Every muscle has a specific function(or a couple). That function is determined by its two end points. Its only job is to shorten the distance between those two end points.
Every muscle and muscle group has a general function. When the distance between those two end points is shortened, you achieve a goal/do a job.
For instance: the core is a group of muscles in the midsection of the body. All of their specific functions are to shorten the distances between their end points. In conjunction, their general function is stabilization. The core is made for stabilization. Some movement, but little.
Sometimes muscles don't do their jobs. That is a problem. If there wasn't a back-up system for that, we would be paralyzed in those movements where the muscle(s) was not doing its job. Fortunately, there is a back-up system. It's called compensation. Compensation is when a muscle does more than its fair share of the work for a muscle doing less than its fair share. That's a great thing because it keeps us moving. The problem with that is that all of our muscles are really only made to do their own jobs. Over time, they are going to tire out. Compensation, much like in the rest of life is really just for emergency situations. Nowadays, our bodies are full of compensation all the time, almost as if it were supposed to be that way. How does that sound to you?
Ex: 8 out of 10 people are going to experience lower back pain at some point in their lives.
While sitting, our hip flexors get tight. Because of a bodily phenomenon called reciprocal inhibition, our glutes(butt) turn off and stop doing their jobs. That's an issue because the glutes are the biggest muscle group in the body. In addition to that, our upper back suffers roughly the same fate as our glutes because it is stretched out all day long, and stretched muscles have a tendency to do less work than they usually do. That means that between our glutes and upper back, we have a lower back that is going to be doing a lot of work (hamstrings, which help the glutes, will also take a heavier load). The issue with that is that the lower back, as stated above, is really not meant to do a lot of work. It's not comprised of big, strong muscles. Thus, a lot of people with back pain. The solution to back pain is to get the midsection stable and doing it's job, and then to get the glutes and upper back(t-spine) going again.
The body is more complicated than just the chest, arms, legs, and back thought of as big blocks of muscle. Here are a couple of exercises you are probably not doing in the gym.