Oct 19, 2017Change of Mind
Historically, rehab from musculoskeletal injury has entailed reducing pain, recovering joint range of motion, restoring muscle strength, and, finally, reestablishing neuromuscular control capability prior to return to play. Yet little attention has been paid to the neuro aspect of neuromuscular control training. Here’s why that should change.
Mounting evidence suggests what we once thought were isolated musculoskeletal injuries that required interventions at the joint or surrounding muscles actually cause a cascade of consequences across the central nervous system (CNS). As a result, the CNS has to compensate through a variety of mechanisms, from locally at the joint/muscle, to the spinal cord, all the way to the brain’s cortex.
These neuroplastic changes are then reinforced during typical rehabilitation. Clinicians often start a rehab protocol by having athletes watch how their recovering limb moves during exercises. This shifts the way the brain processes information — changing its circuitry from sensory to visual motor control. So by the time the athlete gets back to activity, they are relying more on vision than instinct to move the injured joint. In sports that require athletes to constantly make decisions on where to go and how to move, this could be a hindrance.
Fortunately, recent research may have found a way to “reset” the brain back to its original sensory orientation to improve range of motion, increase confidence in the injured joint, and avoid reinjury. The key is including the brain in rehab and intervention efforts.
CHANGES TO THE BRAIN
Before we get into the specifics of rewiring the brain during rehab, we should discuss the ways it adapts following injury. This process begins with a reflexive inhibition or shutdown of the musculature surrounding the injury site, termed “arthogenic muscle inhibition.” In turn, this causes motor compensations at the proximal and distal joints to the injury and even the contralateral limb, which result in further neuroplastic changes due to the development of new motor control strategies to maintain function. The brain then undergoes alterations in how much activity is required to generate movement, how various sensorimotor regions are connected to each other, and the excitability of the motor neurons that control muscles around the joint.
Recent work indicates external focus training activates more frontal, sensory, and memory brain regions relative to internal focus — even when doing the exact same movements. This altered brain activity during external focus likely contributes to improved motor pattern retention and transfer to sport.
For example, in a series of landmark studies, Lepley, Pietrosimone, et al., demonstrated the longitudinal changes that occur in muscle activation and spinal and motor cortical excitability (the ability of neurons to respond) after joint injury. Immediately after the injury, spinal excitability is depressed. Over time, cortical excitability decreases, as well. This means that it will be neurologically harder to generate muscle contractions that originate from the brain, as well as respond with reflex contractions from the spinal cord.
The interaction between spinal and motor cortical excitability is believed to reflect a post-injury joint protection strategy, leading to an increased need for cortical drive to engage in muscle contractions. These neural changes are due to a combination of the lost sensory input from the injured joint, motor changes developed after the injury, and altered input into the motor cortex from the rest of the brain, including pre-motor (motor planning), sensory, visual-spatial, and postural control regions.
In all, this neuroplastic cascade to sustain motor output after injury without the expected sensory input causes the CNS to re-weight sensory processing for motor control. This means the CNS increases the activity of brain regions that process other aspects of sensory-motor function, such as visual feedback.
For instance, as a hypothetical, say the motor cortex required 100 units of sensory inputs to program a particular motor output before injury — it got 34 from proprioception (including joint receptors and muscle spindle/golgi tendons), 33 from vision, and 33 from vestibular input (such as body awareness, upright posture, and gaze control). When one of these categories is disrupted — as is the case following injury — the relative “weights” must change to generate sufficient output to the peripheral nervous system and muscles. So the athlete may instead get 20 units from proprioception, 47 from vision, and 33 from vestibular. It is this disrupted post-injury sensory input and the CNS’ associated sensory re-weighting that likely lead to changes in motor cortex excitability.
REINFORCED BY REHAB
So what’s the connection between neuroplastic changes and rehab? Researchers have found that brain regions dedicated to visual-spatial awareness, memory, and visual-proprioceptive integration become more active after joint injury, along with raised levels of pre-motor and motor cortex activity. These increases are likely due to a combination of the sensory re-weighting already discussed, as well as the rehabilitation and training provided by clinicians.
After injury, athletic trainers typically instruct athletes to stare at their injured limb and generate muscle contractions for weeks or months. This reinforces the use of visual-spatial awareness for limb control. As a result, to engage in joint movement or achieve restoration of muscle contraction capability going forward, the brain will rely on visual feedback to provide input to the motor cortex, as opposed to sensory feedback.
Of course, this is not how athletes control their limbs in sport. After all, a typical soccer game requires hundreds of decisions on where to move next, how to position the body, where the ball is going, what other players are doing, and on and on. With all these thoughts racing through their minds, healthy athletes allot few neural-visual-spatial resources to the control of isolated muscles or limb position in space. But after injury, this constant cognitive stimuli is replaced with a singular focus on reengaging a specific joint or muscle, taking the immerse complexity of athletic activity and distilling it down to simple motor control recovery.
In addition to sensory-visual-motor-related plasticity, changes to motor planning and motor execution occur as a result of standard rehab practices. After injury, increased motor cortex and motor planning region activity is needed to execute the same movements that were done effortlessly before injury. Since motor planning and execution neurons are finite, increased activity to complete simple movements is not ideal, as activity in these regions rises as tasks become more difficult. This means that as complexity of movements increase during athletic activity, the CNS may lose the ability to cope and allow high-risk movement patterns during landing or cutting — putting the athlete at risk for reinjury.
REWIRING THE BRAIN
To avoid the consequences that can accompany neuroplastic changes after injury, athletic trainers can make slight changes to their rehab approach. A good place to start is to determine if an athlete has an over-reliance on visual feedback. This allows clinicians to identify athletes who most need brain-centric therapy after an injury.
One way to ascertain visual feedback over-reliance is to challenge the athlete with specific visual-motor assessments, such as the Dynavision. In our lab, we have been experimenting with new additions to more typical physical function assessments. For instance, during single-leg hop for distance tests, we use timing gates to record reaction time after a go signal to get both visual-motor reaction time and hop distance in response to a variable visual target.
A more clinician friendly test that requires no tools is having the athlete jump three times in place on one leg, going for max height and trying to stay in the same footprint. Then, have them repeat the task with their eyes closed. Those who land in the same spot are unlikely to have a high reliance on visual feedback for motor control. On the other hand, athletes who struggle may benefit from increased therapy in this area.
An easy way for athletic trainers to start to address visual feedback reliance for motor control is by rephrasing feedback. The cues given during exercise instruction can have major effects on the athlete’s subsequent brain activity. External feedback that is centered on the environment (i.e., contract to a set target position, keep your knee over a line, or bend to the chair or cone) versus internal feedback (i.e., contract your quad, knees over toes, or bend your hips) can influence how the brain activates to execute a movement.
Recent work indicates external focus training activates more frontal, sensory, and memory brain regions relative to internal focus — even when doing the exact same movements. This altered brain activity during external focus likely contributes to improved motor pattern retention and transfer to sport (because in sport athletes attend to external factors like the environment, the ball, players, field, etc.).
A more direct approach to avoid sensory re-weighting that is over-reliant on the visual system is to directly knock down visual feedback. The rationale for this is that the feedback into the sensory system is damaged following injury and may never be restored to pre-injury levels. Therefore, the nervous system will permanently continue to weight toward visual feedback for motor control, unless a therapist intervenes. Since it is difficult to force increased utilization of the sensory system directly, knocking down the visual system can be a way to induce sensory re-weighting back to pre-injury levels.
One low-cost and easily accessible method for this is virtual reality (VR). Headsets like Google Cardboard — a piece of cardboard that folds into a box for a smartphone to set in — can be purchased for $10, or more comfortable plastic sets can range from $20 to $30. These tools generally accept most smartphones, and the variety of free VR apps available give clinicians an immense opportunity to perturbate visual-motor function during rehabilitation.
If you don’t want to use an app, you can simply take a 360° picture of the athlete’s field or court and upload it on the VR display. This way, the athlete can complete on-the-table rehab exercises to restore range of motion or strength without visually focusing on the injured joint, as the VR headset forces them to look at whatever is on the screen. However, the motor memory to increase motion or muscle contraction capability is still being created. By viewing the playing surface instead of the clinic during motor training, the improvements gained will be encoded with the visual presentation of the athlete’s sport, possibly improving the transfer of therapy gains to athletic activity.
Once the athlete has progressed in rehab, a more advanced method is to incorporate VR during static balance training. Have them use the VR headset and any of the free rollercoaster apps available to induce not only a visual field distraction, but also a vestibular perturbation, as the visual field will rapidly change without altering the athlete’s static position. This can be quite challenging, so have athletes start by sitting down and progress to standing on both legs before moving to a single-leg balance position.
While VR is a promising way to target visual-motor integration in therapy, its use is limited to static, on-the-table exercises that involve little mobility or environment interaction. But other methods are available to modulate visual feedback during more advanced training. These range from covering an athlete’s dominant eye to involving a ball, target, or other person in the visual field. A wealth of biomechanical data has demonstrated that neuromuscular control during landing, cutting, or other athletic maneuvers decreases when adding these elements of anticipation, dual-tasking, or external targets. Therefore, including them in training may help with the transition back to sport.
Stroboscopic glasses are another great tool for trying to force sensory re-weighting during advanced training. The glasses shutter between opaque and transparent view fields. The time in each condition can be customized by the patient or clinician, allowing progression through the visual feedback perturbation. And because the glasses decrease visual feedback, they may restore sensory weighting back to proprioception.
An aspect of the glasses that makes them beneficial for later stages of rehab is that they can be worn during any therapy or exercise. This gives the clinician the ability to modify visual feedback but leave enough vision for the athlete to complete any activity, including tracking external objects (like a ball) or high-speed movements.
Overall, I believe neuroplastic changes after injury can have implications for how athletic trainers will approach rehabilitation in the future and may play a role in dictating the successful return to high-level sport function. I encourage all clinicians to consider the cognitive and neural control complexity associated with sport activity during recovery in addition to the classic physical components. Remember, in rehab, we must not only restore muscles that have been disrupted but the brain, as well.
This article first appeared in the September 2017 issue of Training & Conditioning.
To view the references for this article, go to: Training-Conditioning.com/References.