to contract you ab muscles while in the plank position in order to straighten and stabilize your back.  While in the plank position transition to the push-up position by lifting one forearm and placing that hand on the ground.  Repeat this with your other arm and you should be in the push-up position. After you are in the push-up position, drop one of your forearms back into the plank position and repeat this with your opposite arm.  One full cycle of plank to push-up equals one rep.  You are going to want to try 3 sets of 20 reps.  For a demonstration check here.

Mountain Climbers

Mountain Climbers is one of the most effective core exercise as it not only strengthens your entire core, but it also is effective for physical conditioning.  You will begin in a push-up position and then proceed to drive your knees to your chest in a running fashion as you focus on keeping your core tight and engaged.  You are also strengthening your shoulders and arms while performing this exercise.  This is also a great exercise to help improve your 40-yard dash time as your focus on driving your feet as you do your mountain climbers.  You should try to do 3 sets of 30 second intervals.  For a demonstration check here. 

Sprinter Sit-Up

This core exercise is most effective for athletes in sports that require explosive speed.  Football is obviously one of those sports.  This exercise simulates the sprinting motion, which helps you train your core muscles in the exact motion that will enhance your running mechanics.  This may take some time to work up to, but do not be discouraged if you cannot finish a full set during your first few tries.   Remember, pain and struggle is often part of the process to becoming great. 

Lie on your back with your legs straight and your arms at your sides. Perform a sit up and bring one knee up toward your chest. Similar to how you would run, pump your opposite arm forward like you are in a sprinter position.  Drive your arm on the same side as your raised knee backwards, alternating your arm drive similar to how you would do while running. After you reach the top of your sit-up, lie back down, and repeat with the opposite leg.  One full cycle is one rep.  You should do 3 sets of 20 reps.  For a demonstration check here.