The Athlete's Core
Whether you're a recreational gym user or a collegiate or professional athlete, you cannot deny the importance of core training. Many cases show that improved core strength and stability are pivotal in improving athletic performance.
The core comprises the abdomen, obliques, back muscles and pelvic-floor muscles. The core stabilizes the entire body; and strengthening the core facilitates and enhances dynamic athletic performance while preventing injuries and improving functional fitness. This leads to improving and supplementing compound lifts like Deadlifts and Squats. (See David Beckham's Core Training: Not Just a Six Pack.)
Core training should be worked around your other strength or plyometric exercises. I recommend performing core exercises at the beginning of your workouts. If you're new to core conditioning, start off with the traditional basics: the Plank, Abdominal Crunches and Oblique Crunches. In the beginning, perform your exercises with minimal resistance, because you want to strengthen your joints before heavy loading. As your core becomes stronger, do more explosive movements with heavier resistance. Do not focus on one area of the core more than others. You should spend as much time strengthening your back as your abdomen.
As you advance, enhance your core stability and strength by performing exercises on unstable surfaces such as foam mats, BOSU balls or balancing boards. This will improve your balance while firming up the muscular stability of your trunk.
Regardless of your workout regimen, you should dedicate as much time to your core as you do to your other muscle groups. Training erratically will never produce the improvements you desire.