Ab Workout for Runners

Many common overuse injuries, from runner's knee to shinsplints, can be traced to weakness in the core, which includes the abs, lower back, and glutes. During running, the core muscles maintain the position of your pelvis. Your pelvis can rock front and back, side-to-side, or rotate. A strong core will prevent excessive motion of the pelvis, which will decrease the amount of repetitive-motion stress.
Do these three moves 3 to 5 times per week after your workouts and you'll increase your chances of getting to the starting line injury-free.

Clamshells

What it does: Strengthens the glute medius, which controls rotation of the femur and alignment of the knee during running.
How to do it: Lie on your side with your head, shoulders, lower back, and feet up against a wall. Keeping your feet together, raise your top knee like a clamshell opening. Repeat 15 to 20 times on each leg. For additional resistance, you can put a circular elastic thera-band around your knees.

Plank to Side Plank

What it does: Strengthens the abs and lower back, which help you maintain neutral pelvic alignment.
How to do it: Lie in a push-up position with your forearms on the ground and your elbows directly below your shoulders. Hold for 30 seconds. Turn to your side, keeping your elbow directly under your shoulder. Lift your hips until your body is in a straight line. Hold for 30 seconds. Repeat on other side. 

Side-Lying Leg Raises

What it does: Strengthens the glute medius, which controls the rotation of the femur and alignment of the knee.
How to do it: Lie on your side against a wall. Raise and lower your top leg 15 to 30 times. Do five to 10 reps with your toe pointing forward, five to 10 reps with your toe pointing up, and five to 10 reps with your toe pointing down. Repeat on other leg.