Ground Reaction Forces and Neuroplasticity

This website concludes with an overview of why, even after we have survived TKR and worked so hard to restore our proper gait, we need to continue to take active responsibility for our new knees.

A good way to think about Ground Reaction Force is to invoke the maxim: “For every action, there is an equal and opposite reaction.”

Every step we take pushes body weight against the ground and the ground (in an equal and opposite reaction) pushes back, sending this force – or impact stress –  up through the body. This stress is referred to as Ground Reaction Force or GRF.  In a body without any structural joint problems, muscles will act as shock absorbers to take in and dispense GRF in the following order:

1. From the outside of the heel to the
2. Bottom of the mid-foot. 
Then the line of force crosses upward over to the 
3. Big toe, The foot is now flat on the ground.  Those forces then travel  
4. From the big toe to the  
5. Outside of the lower leg, then through to the 
6. VMO (Vastus Medialis Oblique) then up and over to the
7. Hip and finally into and through the
8. Abdominal muscles.  

The video below demonstrates the way GRFs moves through a healthy body. Note how the blue circles, which indicate GRF, pass from one foot to the other as the heels strike the ground. The red lines that cross in the middle of the abdomen indicate the upward direction of the GRF. At .06 in the video, the body is in perfect balance

Ground reaction force prediction for gait analysis:
https://bit.ly/2RXyJlv

By weaving from side to side in the neuromuscular path up the leg, the GRFs are absorbed by muscles and soft tissue throughout the entire leg. What is not absorbed moves into the hip and then into the body’s core. The memory for this, and all neuromuscular paths, is located in the brain. The ability of the brain to create and then to modify such neural pathways is called Neuroplasticity as memorialized in the saying, What fires together wires together.

When structural changes or degeneration in the knee joint cause problems in the neuromuscular system, the pathways that absorb GRFs must, by necessity, also change. These changes cause the pathways to become less efficient. Restricted knee flexion results in tight leg muscles and reduced flexibility of the foot, ankle and knee. This, in turn, limits the ability of the soft tissue to absorb stress. This stress is then directed to the joints and bones causing soreness and pain. Pain also occurs when the lack of knee extension limits or prevents the VMO’s ability to absorb and transfer impact stress. As a consequence, the impact force line remains on the outside of the leg where the stress impact will be greater than the ability of the relevant muscles (the outside of the foot, calf and thigh) to absorb it. Again, the result is pain.

Before Surgery

After Surgery

Different knee structures but the same neuropathways unless the patient works to change them

Even after the structural problems in the joint have been corrected through TKR surgery the old pathways, created over time to accommodate dysfunctional changes in the knee joint, persist. These patterns are difficult to modify and may explain why treatment by a physical therapist may have only a temporary effect. In order to ensure lasting change, the patient must commit to an exercise plan that will produce repetitive movement patterns for the new joint  sufficient to create new neural muscular pathways. These new memory patterns will, over time, replace the old ones. It is important to note that the term muscle memory refers to memory For muscles, not memory In muscles. The patterns are located in the brain and created through Repetition. https://learnmuscles.com/blog/2017/08/30/neural-plasticity/

What are the take-aways from this understanding of GRFs and Neuroplasticity for remaining functionally independent?

1.  The brain does not discriminate between good or bad repetitive patterns, which is why it is so important not only to perform your recovery exercises, but to perform them correctly. It is also to your advantage to do multiple, shorter exercise sessions throughout the day, thereby sending more signals to “turn on” or activate the muscles, rather than engaging in a marathon session once a day.

2.  If you have the kind of body system in which GRFs result in muscle tension, it is important to stretch after you have done any activity in which your body has absorbed impact stress. This is one of those realities in life that comes under the heading, ‘playing with the cards you were dealt’, and the reason why you should never compare your recovery process to anyone else’s.  There is, as discussed in the last section, a continuum ranging from Hypermobilility to Hypomobility. Hypermobility refers to joints with a greater amount of stretchiness in the inert tissue enabling these joints to be more flexible. These more flexible joints – in addition to enabling greater knee flexion – also do a better job of reducing GRF stress as it travels through the leg.  In contrast, those of us with body systems that lean towards Hypomobility must remain more vigilant to the consequences of GRFs. Genetics, previous injury or both have left us stiff, dense inert tissue and limited amount of end range motion in our knee capsules. We have both less knee flexion and a reduced ability to absorb GRFs. Impact stress makes our muscles tense; tense muscles are shortened and less flexible and range of motion suffers. It is not a level playing field and we are at a constant physical disadvantage.

3.  Aging is another variable the effects of which cannot be denied. Over time, even lax inert tissue will become stiffer as the body loses water content. We are genetically programmed to desiccate as we age. We are also programmed to lose muscle mass and muscle power. However . . there is a glimmer of hope. The more we exercise particular muscles, the more functionality they are likely to retain and, in turn, the longer we can remain functionally independent