Muscle activation – Are muscles simply reactors to bone and joint motion?

So I have seen various variations on Gary Gray’s view that muscles are reactors. I think this is spot on. Muscles ARE reactors.

I think what Gary meant by this was that generally we see muscles as concentric force producers. Actually during movement we tend to need to decelerate motion by eccentric contraction first. Think walking first we flex to attenuate gravity, ground reaction forces, mass and momentum before creating force to move. So we are reacting to forces acting on the body before we create concentric force.

Some people have added variations on this theme talking about bones and joints feeling movement and muscles being dictated to by to this motion.

At Cor-Kinetic we feel that view is not the full picture. Muscles are reactors, but not to bones and joints but to the brain. We must remember movement is not just a feedback model but also involves feed-forward too.

Bone and associated joint movement will create mechanoreceptor afferent (feedback) signals to the brain based on this information the brain may choose to allow the muscles to lengthen, shorten or even stay the same length. So in fact the muscles may dictate to the bones, something we see all the time when people lack ROM! Muscles can choose to limit the deceleration, in the face of force acting on the body, we go through rather than just dumbly reacting.

The muscle spindles are not just feedback mechanisms but also feed-forward (efferent). The intrafusal fibre tension is regulated feed-forward from the brain. This means that if spindle tension changes efferently then that will change the spindle gain and affect the afferent feedback information (change of length and rate of change of length). The brain can change the sensitivity of the information that it receives back from itself. Amazing.


We see this constant stiffness adjustment as we run on different surfaces. The leg spring stiffness remains constant although the ground stiffness changes. This means that we create changes in muscle tone through efferent changes in spindle gain even though the biomechanical bone motion remains fairly constant. Farley has done some great research in this area. Click here for research paper.

 So the brain will dictate the muscular “reaction” to the bone and joint motion, not simply react by moving. In fact we know that muscle activation patterns changes with pain not simply in response to bone motion. In face we develop protective motor patterns to deal with injury and many times these are not full rehabed back to pre injury movement capacity. In fact over a lifetime, especially if we are involved in sport, we pick up numerous injuries that change our motor patterns and create our individual movement signature.

“Although pain provides a potent stimulus to change the movement strategy to protect the painful or injured part, resolution of pain or injury does not necessarily provide a stimulus to return to the initial pattern"

 Hodges 2011

Great work into pain and changes in our motor control has been done by Hodges click here for his "moving differently in pain paper"

"Adaptation to pain has many short term benefits but with potential long term consequences"

Hodges 2011

So we can use our feed-forward control such as changes in spindle gain to protect against unwanted bone motion as perceived by the brain. Do simple changes in bone motion (especially passively created by external sources such as practitioners hands) create long term adaptations in motor patterns? In my experience not always.

We also know pain is a reaction to a set of patterns in the brain, whether this is a motor pattern or emotional or chemical pattern. It can even just be through the firing of a neuron associated with pain and an area of the body as in the 'neurotag' concept from Lorimer Moseley. This could even be through simply talking about pain or an area of the body associated with chronic pain.

We can force bones to create a reaction in a muscle. This may not happen when the body has to control its own movement based on its stored motor patterns and previous experiences that are much more complex than simple biomechanical movements.

The body is one massive feedback loop. Systems higher up in the brain beyond the spinal cord and cerebellum will also dictate how our muscles react. These can be Ocular, Vestibular, emotion, pain perception and previously stored patterns and responses.


One last thought. If the body was simply biomechanical and force based model then everybody with structural problems would be in pain and everybody who has a “good” structure would not be in pain. We know this is not true. People achieve amazing things and pain-free lives with academically terrible foot types and huge leg length discrepancies. Research supports this.

So what mediates their pain and performance responses? Why the brain of course. So what that means is for someone with a small leg length difference that maybe the cause of their pain, for someone else with a huge LLD it may not be the cause of their pain. Someone’s individual brain and CNS response will dictate this. This maybe why so many scientific study’s are inconclusive on many treatment types and structural abnormalities effect on the body. “Does not always” does not “mean will not ever” Can we have any definitive in the body? So the evidence based approach may disregard the right approach for the right person. It just means nothing is right for everybody all the time. But we knew that right? That's why some of the people I respect so much have such a multi faceted approach to treating the body and learning from a broad range of experts in different fields.

This means that people who tell you that your LLD IS the cause of your pain COULD be right 50% of the time or a certain foot type WILL cause you back pain maybe also right. That fascia is the key to everything POSSIBLY MAYBE right some of the time. But behind their certainty may not be anything real but just personal experience clouded by bias. As all of us are clouded by, myself included.

As the famous Betrand Russell quote goes "The trouble with the world is the stupid are cocksure and the intelligent are full of doubt" I doubt think this is literally true because people are sure AND intelligent. We just need to be more full of doubt in our methodology and be aware of other influences on the body and its pain and performance.

Another approach maybe to start to look at the influential mechanisms in the brain as well and improve the brains confidence and control of areas that it may perceive as a “hazard” to the system. This could happen at a local joint level or a higher cortical level or a bit of both!

Not the most scientific blog but thanks for reading.

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