When I first started learning about muscles in school, I learned the major names for muscles: Quadriceps, Biceps, Hamstrings, Calves, Abs, Pecs. These names are a simplification because they make it seem like there is only one muscle in these places, when actually these are names for groups of muscles. The muscles in our bodies form a complex overlapping, neighboring, and layered system of muscles. Do an image search for "quadriceps cross section", to see what I mean. There is an amazing array of muscles and tendons that are next to one another in the same area.
Having all of these tissues packed next to each other has interesting consequences: strong tissues can compensate for weak ones, tissues can adhere to one another forming what are called adhesions, they can rub or snap against each other, and, through tightness, they can limit the range of motion of one another. This creates a complex system of interrelations between different muscles. If one of your leg muscles gets stuck to another, it can start to atrophy as it no longer is supporting a load. This can further be compensated for by another nearby muscle taking over the job. Eventually, you lose (or never gain in the first place) the ability to activate a particular muscle with your brain, which is called poor motor control.
This problems can also be reversed. Adhesions can be broken up, you can build your muscle control for a particular muscle, you can stretch your muscles so they do not limit each other, and you can develop a training routine that makes sure your muscles get the individual attention they deserve.
Once I started to understand these concepts and work through these problems and interesting thing happened. I started realizing there were muscles that I did not know I had. An example of this was my serratus anterior muscles. I started being able to activate or flex muscles in my body in ways I had never done before. This is especially empowering and interesting. When you see a new muscle sticking out from the bundle of muscles it becomes easier to understand how to train that muscle and how to treat the problems with it. I am not saying you need to look like a body builder, but as you go a little bit toward that, you will start to notice some muscles you see in those pictures.
Going down the path of improving your own body's bundles of muscles requires understanding this concept of the complex systems of your muscles. The more you learn about this subject, the more you can address problems, and improve your body's muscular system.
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