The body is an amazing machine. It has a built-in system that makes things easier if you continuously repeat the same movements for a long enough period of time. In the exercise world this is known as the principle of specificity in training, or the SAID principle, which stands for Specific Adaptation to Imposed Demands.

The SAID principle refers to the way that the human body adapts to specific types of demands and stresses placed upon it. This is a beneficial function of the human body. In many places of the world this is how people survive. Human beings have the ability to adapt to temperature, altitude, harsh UV rays, allergens, and other environmental stressors. But humans also have the ability to adapt to exercise.

If the body were not able to adapt, you would not be able to increase strength and endurance. For example, if a person consistently lifts heavy weights, that person will develop his strength and muscle mass. If a person consistently lifts lighter weights for longer periods of time, however, then that person will increase his endurance and leaner muscles. When your muscles hypertrophy (increase in size), this is just your body adapting to the demands you are placing upon it.

This principle applies to almost any stressor that is continuously placed upon the body. Have you ever developed a skin callus on your foot? This is the skin adapting to the constant friction placed upon it, say from a shoe for example. If too much friction is placed upon the skin too quickly, what happens? A blister will develop, which is considered a skin injury. This same concept applies within the body. Too much stress too quickly can cause injury. This is why you must progress training appropriately.

It is necessary for your soft tissues, nervous system, and skeletal structure to adapt to the applied resistance before increasing to higher levels of training. Otherwise, injuries to these areas are more likely to result.

In fighting systems involving kicks to the legs, such as Muay Thai kickboxing, fighters are known to run rolling pins up and down the fronts of their lower legs/shins. They do this in order to strengthen the bone in the lower leg so that it is able to defend against leg kicks with less risk of injury. And yes, according to the SAID principle, it is a good idea for these kickboxers to train their bones to adapt to this kind of serious impact.