Motor Patterns & Pathologies – Part 2: Motor Learning
One Experiment – Three Perspectives
Wynne Lee’s study in reaction time is virtually unheard of in biomechanics. She is a motor behavior scientist so I guess this is not surprising. The image below triggers such lively discussion that when I was able to confirm the details of the experiment with a real, living, breathing subject, I went “all in” and began reproducing the experiment in my neuromuscular therapy seminars. And, yes, my results are always the same – which is very important in front of an audience!
Figure 1. “EMG tracings during a reaction time study conducted by Wynne Lee”; IN: Anticipatory control of postural and task muscles during rapid arm flexion; Journal of Motor Behavior, 1980 Sep; 12(3):185-96. Reprinted with permission of the Helen Dwight Reid Educational Foundation. Published by Heldref Publications, 1319 Eighteenth St., NW, Washington, DC 20036-1802. Copyright 1980.
Though this was a reaction time study, as you see in the picture the subject is performing a simple arm raise task. The surprise finding is that the left hamstring contracts in advance of the left anterior deltoid. The researcher describes this as part of “anticipatory control” but this name, while interesting, is not the same as an explanation. So let’s look at the surprising data from three different perspectives, Biomechanics, Psychology and Motor Learning.
Biomechanics views the human body as a negative feedback system. An example of a negative feedback process is temperature control: the body heats up, sweat glands are triggered releasing moisture onto the skin at which point cooling begins.
In biomechanics, simply put, tissues are damaged when their tolerances are exceeded. The forces that impact the body may come from outside or inside the body, but reactions happen when actions demand them of the body – and never sooner. So how can a closed loop, negative feedback system explain how the nervous system somehow “knows ahead of time” to stabilize the center of gravity using a distant body part milliseconds ahead of an action that requires the resulting stabilization? This would be equivalent to sweating before getting hot or digesting food that hasn’t yet been eaten. Sure, we salivate in anticipation of food – but to transfer this idea to anticipatory muscle contractions is not biomechanics – it’s more like time travel or being psychic. So is there a biomechanical explanation for the hamstring firing ahead of the anterior deltoid? No. From a purely biomechanical point of view, the data makes no sense. What does make sense is the contra-lateral hamstring firing after the anterior deltoid – which also happens (see diagram above).
So the findings demonstrate that the body only partially follows biomechanical rules. Part of me wishes my findings would have refuted those of the original researcher. So, the difficult choice I had to make was to either ignore the data and stick with biomechanics, or seek another explanation. Fortunately, this study included, quite possibly originated, the term “feed forward” to describe the observation of apparent anticipatory muscle control. This term is used to describe how the system anticipates instability and feeds forward neural drive / electrical nerve energy to contract a stabilizing muscle. How it actually does this is not shared but it might be something anticipatory like shivering at the thought of going outside on a cold day – or salivating at the sight of enticing food. Where the trouble starts is that the stability is exactly appropriate to only that motion – even though there are an infinite number of arm movements possible at any time. In fact, notice how the contra-lateral hamstring does contract afterthe anterior deltoid – confirming the inclusion of perfectly normal mechanical stabilization.
Feed forward is a fitting name for what happens in this experiment. But, again: naming something is not the same as explaining it. So unless we’re psychic time travelers we need a better explanation.
Motor Learning & Neural Drive
As a computer programmer, my son explains a learning system this way: The system makes multiple attempts to accomplish preferred outcomes. It logs and repeats those actions that lead to desired outcomes and drops actions that don’t accomplish this.
The human being is a twenty-four / seven, always on, always adapting, learning system. Motor learning science provides the simplest explanation of the hamstring firing ahead of the anterior deltoid i.e. these actions always occur in this sequence – or always should – since one of the earliest learned motor patterns in the human motor system is the cross-crawl or x-pattern. When the CNS fires the hamstring in advance of the deltoid it does this because the actions were learned together as part of normal cross-crawl pattern. Interestingly, this pattern forms the basis of an individual’s gait pattern.
If ever there was an idea with consequences it is this: no muscle contraction occurs in isolation. Kinesiological sEMG studies repeatedly confirm that when shoulders and hamstrings function interdependently they are functioning normally. To recognize this as absolutely true means that effective rehabilitation of either the shoulder or the hip/thigh/leg complex is dependent on the proper function of the other.
Learned Motor Patterns Underlie the Elusive Twenty-Five Percent
The prevailing view is that biomechanical causes account for roughly seventy-five percent of what presents as biomechanical distress. Most biomechanists I know believe that given better technology, biomechanics will eventually find ways to solve the other twenty-five percent. I assure you this will never happen. My conclusion at this point, having studied and used kinesiological sEMG for two decades, is that when we take a closer look at the learned patterns that underlie the biomechanical events we see every day, we will discover the dysfunctional patterns underlying this elusive twenty-five percent.
In Installment #3 I will describe my approach to motor pattern testing (using natural observation via kinesiological sEMG). I will provide a list of common pathologies and share their underlying learned motor patterns.