So what's a runner to do? Well, there are plenty of core exercises to choose from that are much more functional and benefitial to runners. First of all, when you think of "core" try to think beyond your abs. There are a lot more muscles than just the abdominals that make up the core. Core muscles consist of the muscles that run along the trunk and torso and generally include the following:
Rectus Abdominis—the "six-pack" muscles that everyone strives for
Erector Spinae—three muscles that run from your neck to your lower back
Multifidus—found beneath the erector spinae
External Obliques—positioned on the side and front of the abdomen
Internal Obliques—found beneath the external obliques, running in the opposite direction
Transverse Abdominis—muscles that protect your spine and provide stability; found beneath the obliques
Hip Flexors—a group of muscles (psoas major, illiacus, rectus femoris, pectineus, and sartorius) found in front of the pelvis and upper thigh
The problem with a crunch is that it's only one movement—crunching forward. Think about it. How many times during the day do your crunch like that? Now think about how many times you twist up, down, sideways? Your core exercises need to include more of a full-range of motion. Incorporating the elements of stability and balance into you core exercises is also great. Doing core exercises on a stability ball, medicine ball, core-board, or bosu helps you recruit more than just the targeted muscle group. This "extra recruitment" helps develop a stronger core as well as better balance and stability.
One of the best core exercises a runner can do is so simple, yet extremely effective—The Plank. To do a plank, lay face down on the floor. Put your forearms on the floor so that your elbows are aligned directly under your shoulders. Your forearm and upper arms should be making a 90°angle. Your legs should be stretched out behind you and you should be resting on your toes like you're doing a pushup. Engage your abdominal muscles to help you hold your body in a straight "plank" position. Your back should not be arched and your butt should not be drooping or stuck up in the air. Your body should be making an even diagonal line that's moving slightly up from your toes to your shoulders. These looks simple but they can be tough. Try holding the plank in good form for 30 seconds. See if you can work up to a full minute. Last check I could hold a basic plank for 4:02 minutes. TOUGH!
Once you've mastered the plank try some variations such as the basic plank with one leg lifted, a side plank, or a side plank with a raised leg. When you feel confident, try upping the difficulty even more by putting your feet and/or hands on a bocu, stability ball, and/or medicine balls. Another good plank workout is to alternate plank positions, moving from one to another (never letting your butt hit the ground) for 5 minutes. Mastered that? See if you can go for 10 minutes!
For a complete core workout from RunnerDude using a medicine ball, [click here].