What is your core?
There are lengthy explanations and shorter explanations of what is considered your “core”. Here is a list of the main components of the core: Pelvic girdle/pelvic floor muscles, transversus abdominis, multifidus, internal and external obliques, rectus abdominis, erector spinae, and the diaphragm. Some people will include the gluteus maximus in there too, as it is a huge part of stability in most movements. You can google these terms to see images of what they are and where they are located in the body. The core is 3 dimensional, as it moves through all 3 planes of motion (transverse, frontal, and sagittal).
Why is core important?
Your core is the stabilizer and force transfer station of your body. It is the center of your body and all movement involves it in some way. It is important to have a strong core to prevent injury. A frequent injury due to a weak core, is the lower back. You need to have a strong core so that other areas do not end up compensating when you move. Aside from working out, other simple movements that you do throughout the day involve your core too. The main reason why I am not a fan of using machines to work out on is because the machines stabilize your body for you and do not require you to get those muscles firing. A regular barbell squat is very different than a smith machine squat. Learning how to engage your own core stabilizers will make you a better athlete and human.
What to do?
Do core exercises and do them often. Mix it up, sit-ups does not have to be everything you do. While I do believe that using a pull-up bar for knees to elbow and toes to bar is very effective, not everyone has access to this or has the strength to do it yet. My advice is to mix it up as much as possible. You’re also working your core when you’re not necessarily focused on it. Anything you do overhead requires an immense amount of core control and strength. Engage your core (flex it) when doing just about any exercise and your movement will be safer and stronger. Here are just A FEW of the many core items I do weekly.