The Mind Muscle Connection
Monday 14 November 2016
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No the title of this post is not referring to “muscle memory”, and for the record there is no such thing as “muscle memory”. Muscles do not possess the capacity to remember anything, and it has yet to be shown in any form that muscles themselves have the ability to “get back into the groove” of a motion after a few repetitions. The term muscle memory actual refers to the ability of a previously practiced skill to return quickly due to regrowth of synapses and improved cortical firing of an already developed motor program the brain has stored for a task. The rapid gain in strength seen when returning to lifting weights after a prolonged period off is due to improved neural function, improved motor unit recruitment, and increased central nervous system drive.
As a side note there is a possibility of improved muscle protein synthesis and increased speed of muscle growth in previously trained individuals or those who have previously used anabolic steroids. This however, is due to the increased density of myo-nuclei that form on skeletal muscle cells with high level strength training and/or anabolic steroid use. Skeletal muscle cells are interesting due to the fact that they are “multi-nucleated”, and have the ability to increase the density of nuclei as a response to training stimulus and need for additional protein synthesis capacity. Interestingly, it does not appear that this up regulation in nuclei in the muscle cell drops back down to baseline following cessation of training. Therefore, it is POSSIBLE that muscle growth could in fact happen at a faster rate the second or third time around due to improved muscle protein synthesis capabilities supplied by the increased number of nuclei. This is the closest possible physiological example of what could be referred to as “muscle memory”. However, this still would not account for the rapid increases in strength seen in the first 2-3 weeks of training.
Mind muscle Connection:
The actual “mind muscle connection” refers much more to the actual chemo-electric connection between the primary motor cortex, the corticospinal tract in the spinal cord, the peripheral nerves, and the neuromuscular junctions. It is the physical ability of the brain to generate electrical activity to cause a muscle contraction through the intricacies of the above listed pathways. It is well known that the rapid early strength gains seen in resistance strengthening are much more so attributed to improving the efficiency of this neural pathway as opposed to actual muscle hypertrophy. Anecdotally, in the body building and S&C world it is also well understood that visually watching a muscle contract as you use it and focusing on creating high amounts of tension (higher than necessary to lift the weight) in a muscle under load leads to far superior strength and size gains. There is also a plethora of research to demonstrate this as related to cortico-motor excitability with mental practice and studies on intrinsically generated muscle tension during exercise here, here, here, and here.
Why does it matter?
To anyone in the business of making individuals bigger, stronger, or more aesthetic this topic of mind muscle connection is of the utmost importance. Using mental focus and practice through flexing, posing, concentration reps, etc to improve muscle recruitment and contraction efficiency are principal methods to improve training and muscular development. Yes, you read that right. I just justified posing practice and watching your muscles closely during a workout to achieve maximum results. In additional to increasing central nervous system drive as listed above; creating a strong contraction in the acting muscle one can actually increase the amount of tension generated by the muscle subsequently increasing the work load and training stimulus.
But what about those of us who work with those who are injured? I would argue that this topic becomes even more important when with working with injured clients or when trying to train around an injury yourself. Take for example that immobilization of a limb following injury is common practice, and has well documented acute and chronic maladaptive effects, including loss in strength and muscle atrophy (Berg et al., 1991; Deschenes et al., 2002; Lundbye-Jensen & Nielsen, 2008). Just as with the rapid increase in strength early on in training, neurophysiological factors have been suggested to contribute to the loss in strength, particularly to account for the rapid strength loss within the first 4-weeks. Neural alterations that have been observed with limb immobilization include motor unit firing rates (Duchateau & Hainaut, 1990), rate of force development (Suetta et al., 2004), and neural (corticospinal) excitability (Clark et al., 2008).
Ok, so now we know what happens when we immobilize or stop using a particular muscle, muscle group, or limb. The exact opposite of the beautiful adaptations that occur during early overloading creep in and create significant weakness and a form of neural atrophy. The question is whether or not we can do anything to combat these changes and either save the function of a client or hold on to our precious gains as we work around and through an injury. The short answer is yes. For the longer explanation please read on.
Cross education:
What is Cross education? Cross education is a neurophysiological phenomenon where an increase in strength or skill is witnessed within an untrained limb following unilateral strength or skill training in the opposite, contralateral limb. It is hypothesized that this phenomenon is due to factors both at the muscular, peripheral nerve, spinal, and cortical levels.
As described by Pearce et al 2013:
“Cross-education of strength occurs when strength training of one limb (unilateral training) results in an improvement in strength in the homologous muscle of the opposite limb (Zhou, 2000; Farthing, 2009). The most recent meta-analysis of cross-education research from 16 studies (Carroll et al., 2006) calculated a mean increase in strength in the untrained arm of 7.6%, corresponding to approximately 52% of the strength gained in the trained limb. It has been suggested that supraspinal mechanisms are responsible for the cross-education transfer effect, with two recent studies providing evidence of neural mechanisms following unilateral strength training. Farthing et al (2007) used fMRI in a sub-sample of a larger study of individuals who performed 5-wks of unilateral maximal isometric ulnar deviation. These authors showed an increased area of activation in both sensorimotor cortices as well as ipsilateral activation in the left hemisphere for the left arm. Further, identification of new activation in the left temporal lobe, was also observed, an area involved in the retrieval of motion knowledge and semantic memory. “
“These authors hypothesized that the area of new activation may suggest memory formation related to the specific motor control requirements of the strength exercise (Farthing et al., 2007). In other words, the strength task may have had a motor learning effect that was reflected in improvement in performance (strength) in the untrained left arm. More recently, Kidgell et al (2011) used transcranial magnetic stimulation (TMS) to investigate the corticospinal projection to both the trained and untrained arms following 4-wks of heavy load (80% of 1 repetition maximum, RM) unilateral bicep curl training. A 28% increase in 1-RM right elbow flexion strength resulted in a 19.2% increase in strength of the left untrained homologous muscle. An increase in corticospinal excitability was observed in both the trained and untrained arms. Both these studies, however, did not involve immobilisation of the untrained limb.”
“Cross-education studies have progressed to investigate models whereby one limb is immobilised. Farthing et al (2009) demonstrated the maintenance of strength and muscle thickness following immobilisation of the wrist with unilateral strength training of the opposite arm. Similarly, Magnus et al (2010) have recently presented similar data following immobilisation of the arm. Maximal isometric strength training of the biceps and triceps for 4-wks demonstrated retention of strength and muscle thickness in the untrained arm with no change in maximal voluntary activation or electromyography (EMG). Recently, Farthing et al (2011) using fMRI, demonstrated cortical mechanisms mediating retention of strength and muscle thickness in the immobilised wrist following grip strength training in the contralateral hand. Although the recent study by Farthing et al (2011) did not demonstrate a clear maintenance of muscle size affect, collectively, the Farthing et al (2009) and Magnus et al (2010) investigations demonstrated evidence of muscle size maintenance of the immobilised arm when the free limb was exposed to training.”
Mental imagery:
Mental imagery, mental practice, or visualization is a technique that has been used by sports psychologists for years to improve athletic performance on the playing field. However, there is quite a bit of literature to suggest that the power of mental imagery goes much deeper than merely decreasing anxiety through mental practice. In fact the studies below describe significant neural activity that occurs during imagery that can directly mimic actually physically performing the imagined action.
“We suggest that the IMI training likely strengthened brain-to-muscle (BTM) command that may have improved motor unit recruitment and activation, and led to greater muscle output. Training by IMI of forceful muscle contractions may change the activity level of cortical motor control network, which may translate into greater descending command to the target muscle and increase its strength.”
“Motor imagery can influence muscular abilities such as strength and power and can modify Movement Related Brain Macropotentials”
“The mental practice training program with motor imagery-associated cortical feedback facilitated motor excitability during the production of voluntary motor control. Motor imagery-based mental practice training with movement-associated cortical activity feedback may provide an effective strategy to facilitate motor recovery in brain injury patients, particularly during the early rehabilitation stage when full participation in physical and occupational therapy programs may not be possible due to excessive motor weakness.”
There are twos subtypes of imagery termed internal and external imagery. Using internal imagery (also known as kinesthetic or first-person imagery), one imagines or mentally creates the physical feeling of performing the exercise from within your body. In contrast, using external imagery (or third-person visual imagery), one visualizes performing the task from outside your body—similar to watching yourself in a movie. Interestingly, Internal imagery appears to generate significantly more physiological responses such as changes in heart rate, blood pressure, and respiration rate compared to doing external imagery.
Attentional Focus:
To add an additional degree of complexity to this topic it has been extensively shown that focus of attention during movement has far greater outcomes on performance when attention is directed externally as compared to internally. Simply put, focusing on the target or outcome improves performance to a much greater degree than focusing on the internal components that make up the movement or task.
A great example of this is focusing on the basketball goal when shooting vs focusing shoulder flexion, elbow extension, wrist flexion, etc will result in much greater performance.
Conclusion:
Cross education, mental imagery, and attentional focus are viable and excellent training/rehab modes that can be used to aid in improving performance, decreasing atrophy, retaining and improve skill. It is of course extremely important to determine that goal of the intervention you choose.
- In the case of rehab during or after injury/immobilization cross education and mental imagery can be utilized to decrease the amount of atrophy incurred, retain “crisp” neural pathways, and retain movement skills.
- Attentional focus can be used internally to increase muscle EMG activity which may aid when training for strength and hypertrophy.
- Conversely, attentional focus should be utilized externally when focusing on skill refinement and improvement in performance.
- It is extremely important to recognize how important the brain and rest of the nervous system are to retaining and making improvements in strength and function.
As always thanks for reading,
Jarod Hall, PT, DPT, CSCS