Progressive Loading in Rehab: Part 2

"If I lift heavy weights, I will get too big and bulky." This is the cause and effect statement I hear oh so often, which is often followed by the "I don't need to be big" clause and the "I just want to be healthy and not hurt anymore" explanation. This is the misconstrued vantage point of the effects of progressive loading in rehab.

Muscular hypertrophy is the most widely accepted and acknowledged adaptation stimulated by progressive loading. Though we recognize it, do we understand how it occurs and why it's beneficial? Just like in part 1 on neural adaptations, we will explore the adaptations progressive loading forces; as well as their role in the rehabilitation setting.

Before we get into these adaptations, we are going to take a crash course on Muscle Physiology to appreciate the difference between muscle fiber types. This will allow us to understand what progressive loading means for different fiber types and how adaptations can be achieved in each class.

We have two different classes of muscle fibers: Type 1 (aerobic) and Type 2 (anaerobic). With that said, the Type 2 can be further sub-categorized by their rate of fatiguability: Type 2a (fatigue-resistant) and 2b (fast-fatiguable). Each class and sub-category has specific characteristics unique to its fiber type. These characteristics are then either magnified or mitigated based on our goals and exercise programming.

Check out the chart below to see how each fiber differs from one another.

Now the proportions of Type 1 to Type 2 fibers is relatively fixed, but there is potential for fiber type conversion. The most potential is between the sub-categories of the Type 2 fibers. This is where most of the magic happens with regards to the rehabilitation benefits of muscular adaptations.

What does this conversion look like under the microscope?

First off, it is important to note that the process of increasing Type 2a or 2b is bidirectional and is facilitated by a stimulus. Each muscle is comprised of myofiber fascicles (muscle fibers), which contain fascicles of myofibrils (contracting element). It is at the level of the myofibril where we can appreciate muscular adaptation. Muscular hypertrophy. Now although this can lead to much larger musculature, it doesn't have too. This form of hypertrophy demands several months to years of consistent progressive overloading not the short stent spent in rehab. Recognize we are trying to rebuild a muscle at the molecular level because it has already been torn down to a dysfunctional state.

So what happens?

A myofiber moving towards a Type 2b profile will begin with changes that initially can only be seen under a microscope. When we progressively overload exercise we are using the stress of exercise to create not only a hormonal response, but an adaptive local immune response. We intentionally break ourselves down through exercise because our body's natural response is to rebuild itself stronger to avoid the same breakdown, or so we hope. This should be both our mindset and goal during rehabilitation.

To achieve this adaptation our satellite cells are crucial. The role of a satellite cell is to act as a precursor to a muscle cell when called upon. They work to facilitate growth, maintenance, and repair of damaged skeletal muscle tissue. Their location is also important. They are located on the outer surface of our myofibers. Thus, when needed for adaptive muscular regeneration satellite cells can proliferate and/or split into daughter cells and migrate to the damaged tissue site. This is significant as it allows an increased presence of cellular nuclei to feed our myofibrils. Now adding nuclei does not mean we are adding muscle fibers, but rather increasing its support system via contractile elements. This is what ultimately allows for increased cross-sectional area and thus muscular hypertrophy.

What about in the injured patient? Does this change? No. Our bodies respond in a very predictable

manner. In an non-injured individual, the acute immune response yields positive adaptations as just discussed. However, when the immune response becomes chronic, things get hairy. Initially, when we see a patient in a chronic state of systemic inflammation, our muscles become inhibited and further broken down when stressed. Thus, the use of manual therapies aiming at the reduction of inflammation during the first 2-4 weeks not only supports the initial neural adaptations, but lays the foundation for muscular adaptations to begin by reducing inflammation. This is allostasis at its' finest. Once we achieve this state of dynamic equilibrium within an injured tissue, the adaptations generated by progressive loading begin as if it were not injured.

Okay great, we are ready for muscular adaptations, but why does it matter that we move into a more Type 2a or 2b fiber type? Well this is where we give purpose to our bodies natural stress-response. First off, adding contractile elements via satellite cell stimulation means nothing if we do not utilize them by increasing our force production and/or our ability to sustain power output via progressive loading. Now how we overload our muscle tissue can dictate what type of fiber adaptations develop. If we look back at the graph above we can use those fiber characteristics to dictate our exercise methods.

Consideration for developing Type 2a fibers in Rehab:

1.High Intensity Continuous Training (HICT): increase oxidative abilities of moderate fast twitch fibers

2.Aerobic plyometrics: increases endurance of fast twitch fibers

3.Explosive Repeats: increases endurance of fast twitch fibers

a. Increasing mitochondria

b. Improves elastic energy transfer

Consideration for developing Type 2b fibers in Rehab:

1.Velocity Based Training (VBT): increased rate of force production

Consideration for developing Type 1 fibers in Rehab:

1.Tempo lifting: develops size and endurance of slow twitch fibers

Thus we must know what our goals as clinicians are to ensure we are choosing the proper methodology. This will allow us to achieve our desired muscular adaptations. Muscular hypertrophy in each respective fiber type is different and should be respected. Hopefully we recognize this does not mean we are building bigger bulkier muscles necessarily, but rather more supportive, effective, and efficient musculature.

In the next post we will dive into bone adaptations in response to progressive loading. Stay tuned.