About 8 years ago I gave a presentation titled “Motor Unit Discharge Variability” for an Exercise Neuroscience class I was taking in grad school at UMass.
The precedent for studying variability in neural firing rates stemmed largely from studying variability in heart rate. At the time, I would have assumed that a more variable heart rate would be problematic. Without a background in the area, I deferred back to hearing the steady “beeps” when people were monitored in a hospital setting on TV, and when things sped up or slowed, it was generally a disastrous sign.
Of course, there are more credible ways to gain a deeper understanding of human physiology than ER and Grey’s Anatomy. In reality, increased heart rate variability (HRV) was universally regarded as a positive measure. In fact, a decrease in HRV is predictive of death from a heart attack or heart failure. The rate at which the heart discharges is controlled by the autonomic nervous system, which is subject to feedback/influence from a number of systems within the body. In the interest of simplicity, the brain controls heart rate, and in this instance, increased variability equates to better health.
Read more about how HRV can be used to influence your training here: BioForce HRV
Motor units, for those of you that don’t read about neuroscience on the weekends (or slept through your freshman year anatomy course) is an individual motor neuron and all of the muscle fibers it innervates. It’s referred to as the smallest functional unit of the neuromuscular system. When we need to produce force, or move, the brain sends a signal down to a group of motor neurons, which signals the muscles to contract.
There are several different ways to measure, and therefore discuss, the role of variability within the neuromuscular system. On the scale of an individual motor unit, variability in firing patterns at low force levels may lead to less controlled movements; this is particularly relevant for fine handling skills in older adults.
On the other hand, when a motor unit fires twice within a short period of time, known as a “doublet” discharge, force is increased faster and to a higher level than if those two impulses were spread further apart.
In other words, a more variable discharge allows for a more rapid accommodation/adjustment to force requirements.
Similar arguments can be made about the coordinated firing of multiple motor units (i.e. synchronization), and the rotation of motor units firing to maintain a given force output in any given position/pattern (e.g. if you hold a split squat for as long as you can, your brain will naturally rotate which motor units are active, and to what extent, in an effort to prolong your ability to hold the position). Similar to HRV, the common “controller” of much of this variability is the nervous system, which we’ll uncomfortably oversimplify as “your brain”.
Taken together, these measures of “nervous system variability”, of which there are many more, demonstrate how the body finds different strategies to accomplish a given task. Variability implicates a more dynamic, flexible, and resilient “system”. This idea has important implications for how we program and coach movement in training settings.
Since Gray Cook and Mike Boyle first popularized the Joint By Joint Approach to training (more on this topic here: The Mobility-Stability Continuum), mobility work has received a lot of attention. The term mobility is used in a variety of different ways, but within this context it really just means ensuring that each joint possesses the full range of motion the individual’s structural anatomy allows.
A lack of mobility at a given joint is likely to lead to excessive accessory motion within the same joint (which is potentially pathological) and/or compensatory motion at a neighboring joint.
Another way to view the issue is that by limiting range of motion in one or more planes/directions of an individual joint, the strategies that joint has available to accommodate different movement patterns or force requirements is limited. By repetitively overloading a narrow range of inter-joint movement possibilities, there’s an increased risk of a given structure within that joint breaking down (e.g. overstretching a ligament, wearing through a labrum/meniscus, etc.).
Imagine slowly pouring a bucket of water on a level area of sand. Because there are minimal restrictions to where the water can go, it will spread out and absorb into the sand with minimal changes in the shape/position of the sand. In contrast, if you dug a small trench in the sand with 2 of your fingers and poured the water at the beginning of that trench, the water will flow down the trench, dragging more sand with it, both deepening and widening the groove. This erosion is analogous to what happens at joints that posses very narrow movement opportunities, a concept I refer to as “Micro-Movement Variability”.
Just a small groove in the sand (Image Credit: Here)
This highlights the importance of using whatever strategies you’re competent in to maximize individual-specific joint mobility. Simply, if the joint doesn’t have the motion, it can’t use it. This will not only negatively influence the health of a single joint, it will also limit the quality of more global movement patterns. When you attempt to integrate one or more joints that have compromised movement variability into a more complex movement pattern, your body has very few strategies available to it to accomplish that pattern.
Consider, for example, an individual attempting to perform a reverse lunge with limitations in ankle dorsiflexion or hip extension. An inability to load through the back ankle will cause them to either turn their back foot out more or lean forward more to unload the ankle. An inability to extend fully through the hip will cause someone to arch excessively through their lower back, lean forward more to “cheat” hip extension, or simply to shorten the stride. In every case, the position of one or more joints is being torqued through undesirable positions and various soft-tissue structures are being predictably overloaded as a required compensation for the lack of individual joint mobility available to pattern the movement around.
From a more athletic perspective, one thing common to many elite performers is their ability to execute a specific skill in a wide range of positions/environments. Using a variety of strategies to accomplish the same outcome is what I refer to as “Macro-Movement Variability”.
For example, consider a quarter back that has to hit a receiver on the same spot of the field under different conditions: sitting in a well-protected pocket, adjusting to a collapsing pocket, escaping a collapsed pocket. All of these scenarios lend themselves to further variability: Is the defense blitzing? Are their hands in the way of the optimal flight path? Where are the defenders on the field? All of these things need to be accounted for to execute the pass, and many of these things will be entirely different the next time the same play is run.
Very different foot placements, movement strategies, arm motions, and defensive looks integrated into these highlights.
In hockey, the best shooters can pick their spots while facing the net, facing perpendicular to the net, while stationary, while moving, with sticks in the way, under pressure, etc. All that matters is that the “task” is executed effectively. Those players with better Macro Movement Variability will be able to produce a similar outcome under much more variable conditions.
Pay particularly close attention to the different areas on the ice, different challenges from defenders, and different body positions he scores backhand goals from.
Understanding micro- and macro-movement variability should inform a more strategic approach to both training and skill development. Micro-movement variability helps highlight the importance of assessing athletes before writing their programs, an idea I discussed in depth in my DVD Optimizing Movement.
You need to know whether the individual can get into and control the positions you’re asking them to overload with exercises.
It’s also important to recognize that the consequences of bad exercise selection aren’t always immediate. Similar to erosion, sometimes things will break in big chunks, but issues are more likely to accumulate over time. Having a team of athletes with significantly compromised ankle mobility back squat to full depth is foolish, even if no one gets injured on the first day. Micro-movement variability should drive exercise selection, and ultimately how you teach/coach an individual to perform the exercise correctly.
Macro-movement variability further highlights the importance of integrating variation into exercise selection, while still sticking within the individual’s movement capacity. Exercise variation teaches the body to produce force in different positions, requiring variable stabilization strategies.
It also changes which structures are being loaded, therefore minimizing the risk of overloading any one area of a structure (e.g. a specific section of your hip labrum). This strategy produces a stronger, more resilient and durable athlete, even if 1-RMs in a specific lift are compromised by rotating it out for a phase or two.
Macro-movement variability also has implications for skill development. Athletes should be practicing rehearsed skills in a variety of body positions and movement directions. For example, many youth hockey players will set up a pile of pucks and shoot, stationary, from the same location. This is not bad, only limited. Instead, they should practice shooting from a variety of different positions on the ice while directly facing the net, “off-centered” so they’re turned slightly away from the net, with weight shifted more on one side, with the back leg dragging behind them or down on the ice, etc.
Challenging athletes to execute their skills in unpredictable environments better prepares them for the open-loop nature of team sports.
This is also one of the most overlooked arguments for the importance of playing multiple sports through adolescence. Different sports not only require different skill sets, but different pattern recognition tendencies that can be transferred to specific situations within a different sport.
Variability is a symptom of a dynamic, flexible, and durable system. Micro- and macro-movement variability create the foundation for both the development and execution of elite athletic skills, as well as an athlete’s durability. Understanding these concepts should therefore underlie the entire athletic development process – from intake assessments, to designing training programs, to in-sport skill development and practice planning.
To your success,
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Kevin has rapidly established himself as a leader in the field of physical preparation and sports science for ice hockey. He is currently the Head Performance Coach for the Boston Bruins, where he oversees all aspects of designing and implementing the team’s performance training program, as well as monitoring the players’ performance, workload and recovery. Prior to Boston, Kevin spent 2 years as an Assistant Strength and Conditioning Coach for the San Jose Sharks after serving as the Director of Performance at Endeavor Sports Performance in Pitman, NJ. He also spent 5 years as a Strength and Conditioning Coach with USA Hockey’s Women’s Olympic Hockey Team, and has been an invited speaker at conferences hosted by the NHL, NSCA, and USA Hockey.