You may think that you're in good shape, but if your bodybuilding routine doesn't include core strength training, you are missing out on something that will improve your overall fitness.
The "core body" refers to the muscles of your torso that help you maintain balance and posture, whether you are running, standing or sitting. Strengthening these muscles will improve your ability to perform your specific, specialized exercises for your arms and legs.
Fitness balls will help strengthen your core muscles. They are fun to use, but that doesn't mean they don't work. If you are new to doing crunches on a fitness ball, you will be surprised at how much more your abdominals are engaged compared with standard crunches. Sitting on a fitness ball engages your abdominal and lower back muscles; they're working hard just to keep you from falling off! But you'll strengthen your core even more with some basic ball exercises.
Start by sitting on the ball and roll back so the ball is centered under your shoulder blades and your head is dropped back towards the floor behind you. Place your hands on your ears and curl up into a sit-up. Release. Start by repeating 10 times and work up to 30. If you are new to using a fitness ball, sit on the ball with your legs wide, between three and four feet. Once you've mastered that, move your feet in so they are shoulder-width apart.
Many gyms have fitness ball available, but if you have a workout space at home, consider buying your own from any sporting goods store or sports equipment web site.