The general "definition" of function can be summarized as an outcome that is derived from one or more interdependant sources. For example, force (F) is a function of mass (m) and acceleration (a). In the biomechanical sense, "function" is essentially the same...however this concept is often lost.
Therefore, a focused "re-learning" of the component parts of function will ultimately provide an expanded perspective into how this understanding can translate into more efficient and productive treatment strategies. This can be done via a very simple formula:
Function = Force Activation Source + Force Transferring Source + Force Limiting Agent
The Force Activation Source refers specifically to the muscles themselves. Although forces are also generated from interactions with the ground, exposure to environmental stimulii, and autonomic activity, the musculoskeletal system is the primary catalyst with respect to function in the "locomotive" sense.
Force Transferring Source is likely the most underappreciated component of this equation. The common error is to consider the source of force only...however how the generated force is transmitted and transfered effectively determines the overall outcome. There are 3 primary considerations within the transferring context:
1) Tendons 2) Myofascia / Periosteum 3) Adjacent Tissues
During muscle activation, tensional force is subsequently translated into the tendon which is intimately related to the periosteum at the insertion point (in fact, tendon and periosteum are continuous with each other and are actually a singular tissue which has been characterized as 2 different tissues out of convenience). This myotendinal / periosteal action is governed by the myofascial "architecture" that provides a paradoxical connection and "disconnection" of the working / non-working muscles...which essentially perpetuates metabolically efficient operation. The considerations of adjacent tissues is a relevant consideration because they are directly involved in either the loss or gain in energy. Adjacent tissues that manifest some dysfunction (fibrosis, edema, etc) can reduce the overall transmission of force...and perhaps more relevant, contribute to chronic conditions due to inefficient muscle activation and force transmission.
Limiting Agents are the vast array of ligaments and joint capsules that surround and contribute to biomechanical operation. These specialized connective tissues are designed to absorb forces and therefore stabilize and protect excessive impacts and ranges of motions.
In summary, the Function Equation should be well integrated and implemented when any functional assessment and evaluation is in effect.
FeQ = F(a) + F(t) + L
In more practical terms, the careful evaluation of muscular performance, fascial health, tendonal integrity, adjacent structures, and joint integrity of at least 4-5 adjacent joints should be fundamental minimal "starting point" for any sage rehabilitative strategy.