Put a muscle at a mechanical disadvantage or isolate it and you will get high degree of muscle action on an EMG. Put that muscle into a movement where it is has to work with other muscles and now watch what happens. The pattern of activity will be quite different.
In the first case the muscle is screaming at you to set it free to let it work to its fullest capabilities as part of a team. In the latter case the muscle is singing because it is being the used as part of a team to work to move or resist movement of a body part. It is doing what it is designed to do work together with other muscles to produce smooth efficient coordinated movement. Muscles work in synergistic patterns, it is too easy to revert to reductionist thinking and forget that. What is convenient is not always right. Think movements not muscles in evaluation of movement and in designing effective training programs. Ask yourself is the muscle screaming or singing?