Within the body, structure and function are intricately linked. Structure refers to the shape of a particular area of the body and how it grows. Whereas function refers to its purpose, or what that part of the body does. It is part of an equation that can be read in both directions – the requirements for a certain part of the body also dictate the way it forms. As a simple example, there are two bones in the lower leg; the tibia and the fibula. The tibia is much larger than the fibula because its function is much different. The tibia is the primary weight bearing bone in the lower leg, accounting for approximately 95% of the body’s weight when you are standing. The smaller fibula serves a much different function that does not primarily involve weight bearing, so it has a size and a shape that is much different than the tibia. It is important to understand the relationship between structure and function in the body to ensure proper treatment of injured areas.
Another example is the shoulder – a ball and socket joint that is built for movement. The shoulder has a greater range of motion than any other joint in the body because of the way it is oriented. A mobile shoulder blade with a shallow "socket" to receive the upper arm bone significantly increases the mobility at the joint to allow the shoulder to move in almost any direction. Instead of strong ligament support that may restrict mobility, the shoulder has weak and mobile ligaments with the majority of stability coming from the rotator cuff muscles. This increase in mobility does come with a cost; the shoulder is also the least stable joint in the body and is therefore prone to dislocations. The function of the joint to provide as much range of motion as possible is dictated by the structure of the bones, muscles and ligaments that make up the joint. Conversely the structure of the joint is guided by what we need the joint to do.
Let’s take a look at a more detailed relationship in the abdomen. The organs and viscera in the abdomen (stomach, intestines, liver, kidneys, etc.) are surrounded by layers of connective tissue (or fascia) that anchor them onto the low back, diaphragm, or pelvis. The general name for all this connective tissue is the peritoneum. The purpose of the peritoneum is to somewhat secure your digestive system in place. All of the different organs can be divided into two categories – structures that are intraperitoneal (or inside the peritoneum) and structures that are retroperitoneal (or behind the peritoneum).
Intraperitoneal structures are much more mobile because their function is different – they are primarily involved in digestion and need to be mobile to accommodate that function. We fill the stomach up with food, so it needs to stretch to accommodate the extra weight. The food then moves into the small intestine where it continues through all the coils of the small and large intestines to complete the digestion process. All of these structures are secured in place by fascia but they should remain mobile. You should be able to take the bulk of the tissue in your abdomen and move it from side to side without any discomfort.
Retroperitoneal structures are anchored to the low back and are not freely mobile. This is not to say that they can’t move - everything in the body is built to move – but they do not have the same mobility as the intraperitoneal structures.
Now we make the scenario more realistic and add in the effects of inflammation on the fascia of the abdomen (see article about inflammation). As prolonged inflammation causes the abdominal fascia to lose its structural integrity, the layers begin to become tighter and bind to each other in non-functional ways. The end result is a change in the ability of the digestive organs to move. This decreased mobility is not readily apparent in our lives but it can manifest in different ways; bloating after meals, fatigue after consuming food, poor posture as it begins to affect the thoracic spine, or back pain because the connective tissue surrounding the organs is constantly pulling on the low back muscles. Essentially we have taken an area of the body that was designed to stay mobile and we have changed its structure. Can we really expect that it’s able to perform the same function? If we changed the mobility of a knee, elbow, or wrist we might expect some discomfort performing movements with that joint. We should expect the same response in the abdomen.
While this is one example, it is a common occurrence we see in clinic. Structure and function are intricately linked in the body. It is important that a therapist understands the requirements and functions of the different areas of the body to ensure that area is properly rehabilitated; not just to the point where it is pain free, but to the point where it will not bother you again.