The body is a complex system composed by different parts. Although current medicine tends to see every part individually, the reality is that every part has mutual relationship with the others, also when anatomically distant: what happens to the head has repercussion on the feet and vice versa. If you have pain or injuries in one spot of your body, probably the cause is in a complete different place than you think.
The standard analysis sees individual muscles in isolation, divided from their connections above and below. The reality is that whatever else they may be doing individually, muscles also influence functionally integrated body-wide continuities within the fascial webbing, forming traceable meridians of myofascia. Stability, strain, tension, fixation, resilience and postural compensations are all distributed via these lines that run from head to feet.
Thomas Myers in his fascinating work of Anatomy Trains  describes myofascial meridians that run throughout our body, linking the individual muscles into functional complexes (Figure 1). The myofascia is a thin layer which covers and wraps around the muscle, connecting a variety of structures in the body together.
Consider the bones. These are held in place by the soft tissues of the body, i.e. muscles, tendons, ligaments and fascia. Take all the soft tissues away and the bones would pile on the floor, as they do not interlock or stack in any kind of stable way. Thus, the position of the bones depends on the tensional balance among soft tissue elements.
In order to guarantee body balance, the myofascia in a human body has intrinsic tone and is never completely lax (stress/strain can never reach zero): the integrity lies in the balance of tension, named tensegrity. Forces are distributed throughout the system rather than locally concentrated and the musculoskeletal system therefore functions as a single unit. This is where the compensatory mechanisms of our body lie, how the scoliosis arises from occlusion problems and how a forward head posture affects the pelvis.
In Figure 2 we can observe two of the twelve meridians described by Thomas Myers, the Superficial Front Line and the Deep Front Line. See how the top of the foot is connected to the skull or how the legs are connected to the jaw. It is really interesting to see for example how the ebb and flow of breathing (dictated by the diaphragm) is connected to the rhythm of walking (organized by the psoas). The body really is an unique system!
Due to these myofascial connections, in case of pain or injuries, areas of the body seemingly unrelated to the area you are concerned about may actually be the predisposing factor. Read about how malocclusion influences total body posture, or how neck muscles tension propagates to the head causing blood vessel compression and consequent hair loss, or how being shod gives higher predisposition to knee injuries than being barefoot. If you have a problem somewhere, more than probably the cause lies in some other anatomically distant part of your body.
Another concept that provides a framework to understand the body in a holistic way is the concept of trigger points. These are described as hyperirritable spots in the fascia surrounding skeletal muscle, associated with palpable nodules in taut bands of muscle fibers .
The trigger point model states that unexplained pain frequently radiates from these points of local tenderness to broader areas, sometimes distant from the trigger point itself. Practitioners claim to have identified reliable referred pain patterns which associate pain in one location with trigger points elsewhere. An example is shown in Figure 3, where we can see how trigger points in cervical muscles propagate pain in distal points in the head.
The body is an unique system: every part cannot be seen in isolation, since they mutually influence each other. Muscles and their myofascial meridians are an example of how a problem in the head can propagate to the feet and vice versa. So, next time you have pain in the knee is better that you look somewhere else to find the problem!
 Myers, Thomas W. Anatomy Trains E-Book: Myofascial Meridians for Manual and Movement Therapists. Elsevier Health Sciences, 2013.
 Simons, David G., Janet G. Travell, and Lois S. Simons. Travell & Simons’ myofascial pain and dysfunction: upper half of body. Vol. 1. Lippincott Williams & Wilkins, 1999.