The musculature and structures between your shoulder girdle and your hip; the superficial and deep abdominal muscles.

What is the function of the "core" and what is "core stiffness"?

Ask a performance coach and they'll give you one answer. Ask a physical therapist and they might give you a different one. From a physical therapy or rehabilitation setting, core stiffness may refer to increased muscle tone that leads to lower back pain and movement restrictions. In the performance sports setting, core stiffness may refer to the core's ability to maintain stability in response to the influence of external forces, which may vary in magnitude, direction and speed. For the purpose of this write up, I'll be talking about the latter.

To my knowledge, the function of the core is to transmit force, prevent force/power leaks and maintain the position of certain joints and body segments. In the words of professional lower back and spine expert Stuart McGill, he describes the function of the core to "stiffen the torso to prevent motion, [and] enhance motion of the distal limb segments".

In the majority of sporting performances, the ability to accelerate distal limbs is the key to high power output: think about throwing a baseball, kicking a soccer ball, jumping, pushing, punching. Stiffening the torso to prevent excessive motion and enhances force production and power output because it acts as a medium for force transmission - ensuring all the force you generated is directed towards your intended direction. A floppy and weak midsection results in a weak kick, a weak throw, a weak jump and a weak landing.