Keeping fit means moving, but if you are looking to strengthen your body and define your muscles, that's a whole different routine. Fitness experts say you need to work the core muscles. A lot of times people think when you say "core" what you mean is "abs", and that's just not the case. Abs are actually just one section of the core muscle group.
The core is the foundation of your house. It's a group of muscles all working together, and it provides us with stability and helps us with every motion we do. When we think about abs, we are really thinking about a few muscles that only work in one direction.
Core strength includes the muscles of your trunk, both stomach and back, and their ability to support your spine and keep your body stable. It's important to work all these muscles because when the core muscles are weak there's an imbalance. That imbalance can lead to back pain, muscle spasms, and other injuries, so work more than your abs. If you only isolate one muscle, you are hurting yourself because you are not going to be very stable, you are not going to have strength and you'll be lacking flexibility.