I used to think that physical therapy would be easy to practice. Learn the anatomy. Focus on learning what each muscle did. Learn the exercises that work each muscle optimally. Find the weak muscles in your patient. Prescribe the correct exercise, and presto! Problem solved.
This would work perfectly if physical problems were simply hardware problems. Unfortunately, we humans are a bit more complicated than that (and, aren't we always)?
Every physical action we take is governed and controlled by our central nervous system. So, when we move, we don't just "move". Our central nervous system initiates a motion and performs complex monitoring of what is happening, along with constant minor adjustments to how we are moving, until the task is completed.
Now, here is the cool and complicated part: each motion, in turn, molds the central nervous system so that the next time the motion is performed, we can do it more smoothly, quickly and efficiently. This is how we ultimately improve our physical performance. We don't just get stronger; our neural firing patterns get more refined. It is not just our bodies that change with exercise, our minds do as well.
Unfortunately, this mechanism cuts both ways. When some part of our hardware is unable to perform its task, our central nervous system needs to find a new motor pattern to get the job done. We essentially compensate for the injury or weak muscle and come up with an alternate neural firing pattern, and an alternate way of moving, to complete the task. These firing patterns become a "new normal", but they load tissue abnormally, and usually generate other problems somewhere down the line.
A simple example of this: a sprained ankle causes a limp. The person can still walk, but the gait pattern is not normal. Even after the ankle heals, some abnormality in the motion remains. Over time, the ankle feels fine, but now the person develops lower back pain from the persistent gait compensation.
Rehabilitation, therefore, becomes not simply about strengthening the weak muscles, but about identifying the abnormal substitution patterns that have developed and correcting those as well.