The Main Function Of Muscle Spindles Is – The concept of muscle memory is vital in the world of manual and movement therapy. However, there seems to be some controversy about what it is and how it works. To better understand this concept and its application to clinical work, let’s explore the concept of muscle memory.

Simply put, muscle memory describes the idea that the body’s muscles contract in patterns, both for posture and body movement. And these patterns are memorized on some level, operating without our conscious control. Hence the term muscle memory.

The Main Function Of Muscle Spindles Is

The Main Function Of Muscle Spindles Is

As for the memorized muscle contraction patterns for movement, there is little controversy. By practicing certain activities such as walking, swinging a tennis racket, playing a piece of music on the piano, or driving a car while drinking a cappuccino and adjusting the radio dial, we learn to perform these complex and coordinated activities with little or no input . from the part of the brain that controls voluntary muscle contractions, the cerebral primary motor cortex. Instead, once a movement pattern is initially learned in the cerebral motor cortex, it is transferred to the basal ganglia, located deeper within the brain. Consequently, it is possible to perform a complex coordinated movement pattern while paying little or no attention to it and thinking about something else entirely. So if the question is: What is muscle memory for movement? The answer would be that it is a stored memorized movement pattern that can be released from the basal ganglia of the brain.

Spinal Cord Tracts And Reflexes

More controversial and more relevant to the world of massage and bodywork is how the concept of muscle memory relates to posture. Our body posture is largely determined by the memorized pattern of our body’s basic muscle tone. By exerting a pull on the bones and joints, the basic resting muscle tone determines the position, and therefore the posture, of our body. Indeed, when clients come to a manual therapist, their common complaint is that their resting baseline muscle tone is too tight. Musculoskeletal, the most common goal of soft tissue manipulation (massage) therapy is to change the muscle memory of the baseline resting tone of our clients’ tight muscles.

So, let’s explore the idea of ​​muscle memory in this context. The first misconception is the belief that postural muscle memory resides in the muscle tissue itself. The muscular system is an incredibly complex and awe-inspiring system of the body, but it does not hold the key to its own memory. Of course, the adhesions present within the musculature can determine its ability to stretch, and thus affect the degree of passive tension and therefore posture. But if we refer to active muscle tone, in other words, muscle contraction, the memory for this lies elsewhere. This can be easily understood by considering a person who has suffered a traumatic injury that cuts the lower motor neurons (LMNs) that synapse and control a muscle. In these cases, the muscle will be loosely paralyzed and will not have the ability to contract (unless electrical stimulation is applied from the outside). If the memory for the muscle contraction actually resided within the muscle, then it would be able to contract, even if its nerve was not working.

So where is the muscle memory for the basic resting tone? Similar to muscle memory for movement patterns, it resides in the nervous system. The nervous system is the great master controller of all muscle functions. However, instead of being located in the basal ganglia, it is located in a different area of ​​the brain known as the gamma motor system.

Figure 1. Spinal muscle fibers are a specialized type of muscle cell. They are found within a muscle and lie parallel to normal muscle fibers. License Joseph E. Muscolino. Kinesiology – The Skeletal System and Muscle Function, 3ed. Elsevier (2017).

The Spinal Cord

The way the brain’s gamma motor system controls resting muscle tone is through the fibers of the muscle spindle. Spinal muscle fibers are specialized muscle cells that are sensitive to stretch. They are located within the belly of the muscle, lying parallel to the normal muscle fibers (Figure 1). When the muscle is stretched too much or stretched too quickly, the fibers in the muscle spindle are excited and send an impulse into the sensory neurons in the spinal cord.

Figure 2. Cross section of spinal cord and muscle spindle stretch reflex. The muscle spindle reflex is triggered to occur when the muscle is either stretched too far or stretched too quickly, such as being stretched too quickly by reflex hammering as shown. This reflex causes lower motor neurons (LMNs) to direct the muscle and its antagonists to contract. License Joseph E. Muscolino. Kinesiology – The Skeletal System and Muscle Function, 3rd. Elsevier (2017).

These sensory neurons synapse with LMNs and cause them to send a signal for contraction to the normal muscle fibers of the muscle and its synergists (Figure 2). When the muscle contracts toward its center, it no longer stretches and therefore cannot be overstretched and torn. For this reason, the muscle spindle reflex, also known as the stretch reflex, is considered a protective reflex that prevents muscles from being stretched and torn excessively.

The Main Function Of Muscle Spindles Is

Figure 3. Cross section of spinal cord and muscle spindle fibers within a muscle. The sensitivity of the muscle spindle fibers to stretch is determined by the lower gamma motor neurons (LMNs) that direct the muscle spindle fibers to contract and shorten. These gamma LMNs are controlled by brain-derived gamma upper motor neurons (UMNs). License Joseph E. Muscolino. Kinesiology – The Skeletal System and Muscle Function, 3ed. Elsevier (2017).

Diverse And Complex Muscle Spindle Afferent Firing Properties Emerge From Multiscale Muscle Mechanics

The critical aspect of this mechanism is that the sensitivity of muscle spindle fibers can be regulated by specialized LMNs known as gamma LMNs. These in turn are controlled by gamma upper motor neurons (UMNs) located within the brain and operating subconsciously (Figure 3). When this gamma motor system of the brain commands the muscle spindle fibers to contract and shorten, they become less tolerant of stretch and therefore more able to trigger the muscle spindle stretch reflex. The stretch reflex will then cause the muscle to contract and tighten to match the tone of the spindle fibers within it. On the other hand, if the gamma motor system does not contract the muscle spindle fibers, then they will be longer and more tolerant of stretch and less likely to trigger the stretch reflex. As we move our bodies through normal activities of daily living, we inevitably stretch our muscles to some degree because when we command a muscle on one side of the joint to contract and shorten to cause movement, its antagonists on the other side of the joint must lengthen and stretch to allow this movement to occur. If and when this stretch exceeds the tolerance of the fibers of the muscle spindle, they will trigger the onset of the stretch reflex. Therefore, resting muscle tone reflects the tone of the muscle fibers of the spindle. In this way, the tone of the muscle spindle determines the muscle memory of the basic resting muscle tone in our body.

So when we work on a client’s tight muscle, what we’re really trying to accomplish is to reduce the activity of the gamma motor system so that the muscle spindle fibers relax, allowing the normal fibers in the muscle to relax. (It should be noted that this does not apply to trigger points, which are essentially a local phenomenon.) We may be working directly on the muscle, but we are doing so to affect gamma axis activity in areas of the brain that act unconsciously. Actually we are trying to change the spindle tone patterns that are set. Simply put, when it comes to changing muscle memory for tight muscles, the purpose of massage, as well as stretching, is to change the gamma motor activity of the brain so that we can change the tone of the muscle spindle and thus change muscle tone at rest. this pattern has existed for months, years or even decades, it is extremely important. As a general rule, it is the chronicity, not the severity, of a tight muscle that is the primary factor that determines how long it takes for the client’s muscle mass to relax, because according to the concept of neural plasticity, neural patterns have been activated for a long time. over time, they become entrenched in their firing pattern with functional and structural changes.

This blog post article is adapted from an article originally published in the Spring 2009 issue of theAMTA Massage Therapy Journal.

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