The core comprises the rectus abdominis, which are the top layer of stomach muscles that form a six-pack, the internal and external obliques of the waist, the muscles in the diaphragm, the back and the pelvic floor.  
 
A strong core is essential for everyday movements such as standing, squatting, walking and reaching for things.  It also helps to maintain good posture, improve breathing, and optimising movement to prevent knee and lower back pain.
 
The traditional core exercises like sit-ups and crunches are linear in movement, but adding rotation to work the obliques and the other muscles will improve your overall strength.
 
The article suggested the three best exercises to develop the core, all of which are regularly included in my Pilates classes.  They are push-ups, the plank and bicycle (lying on your back and turning from side to side as you draw the knee to the chest). 
 
Aim for 90 seconds a week if you start by doing 10 seconds of each exercise three times a week.  Now that can’t be bad!!