For instance, imagine that you decided to start a swimming program and over the period of a few months are able to increase the amount of time that you can swim until you are able to swim continuously for 30 minutes. At that point you feel that you are in such good shape that you want to add running to your program. Can you start your running program at the 30 minutes? Your cardiorespiratory system might be in shape to be able to sustain that new workload, but you would be working your body in a new way and your musculoskeletal system would not be adapted to the challenge.
This is an extreme example, but the SAID principle applies even to more similar activities. In fact, as we age the body gets progressively better at narrowing its response of adaptation...even down to the muscle. If you are on a resistance training program to strengthen your biceps and are performing bicep curl exercises with dumbbells, your body will respond to that stimulus by making the muscles that are needed for the movement stronger. But if you only move the arm through part of its range of motion during the exercise, then the body will only get stronger through that range. Go all the way straight to all the way bent (keeping the elbow from coming forward) and the full muscle benefit can be realized. Stop before one of the two end points (or allow your body to find another way to make the exercise easier, like letting the elbow to come forward) and the benefit to the muscle will only be at the point in the range where the muscle was challenged.
Your body is very good at finding ways to make work easier. It 'thinks' in terms of performing the action that is asked in the most efficient way. I know that this can be one of the most frustrating things about exercise. It doesn't seem fair that if you are spending the time and it feels like you are doing the work that you shouldn't get the most benefit. I agree whole-heartedly, but I also know that I have to work within the given parameters. Our bodies will find a way to perform the action in the easiest way and things like momentum, positional shifts and shortening the range of movement, all make the exercise easier and limit the benefit to be gained. Paying attention to what your body is doing can go a long way toward increasing the results you can gain.
As always, let me know how I can help,