Resting muscle tone is considered the passive tension in our muscles when not loaded or in use. This helps maintain enough tension in the muscles to quickly respond to a stretch and to maintain postural stability in balanced equilibrium positions. This is different from strength or co-contraction of muscle which would be what a muscle can do when not at rest, though this can often build off the baseline resting muscle tone.
One might notice that if you were to jerk someone’s elbow straight, quickly stretching the bicep muscle, it would often tighten up to prevent injury or overloading of tendons and ligaments at end-range of motion for that joint. This often happens automatically without any input from the Central Nervous System (CNS) or brain.
Some of the great effectors of resting muscle tone are the muscle spindles and golgi tendon organs (GTOs). The muscle spindles are located deeper in the muscle between flanking intrafusal muscle fibers consisting of elements of annulospiral mechanoreceptors and flower-spray mechanical stretch receptors. Each of the two muscle spindle receptors perform similar tasks of measuring changes to muscle length and velocity, with the former having a greater dynamic sensitivity. This allows the muscle to get instant feedback of degree of stretch in what is called the spinal reflex, or a signal that loops on itself without interference from the CNS. This can act much faster than signals that would have to go to the brain first.
GTOs are situated in the tendon of the muscle and act as a tension receptor. Their main purpose is to measure tension in the muscle and to assess whether it is close to failure. If the GTO assesses that a muscle is at risk of injury it will trigger what is called the inverse myotatic reflex signaling inhibitory interneurons in the spinal cord to reduce force output of the affected muscle. A second function of the GTO is to fire the antagonist muscle, in certain circumstances, often to distribute an overwhelming load or cause a release to prevent injury.
A potential example of GTO firing could be the numerous hamstring injuries we see across the NFL. If the GTOs in those individuals were functioning correctly, the hamstrings would relax or be inhibited during the run, causing the player to collapse prior to injury. In the second scenario the quadriceps would contract during the run, when it is a primary hamstring movement and the hamstrings have already been inhibited via the inhibitory interneuron, thus causing the player to collapse but preventing the injury in the first place.
I often use the example with many of my patients that if you were to feel an Olympic athlete’s muscles at rest you would frequently be able to flick their muscles at rest and watch a ripple due to the low resting muscle tone when not under load, whether it be their calf or upper trapezius, etc.
Densifications in the fascial extracellular matrix (accumulation of the chemical hyaluronan, part of the lubricant that runs between the layers of the fascia) can cause tension locally signaling to nearby the muscle spindles that there is a greater force on them than is present, increasing local resting muscle tone. This will change with position but often we maintain similar positions throughout our day. As this continues to spread throughout the associated myofascial sequence this can continue to increase resting muscle tone along that plane or sequence. This will often result in muscles that you are not able to see a ripple in when flicking them in an unloaded state.
If you notice your calves, forearms, or any other major muscle in your body are not loose and able to be manipulated with your hands when in a balanced resting position, there is a good chance it is due to fascial densifications causing an increase in resting muscle tone. This can often result in feeling like you just got done at the gym with a slight to full on ‘pump’ in them that doesn’t seem to go away many hours or days after working out. This can also present itself as chronically tight muscles, some of the most common I see are upper traps/scalenes (causing neck pain) and hamstrings or quadratus lumborum (causing low back pain). These densifications can also lead to partial or greater inhibition of the GTOs associated with those muscles and it’s associated myofascial chain, increasing the chance for injury.
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