Neuromechanics Weekly: PART 2:

Stretching increases strength in contralateral muscles?

Lots of cool links in this post. please try and find time to check them out.

Figure it out?  Ever wonder about some of the magic behind some of those manual therapy techniques that are out there ? Sometimes it is not magic at all !

There are 2 related reasons we can think of to cause this seemingly odd length-strength phenomenon (OK, there are more, but this is what we are going to cover today):

• Reciprocal Inhibition
• Crossed extensor reflexes/responses

We remember reciprocal inhibition (as demonstrated in LEFT picture above) is when we activate or stimulate a muscle, the Ia afferent from that muscle stimulates that same muscle to contract (this is how a simple reflex arc works) and, through an inhibitory interneuron, inhibits the antagonist muscle on the opposite side of the joint.

Remember, that Ia afferents go to muscle spindles (don’t remember? look here); they respond to LENGTH changes. Wouldn’t you say stretching affects length? If we were talking about the R tricep surae group, we would be inhibiting the R anterior compartment.

But wait, the article said it affects the opposite side….Of course, there is more…

The picture on the right shows the crossed extensor response or reflex (don’t remember? look here). In a nutshell, when you FIRE the flexors on one side, you INHIBIT the extensors on the same side (sound like reciprocal inhibition? It should… it is : ) You also FIRE the extensors on the opposite side while INHIBITING the flexors on the opposite side. (Yes, the opposite side extensors will inhibit the opposite side flexors as well. Yes, this is also reciprocal inhibition).

But wait, that means the opposite calf would be weaker, not stronger, right?

It would be weaker if being called upon to be used at that moment in time, BUT in the study, stretching increased ROM of the stretched calf 8%, with a 1% loss of ROM of the opposite calf (study summary).

Hmm… sounds like shortening to me. That would mean that those spindles (ie the opposite calf)  would be MORE RESPONSIVE to stretch (ie a change in length; and coincidentally, the Golgi’s more responsive to the tension change) . And what happens when we preload a neuronal pool? The likelihood of firing is increased (like doing a Jendrassik maneuver to increase a reflex). The rest is neural adaptation (strength gains initially are due to increased efficiency of the nervous system. For a review to see our video on this, click here)

Interesting that one of the comments on the article was “I don’t have the full text of the paper but a summary prepared by Chris Beardsley and Bret Contreras states that one of the mechanisms for crossover in the case of unilateral strength training is thought to be modulation at the spinal cord level.”   Could they be talking about reciprocal inhibition and crossed extensor responses?

Wow! Very cool! And to think, you knew the answer. We are proud of you!

Ivo and Shawn…Neuro Geeks too!  And applying it to gait, running and motor patterns of all types !