Grass theory/ Muscle adaptation
Imagine a field of tall grass through which you have to walk to reach the other side. On the first day, it is difficult as you need to create your own path by moving the grass. But as the days progress, you would have created a clear cut path to the opposite side. This is Grass theory
Our muscles behave in the same manner. On the first day in a gym, all movements feel new and awkward. But with each passing day, with repeated movements, we create a pathway to the brain . This is called muscle memory. The same exercise which was awkward and almost impossible to do on Day 1 will seem very easy by the end of the week. Your muscles start responding involuntarily to the workout.
This is also the reason that muscle need to be constantly overloaded. We need to constantly create new pathways to the brain for optimal functioning of our muscles