Progressive Loading in Rehab: Part 3
Progressive loading has yet another physiological adaptation that occurs within the rehab setting. In part 1 we looked at how the stress of progressive loading can influence our nervous system, then in part 2 we looked at the muscular adaptations that occur under the microscope, and today, in part 3, we will look at the levers that our muscles attach to, bone.
Bone is a living organism that is responsive to its' environment. There is an allostatic process called remodeling that is always occurring within our bones. It starts before birth and continues throughout life and is regulated at the cellular level by our osteoblasts (bone builders) and osteoclasts (bone absorbers). Remodeling is our bodies active expression of accommodation to the environment and consists of 4 phases: activation, resorption, reversal, and formation. Activation and resorption are defined by osteoclast activity whereas the reversal and formation phases rely on osteoblasts.
Although remodeling occurs at regular intervals throughout life, our behavior can further facilitate this process. If we did nothing and just lived we would remodel about 10-15% of our bone each year. But this isn't how life works or at least shouldn't be. We constantly face physical stressors whether it be gravity, ground reaction forces from locomotive movements, or from progressive resistance training. These all demand adaptation to be able to consistently endure. This is specifically true of progressive resistance training and is evident in those with osteopenia, osteoporosis, and/or within the geriatric population. It simply forces an increase in bone density as a means of survival. But what about beyond that, what about in the rehab setting after an injury?
Is there a different role?
I would not say there is a different role, but different parameters to be considered to appreciate the possible adaptations of progressive loading. The first thing to consider is the type of bony injury: fracture, osteoarthritis, osteochondral implant, etc. Each of these effects how and where remodeling will occur in a bone. Thereafter we have to consider bone class, type of bone, and the region of the bone. Finally, we have to consider our load application because the loading pattern directly influences how a bony adaptation is produced.
Look below for a brief review of bone:
The remainder of the article I want to discuss how we can manipulate the loading pattern. First off what are the types of loading patterns: compression, tensile, shear, and torsion. However it is
important to note that these are not equals and don't always yield positive adaptations as these forces can also generate injury. Compression is a force that facilitates the approximation of the ends of bones. Tension is the opposite, it aims to elongate the ends of bones. Shear forces act perpendicular to a bone and often result in fractures, such as a tibial plateau fracture. Whereas torsion tends to be most stressful as it works to fixate one end of a bone while twisting the other. It is important to note however that these forces can also occur in combination further increasing the demand placed on a bone.
Understanding how these different forces effect a bone requires special expensive gadgets, but an analysis of the load-deformation curve proves to be valuable. This graph illustrates how a material reacts under load. In other words it helps to depict Wolff's law from 1892, which states that any bone in a healthy organism placed under an external load will adapt internally. Therefore when an external load increases a bone adapts internally by increasing osteoblast activity. This allows improved resistance to loading in the future.
The manner in which this internal adaptation occurs can be explained by can the bone characteristics of anisotropic and viscoelasticity. These are 2 of the most important bone properties. Anisotropic reflects bone behavior with regards to the direction of load application; whereas bone viscoelasticity shows the response based on the speed of a load. Regardless of which characteristic we are examining when a load is introduced 1 of 2 things occur: failure or deformity (acceptance). Bone is categorized as a flexible-weak material, meaning it has the capacity to deform. This is why we can endure progressive loading. When loaded bone immediately deforms 3%. This is its' elastic capacity and yields no permanent damage to the bone. This leads into the point of deformation or the yield point, which is followed by the plastic phase. In this phase of deformation, permanent changes occur and bone becomes at risk. So the question becomes what can we do to raise the yield point to delay the onset of permanent changes during the plastic phase? The answer, progressively load!
Why?
Well, let's consider both viscoelasticity and Wolff's Law. First viscoelasticity. This is where the layering of parts 1 and 2 of this series comes into play. Remember this deals with the rate of load application. The best and safest way to increase the rate of force is through the rate of muscular contraction. When a muscle contracts it not only imposes forces at the musculotendinous junctions at either end of a bone, but an overall compressive force to the bone. This in turn increases osteoblast activity at the epiphyses as well as within the diaphyses to improve its compressive strength. With that said the strength of a contraction will also increase bone stress and can be correlated to the overall external load. Again this will increase osteoblast activity, but more so within the diaphyses because it challenges the normal distention of a bone, especially long bones. Simply put Wolff's Law is in effect.
Remodeling is our bodies active expression of accommodation to the environment and consists of 4 phases: activation, resorption, reversal, and formation.
Now it is important to note that these adaptations don't come easily. Repetition and time are needed to achieve adaptation. The good thing is though, if you are in rehab for a bony injury the chances are you have the time as bones take 6-8 weeks to heal at the short end of the spectrum.
Now I believe this is such an under appreciated or even neglected process by rehab specialists. Consider the "boring ankle fracture" with an Open Reduction Internal Fixation (ORIF). They come in non-weight bearing and/or in a boot and want an explanation of their condition/status. Our quick explanation for why they are in a boot is, to let it heal. I am afraid this is not just our explanation, but also the extent of our retention of the healing process. How can we rehab something we aren't giving respect and understanding too. Oh that's right we were given a protocol so we don't have to think, we can just skate by and come up with "cool" exercises and post them on social media.
No! Are you kidding me, we need to appreciate and explain the process we are going to take the patient through. This knowledge can be liberating and foster motivation in a recovery that is often lengthy and drawn out.
So what is my thought process?
1a) Non-weight bearing(NWB)/weight bearing as tolerated(WBAT): the bone exceeded its point of failure recently and the system is highly sensitized and graded exposure is needed to reduce the threat of full weight bearing. Additionally, WBAT allows for an increase in osteoblast activity to occur for the remodeling of bone about the ORIF.
1b) Exercise selection: there must be consideration of how the muscle shortens in an open chain environment with respect to the fracture site and type. We should not just give 4-way ankle theraband drills to strengthen the ankle. We need to think about the line of pull of each muscle and how this can facilitate approximation of the fracture site to stimulate osteoblasts activity.
2a) Weight bearing (WB): Weight-bearing results in osteogenesis through mechanotransduction, which will act to achieve a homeostatic condition at the fracture site. Furthermore, gait abnormalities tend to develop as a means of protection. Continued graded exposure and varying surface types allows confidence to be gained and compensatory behavior begins to be mitigated.
2b) Exercise selection: We should aim to stimulate the most osteoblast activity possible with respect to the healing time table. This is the longest phase of rehab. This is where we see the formation phase of remodeling at its' highest because we can progressively load the ankle. We move through weight shifting to stance phase drills to closed chain exercises: calf raise variations, squats, lunges, and step-ups. The goal is to increase BMD because denser bones are more resilient bones in the future.
3a) Performance development: Regardless of who you are working with an athlete or a member of the general population, high velocity low amplitude and plyometric work are necessary. This is where reaction time and power are developed. Both of these increase the demand on bone by increasing both the overall load and the speed of load application. In other words this is where we truly shift the yield point and achieve our physiological bone adaptation.
3b) Exercise Selection: I like to begin with step, hurdle, and ladder variations for my high velocity movements. Then logically a progression would be into aerobic plyometrics and explosive repeat variations. Then ultimately it is depth jumps, bounding variations, and sprints that will generate the greatest demand on the fracture site.
Is this how you or your PT thinks? Respect of human physiology is powerful and can help generate positive results. Don't loose the fundamentals as they are the foundation of everything we do.
Boney injury? Progressively load it and vary its load application and speed of application and obtain the adaptation you are desiring. Stay tuned for the final part of this series, ligaments, tendons, cartilage, oh my!
Stay Moving Well!
Keaton
