A framework of tissue homeostasis with the example of a marathon runner.
The envelope of function concept of tissue homeostasis was originally developed for patellofemoral joint pain (Dye 2005). The “envelope” is simply a zone in which the joint can function optimally to maintain its current function (homeostasis) or adapt to loading in a positive manner (productive overload). If the joint is excessively overloaded from a single bout of activity or cumulative activities, injury may occur. This can be helpful in demystifying the perceived cause of an injury - running injuries almost always occur when the cumulative load of a running session exceeds the load capacity for the specific structure in question (Paquette et al 2020). There is a compounding effect for multiple sessions day after day or week after week where our tissue capacity temporarily decreases as we adapt to the previous days work.
On the other end of the graph, we see that when the joint is underloaded, either by a period of planned rest or after forced rest imposed by injury, it will take time to build training volume and intensity back to desired levels.
We can extrapolate this concept to include other commonly injured tissues in the body whether a joint, tendon, or bone. It is a helpful way to explain why many injuries happen, and can help us understand the goals of the rehab process. For most running-related injuries we want to relatively offload the injured tissue (usually not complete rest!) with injury specific strategies while using various exercises (strengthening, stability training, plyometrics) to load the injured tissue in a way that promotes healing via mechanotransduction. Concurrently we want to work with the athlete and coach to program running load in order to guide them back to full training in an efficient manner.
It is important to remember that injuries are multifactorial and typically result from a combination of external and internal loads as we focused on here, as well as our ability to adapt to those loads which is affected by stress, sleep, diet, energy availability, hormone status, biomechanics, genetics, to name a few.
Start working from your current ability, be consistent, nail the basics and gradually increase your capacity to handle more training over time.