Qualities Of Muscle And Changes In Sensation
Once in a while, I’ll get called out on something in a blog post.
Fortunately, I don’t take it personally anymore, cause well…we all make mistakes.
In one particular instance, it wasn’t so much a mistake as it was just something I hadn’t remembered learning about, so the post wasn’t as clear and concise as it could or should have been. So in doing some research on the topic, it essentially led to writing this post (but I’ll still need to go back and edit the other one afterwards).
The post was about whether we lengthen a muscle in a treatment and the person said that I didn’t take muscle extensibility into account, so it made the information sound wrong (that’s not what was said verbatim, but it’s the general gist of it).
But there are some other physiological properties of muscle we should take into account as well so that we can always give our patients an accurate description of what is going on with them.
Or more accurately, electrical excitability.
Action potentials, or, an impulse travels along a cell’s membrane due to voltage-gated channels. In a muscle, there are autorhythmic signals that come from the muscle itself (think the heart). The other is chemical stimulus like neurotransmitters released by neurons, hormones from the blood, or local changes in pH level.
This quality of excitability is one of the things that generates movement, the muscles respond to these electrical, or chemical stimulus and produce tension or movement via a contraction.
Contractility is the ability of muscles to contract forcefully when stimulated, thus generating tension. If the tension generated is enough to overcome the resistance of an object, the muscle shortens and movement takes place.
As we mentioned with excitability there is an electrical stimulus, when a stimulus is generated it then creates a twitch response (or contraction), one contraction or twitch response occurs for each stimulus. So, contraction begins as an electrical stimulation, which in turn becomes a mechanical response, thus overcoming the resistance of that object you are trying to lift.
This is probably one of the main things we are trying to have an effect on when working through rehab exercises with our patients, making contractility easier, so they can continue on their activities of daily living pain-free (and yes I realize we are having an effect on the other qualities as well).
Elasticity is the ability for a muscle to return to its original length and shape after a contraction, or extension.
When looking through textbooks most refer to plastic and viscoelastic properties which enable this return to length to occur. However, as you will see when we talk about extensibility below, this may not be the case.
So, this is where I got called out!
Extensibility is the ability of a muscle to be stretched to a predetermined endpoint of sensation and then return to its original shape when the stretch is removed, without being damaged.
If you remember talking about agonist and antagonist contractions in college, this demonstrates an example of extensibility. As you flex your biceps (agonist), the triceps (antagonist) have to relax and stretch (or lengthen) in order for the contraction to take place.
But in order for a muscle to have extensibility, it also has elasticity, which enables the muscle to return to its original, normal length.
This is where research must come in. There are a number of theories used in trying to explain how muscles can be lengthened, or have extensibility increased. Most of them are trying to say there is a mechanically increased length of the muscle but is this really possible?
There are four of these theories and here are their explanations:
- Viscoelastic Deformation
- A decline in resistance to stretch after being held in a stretched position for a period of time.
- Plastic Deformation Of Connective Tissue
- Increase in muscle length, immediately after stretching that causes a muscle to remain in a permanently lengthened state.
- Increased Sarcomeres in Series
- If muscles are immobilized in fully extended positions, there is an increase in the number of sarcomeres in series.
- Neuromuscular Relaxation
- Slowly applied stretch causes neuromuscular reflexes to relax the muscle.
However, most of the above theories have been done as animal tests, could not be performed on humans, provided no evidence of length change according to the description, and provided no lasting change.
Ironically I would have thought neuromuscular relaxation would be the best explanation, however, the studies showed that increased movement after short-term stretching could not be attributed to this. I’m wrong again!
As this review points out, there is a better theory with a better explanation.
The studies reviewed show that increases in extensibility are more likely due to a change in sensation, not an increase in muscle length. It even suggests there is a psychological role in allowing for greater extensibility.
One study showed that an intensive four-week stretch program actually didn’t make a difference in extensibility, but rather increased the subject’s tolerance to uncomfortable stretching sensations and also points out the underlying mechanism may be psychological. The argument was also made in this study, that the four-week stretching program may not have been long enough to make a noticeable change.
This would explain why when I go to a yoga class once in a while, I don’t see any real difference in the amount of movement I can do from one class to the next. My tolerance to the stretch or movements hasn’t increased enough for me to do that damn supta virasana pose (and I don’t think I’ll ever be able to, my body doesn’t bend that way!). However, those folks who go a couple times a week and make a practice of it have a far more increased tolerance to the movement after doing it regularly for an extended period of time.
For some of you, this may be a simple review and you already know this stuff while some of you may be wondering how or why it’s applicable clinically. Well, we quite often tell our patients we are trying to “lengthen” the muscle when we apply a stretch after treatment. We even use the comparisons of people who do yoga and refer to their “long” muscles.
Well, the reality is there is no length difference in the muscle, but there could be a change in their extensibility. Even as we try to explain to them what extensibility is and the changes we make, we need to be accurate in our description of what’s happening. Just like the when we begin rehab exercises and introduce graded exposure to fearful movements, all we are doing when we provide a stretch is a graded exposure to the stretch sensation. We aren’t even going to make a massive change to extensibility in one treatment, but maybe we can increase a little more tolerance to certain movements. Great, now I have to go back and edit that other post, and we’ll see if I get called out on this one!
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