Windhorst U. Muscle proprioceptive feedback and spinal networks. Brain Research Bulletin 2007, 73:155-202.

This article, which provides an amazing review of certain aspects of afferent information from sensory mechanoreceptors, has been discussed in a blog in the Proprioceptors Series from Gray Institute®. The article covers experiments primarily in animals, but also some studies on human movement. In spite of the title, it covers a lot of research on the information provided by the Golgi Tendon Organs (GTOs). It is this aspect of that content that will be the focus of this blog.

Specifically addressing the effects of signal from the GTOs on spinal inter-neurons, Windhorst writes that “…these effects are state-dependent, that is, dependent on the context and task of motor acts.” The simplistic view of the information from GTOs from early research studies was that discharge from a GTO would send information to the spinal neuron that would then inhibit the muscle connected to that tendon. This is true is the limb is passively lengthened. This is referred to autogenic inhibition. It was postulated that this inhibition of the muscle contraction would be a protective mechanism.

More recent work has determined that the discharge from the GTO and the influence on the muscle is not so straightforward. This is where the context and task come into play. If the animal is in a weight-bearing position, the GTO discharge would produce a facilitating effect to enhance the force production to maintain stance, not inhibitory. So it appears that other proprioceptive inputs about the weight-bearing limb alter / modulate the response to the signal from the GTO!

Why would this knowledge about GTOs in an animal “matter for function”? From the limited experiments on humans that monitor neural discharge, there is much agreement with what has been learned from animal experiments. If the spinal inter-neurons respond the same way in humans, then this information combined with knowledge from motor control and dynamical systems theory suggests how critical it is for our testing and training movements to be authentic to the activity.

For many years now, Gray Institute® has “sounded the call” that function is task-specific and context-dependent. Once we understand the Chain Reaction® Biomechanics of any activity, the movements selected to challenge our patients / clients must be as similar as possible to the activity itself. If we want a specific output (motor response) the movements must provide the specific sensory input.

The task of writing a word can be used as an example that might provide some insight to this point. In order to write a specific word, the writer must have the ability to write each letter that makes up that word. Knowing how to write the letters C, A, and T will not help you write DOG. It is task-specific. This is analogous to having the joint motion and muscle force resources. Once all the muscle actions and joint motions to write D, O, and G have been learned, the task is complete but then context dependency may come into play. Writing a word on a piece of paper with a pen has a very different context compared to writing the same word on a blackboard with chalk. The joint motions and muscle actions required are very different.

Similarly, flexing your knee sitting in a chair is very different than flexing your knee against a resistance, and completing dissimilar to flexing your knee in a standing weight-bearing position. Task and context are critical elements to accomplishing any movement. Movements for assessing and training our patients / clients must try to replicate (as much as possible) the function that our patients / clients desire. The proprioceptors will provide the proper sensory input to produce the desired motor output. But the joint motions that provide that the sensory input must be part of authentic global movements. What gives a movement greater authenticity can be surmised by looking at the Gray Institute’s Functional Movement Spectrum. Based on the Principles of Applied Functional Science®, the Functional Movement Spectrum document serves not only as a litmus test for the “functionality” of our movements, but provides unlimited “suggestions” of how to make a movement more functional.