There are two different types of muscles. Those that support movement and those that produce movement. These are often termed stabilizers and prime movers. The movement being performed will determine the purpose and use of specific muscles. This means that all muscles are both stabilizers and movers based on the specific movement.
When I was learning I was given long lists of muscle insertion, origins. When I was learning Kinesiology (study of human movement) I was given lists of muscle movements based on their origin and insertion. While this is a great way to memorize all the muscles, movement is not this straightforward. Movement as with most other functions of the human body is very complex.
When we go to perform we don’t think about what muscles we are contracting and in what sequence. Based on this principle if you are looking to improve movement and movement patterns (which you should be!) you should not look to isolate single muscle groups. Movements are stored and processed in patterns in the brain. Through motor learning our brain will organize and sequence often used movement patterns. The more often we use a pattern the more organised and efficient it becomes.
The process of movement is not under control of only our conscious brain. Many reflexes cause involuntary changes and and adjustments in our movement. These minor changes and adjustments are made to make us as efficient as possible. One way that the body and muscles have become efficient is insufficiency.
Active insufficiency is the inability of a at multi-joint muscle to create enough tension to cause complete full ROM in both joints simultaneously.
Passive insufficiency is the inability of a multi-joint muscle to stretch enough to allow for full ROM in both joint simultaneously.
These multi-joint muscles are vital to human movement. In order for the muscles to be most effective they rely on other muscle to help support and synergize.
The deepest layers of muscle are close the bones and joints making more prone to stabilization. As muscle is layered on top of each other vast networks are created. These layered network of muscle work to provide movement and support in all 3D planes.
The purpose of the muscular system is to support and move the skeletal system. Muscles are constantly in a to fight from allowing your skeleton to fall and crumble to the effects of gravity. The small, deep muscles primarily perform these important stabilizing tasks. Stabilization muscles also enhance the efficiency and power of the prime mover muscles.
A great example of muscles not behaving the “way they are supposed to” during movement is a phenomenon known as Lombard’s paradox. The quadriceps (muscles on the front of thigh) and hamstrings (muscles on the back of the thigh) are antagonists. Being antagonists supposedly means that when the muscles fire they perform the opposite movement on a joint. When performing a squat or raising from a seated position both the quadriceps and hamstrings contract at the same time.
I’m going to nerd out a little here on the anatomy of Lombard’s paradox so bear with me. The rectus femoris is the only quadriceps muscle that crosses both the hip and knee. This opposesallf the hamstring muscles which cross both the knee and hip joints. The moment arm for the rectus femoris is smaller at the hip than the hamstrings. This causes hip extension. The exact opposite happens at the knee when the rectus femoris moment arm is larger than the hamstrings. This causes knee extension. When both these joint motions are combine (hip extension and knee extension) you will be in an upright position.
This is just one example to show that muscles cannot always be classified based on their anatomical properties. If you are looking to improve you efficient and function then you need to take a movement based approach. We are made to move. Everything in our body from head to toe is there to help us move and explore our environment.
Keep moving and do it well!