Embodied Cognition and Muscle Memory
Embodied cognition is the idea that our bodies are integrated with learning and memories. In other words, it isn't just your brain that is capable of memories, but even your muscles and cells may have the capacity to learn and remember. When we move our bodies and interact with our environments, we experience dynamic interactions that lead to cognition.
So, how can we see learning in the body outside the brain? Chances are you've been told that light in our environment highly affects the release of melatonin and other hormones involved in regulating our internal "clocks" and to avoid looking at blue light before bed. While this is not wrong, your eyes aren't the only organ paying attention to the light in your environment.
A great example is the circadian rhythm of blind people. Even if there is damage to the nerves leading from the eye all the way to the brain, where the visual stimulus is processed blind people still have the capacity for their circadian cycles to be regulated by light. If the brain cannot receive the stimulus of light from the eyes, how is the information reaching the brain?
Scientists have found that the skin (our largest organ) is responsible for regulating circadian cycles in some people with visual deficits. The typical intrinsic circadian period is 24.2 hours long in people who can see. In blind people, the circadian period is still 24.2 hours long. Therefore it isn't sight (or rather the eyes) that is responsible for registering light, but rather skin cells. These skin cells have a molecular mechanism that tell the brain if there is light or a lack there of to regulate circadian cycles (Huang, 2018).
We also see examples of embodied cognition in heart transplant patients. Although it may sound a bit like pseudoscience, there are instances where a person who has received a heart will develop characteristics of the donor. Patients have reported personality changes; some even reported developing the same fears as their donors. Some research focused on planaria (a type of flatworm) found that memories reside outside the brain through transplanting memories from one flatworm to another (Glanzman, 2018), while other research has demonstrated that memories can be transplanted epigenetically across generations through DNA (Hoshide & Jandial, 2018). While no empirical work clearly answers if this is the case for humans, there are plenty of anecdotal cases of embodied cognition.
A famous case is Claire Sylvia, who developed the same interest in food cravings as her heart donor. Claire was a professional dancer who began to crave KFC chicken and beer and experienced a change in her gait (described as "walking like a man") after her heart transplant. Upon inquiring about her donor, Claire discovered that it was an 18-year-old male who enjoyed KFC and beer. This suggests that some sort of food memory was transferred by receiving the heart.
So, what is muscle memory? If we use this model of embodied cognition, muscle memory becomes a bit more in-depth. We've all heard the term, but it basically means that you've learned a motor task through repetition. This is due to procedural memory (learning through repetition). But, if we take a more wide-scoping lens and analyze muscle memory, we might say that muscles are actually learning (not just the brain). If other body organs can hold memories, why wouldn't muscles?
Theoretically, if I were to transplant an athlete's muscle tissue into a sedentary person, that donor muscle would act differently than the person's own muscles. We actually see this in an ethical dilemma facing the sports world: transplanting "better" muscle tissue (i.e., faster, stronger) into athletes for performance gains. But is this transplant purely physiological characteristics, or could we argue that the muscle memory is being transferred? Muscle cells have an interesting property, they have multiple nuclei that all store the genetic information of the cell. When studying these nuclei, it has been found that even if an athlete were to stop training and the muscle cells shrink, it is easier for an athlete (than a non-athlete) to rebuild this muscle back to it's original larger size even if they were sedentary for years (Staron et al., 1997). In a sense, these nuclei hold memories and can be recalled later on. This is also seen in mitochondria. Once you've taught your muscles to produce more mitochondria, regardless of how long you go without training, your muscles are primed to produce more mitochondria (Lee et al., 2015). Therefore, muscles seem to hold memories.
When we train strength we are in theory causing adaptation that promote muscle tissues that sustain this goal (i.e., type IIb and type IIa muscle fibers). Through repeated exposure to movement and stimulus, the muscle transforms to support survival. The same way we can teach our brains something new through repetition, we can teach out muscles. In fact, a good chunk of motor movement is supported by implicit memory. Meaning, memories that we can't actually recall but can perform.
For example, imagine you are driving a car and want to change lanes. Let's say you're in the left lane and you merge into the right lane. Close your eyes, think about what our hand movement will be. For the majority of us, we pictured ourselves pulling the steering wheel right and then putting the wheel back into a neutral position. This actually isn't correct. If you were really driving, you would have pulled the steering wheel right then gone back to the left equally as far. This is what straightens out the car. Had you actually been performing the task, you likely would have done it perfectly. You just can't always recall a memory accurately (especially motor task memories). Your muscles know what they need to do, so your brain doesn't have to hold memories of each specific step.
So, we have these implicit muscle memories of how to do things. How much of this "memory" is within the brain or muscle is debatable. But it is an interesting thought that donating an elite athlete's muscle fiber into a novice might give them an advantage not just from an anatomy stand-point, but also due to a learning-curve. That muscle already knows how to be explosive or exert force. Whether or not the brain could create this action is also unclear (motor units between the brain and new muscle would complicate this). But if a transplanted heart can create a craving for KFC, maybe a transplanted muscle could teach someone how to squat better.
Considering all of this, when we train, we aren't just causing physiological changes. We're also creating learning opportunities, not just for our brains, but for our entire body. Your muscles are forming brand new memories alongside your brain each training session. We see evidence of these muscle memories in the nuclei and mitochondria, as well as in implicit memory. It is likely that muscle memory plays a key role in embodied cognition, and heavily influences how we perceive and interact with our environments. If it didn't, our bodies arguably wouldn't have evolved for muscles to have the capacity to hold memories.