Having a strong core is one of the most important things in any skilled athletic movement, and tennis especially. Strong supporting muscles around the spine will decrease the risk of low back pain or injury. And besides that, if you add a good and healthy diet into your training regimen, your strong abs will look terrific as well. On the other hand, if you have well defined abs it doesn’t mean that you have a strong core. Abdominal muscles are just a part of the core which consists of more than 10 muscle groups. One of the simplest methods to test the core strength is the plank. There are many variations of a plank: plain, plank with leg and arm lift, side plank with leg lift, plank with knee touches, and many more.