To develop well-rounded athleticism means to develop skill in several attributes simultaneously.
Some of these attributes include strength, power, endurance, flexibility, mobility as well as concentration, patience, self-control, body control and so on.

This is what we focus on in the IKFF courses, the comprehensive development of numerous attributes, all working together in concert to produce a well-rounded, holistic health and fitness program.

Often times, athletes and fitness enthusiasts will train one or two attributes at the exclusion of others.

Here is an interesting demonstration, that combines strength with flexibility, which are actually two sides of the same coin. Strength training focuses on development of the agonistic muscles, the muscles on the same side of the joint being worked. Flexibility focuses on development of the antagonistic muscles–the muscles on the opposite side of the joint. So, in actually, flexibility training is a form of strength training, but sort of the mirror-image to muscle development.

The clip shows not just the getting into the position, which is flexibility, but also getting out of the position, which is strength. This is not the first progression, but if you have a good basic flexibility level you can try it.