The intrafusal fibers of the muscle axis run parallel to the extrafusal fibers of the muscle, joining them along short segments.
Muscle spindles are stretch receptors within the body of a muscle that primarily sense changes in the length of the muscle. They transmit length information to the central nervous system through afferent nerve fibers.
Sensory information transmitted by primary type Ia sensory fibers and type II secondary sensory fibers, which revolve around muscle fibers within the spindle.
Also motor action by up to a dozen gamma motor neurons and, to a lesser extent, by one or two beta motor neurons that activate muscle fibers within the spindle.
When the extrafusal fibers of the skeletal muscle are stretched, the intrafusal fibers of the spindle are stretched as well. This stretch of the muscle spindle indicates the length of the muscle.
When stretching occurs, the sensory neuron in the muscle spindle signals motor neurons located within the ventral horn of the spinal cord. This signal causes motor neurons to fire, resulting in muscle contraction.
This reflex arc provides negative feedback. Stretch-induced muscle contraction works against or negates further muscle stretching. This mechanism helps maintain proper muscle tension or tone.
Although the muscle axis helps maintain proper muscle tension, unlike the Golgi tendon organ, it is not an indicator of muscle tension, but rather of muscle length.
Structure of the muscle spindle
The specialized fibers that make up the muscle spindle are known as intrafusal fibers (as they are present in the spindle), to be distinguished from the fibers of the muscle itself which are called extrafusal fibers.
The fusiform shape of the muscle spindles underlies the labels given to the two types of fibers that make up skeletal muscle. Intrafusal fibers are named for their contribution to the structure of the spindle.
Extrafusal fibers are those located outside the structure of the spindles. Muscle spindles are made up of very fine muscle fibers of lengths that vary between 4 and 10 millimeters.
The intrafusal fibers that make up the muscle spindle are so small that they contribute little to the force of contraction. Some specialized muscle spindles are sensitive to the rate of change in muscle length.
The density of the muscle spindles is greater in those muscles that participate in precise movements, such as those that control the hand.
Types of muscle spindles
There are three types of muscle spindles. They are distinguished based on the arrangement of the nuclei within the intrafusal fibers and the function of the fibers.
The first type, or nuclear chain fibers, are relatively thin and have cores that are arranged in a single row along the length of their fibers.
The second type, called nuclear sac fibers, has an expanded region midway along its length. Nuclei are grouped with the highest density in these expanded regions.
These fibers are thicker than the fibers of the nuclear chain.
Finally, the dynamic fibers of the nuclear sac are sensitive to changes in the stretch of the fibers, in contrast to the static fibers of the nuclear sac that are sensitive to the stationary stretch of the muscles in steady state (the new stabilized length of a muscle) .
Composition of the muscle spindle
Muscle spindles are composed of three to twelve muscle fibers, of which there are three types: dynamic nuclear bag fibers (bag1 fibers), static nuclear bag fibers (bag2 fibers), nuclear chain fibers, and afferent nerve fibers.
The sensory fibers spiral around the intrafusal muscle fibers, terminating near the middle of each fiber.
These fibers, primary type Ia sensory fibers and type II secondary sensory fibers, send information through ion channels sensitive to axon stretching.
The motor part of the spindle is provided by motor neurons: up to a dozen gamma motor neurons and one or two beta motor neurons, collectively called fusimotor neurons.
These activate the muscle fibers within the spindle. Gamma motor neurons only supply muscle fibers within the spindle, whereas beta motor neurons supply muscle fibers both within and outside the spindle.
As the name implies, the muscle spindle is a spindle-shaped sense organ, which means it is thicker in the middle and tapered at each end.
It is a stretch receptor that is widely dispersed in most skeletal muscles.
Our muscle spindles function as a mechanism to help protect a muscle from overstretching, triggering a stretch reflex when necessary.
Muscle spindles are specialized to detect changes in muscle length, particularly when the muscle changes length rapidly.
Each muscle spindle is enclosed within a capsule and runs parallel to the extrafusal fibers (normal skeletal muscle fibers).
The muscle spindle contains specialized muscle fibers called intrafusal fibers. These intrafusal fibers have contractile proteins at each end (actin and myosin) and a central region that is enveloped by sensory nerve endings.
Because the intrafusal fibers of the muscle spindle are parallel to the extrafusal muscle fibers, a stretching force applied to the muscle will stretch both the intrafusal and extrafusal muscle fibers.
This will cause a sensory discharge from the muscle spindle that is carried into the spinal cord. This then leads to a motor response, the activation of the muscle that was initially stretched.
From a practical point of view, static stretching exercises are normally performed in such a way as to avoid activation of the muscle spindles. Moving slowly to a stretched position prevents activation of the muscle axis.
This is important because muscles stretch more easily when relaxed. However, there are other times when muscle spindle activation is desired during training.
For example, plyometrics are performed by rapidly stretching a muscle, and this is immediately followed by the concentric action of the same muscle. This rapid stretch of the muscle will activate the stretch reflex, leading to a more powerful concentric action.
The Golgi tendon organ and muscle spindle
To keep muscles healthy and safe, we must have a good understanding of the most basic underlying structural components of the body and how they work together, as this knowledge provides the foundation for effective exercise instruction.
Two of these components, the Golgi tendon organ (OTG) and the muscle spindle, belong to the nervous system and function to influence movement.
Two important proprioceptors that play a role in flexibility, the Golgi tendon organ and the muscle spindle, work together reflexively to regulate muscle stiffness.
When an organ of the Golgi tendon is stimulated, it causes its associated muscle to relax by interrupting its contraction. When a muscle is inhibited by a Golgi tendon organ, the process is called autogenic inhibition.
The function of the Golgi tendon organ can be considered the opposite of the muscle axis, which serves to produce muscle contraction.
Imagine a muscle spindle as if it were a spiral thread (or wrapped around the muscle fibers near the muscle belly; as the muscle lengthens or stretches, it pulls on the spindle causing it to lose its spiral shape and stretch as well.
This signals the muscle to contract (after which the coil returns to its shape), which in turn protects the muscle from overstretching.
When the muscle associated with a muscle spindle is stretched rapidly, the spindle can do two things:
- It can signal your muscle to contract to keep you from going too far, too fast in the stretch.
- It can inhibit the opposite muscle (the antagonist of the stretched muscle) to prevent it from contracting, so that it cannot contribute to any further stretching.
The relaxation of the antagonist that occurs simultaneously when a muscle spindle of its associated muscle contracts is called reciprocal inhibition.
Ultimately, the muscle spindle works to alert the brain that nearby joints and soft tissues are in danger of being overstretched.
These are important concepts for understanding body awareness (also known as proprioception and kinesthetic awareness).
The Golgi tendon organs sense muscle tension within muscles when they contract or stretch.
When the Golgi tendon organ is activated during contraction, it causes inhibition of contraction (autogenic inhibition), which is an automatic reflex.
When the Golgi tendon organ is activated during stretching, it inhibits the activity of the muscle spindle within the active muscle (agonist), so a deeper stretch can be achieved.
The Golgi tendon organs are sensitive to changes in tension and tension velocity and, because they are located at the musculotendinous junctions, they are responsible for sending information to the brain as soon as they feel overload.
Static stretching is an example of how muscle tension signals a Golgi tendon organ response.
Therefore, when you hold a low-force stretch for more than seven seconds, the increased muscle tension activates the Golgi tendon organ, which temporarily inhibits muscle spindle activity (thus reducing tension on the muscle) and allows more stretch.
It is also worth mentioning that autogenic inhibition can be induced by contracting a muscle just before it is passively extended, which is a method called proprioceptive neuromuscular facilitation (PNF).
Proprioceptive neuromuscular facilitation is a stretching practice that promotes the response of neuromuscular mechanisms through the stimulation of proprioceptors in an attempt to gain more stretch in a muscle.
A practical example of this method is to produce a low-grade contraction (50% of maximum force) within a muscle for six to 15 seconds immediately before a partner passively stretches the muscle.
The pre-stretch contraction reduces the activity of the muscle spindle within its associated muscle (the muscle that is about to be stretched) so that the brain willingly accepts an increase in range of motion during the impending stretch.
The muscle spindles and organs of the Golgi tendon go through this cycle to help you stretch safely and effectively.
This is also the reasoning behind holding a seven to 10 second stretch to allow the stretch to deepen.
The Golgi tendon organs and muscle spindles work together through their reflective actions to avoid injury.
The basic physiology of stretching
Stretching a muscle fiber begins with the sarcomere, the basic unit of contraction in the muscle fiber.
The actual length of the entire muscle depends on the number of fibers stretched (similar to the way the total force of a contracted muscle depends on the number of recruited fibers that are contracted).
One way to visualize this is to think of small groups of fibers throughout the stretch of the muscular body, while other groups of fibers are simply “going along the way.”
As such, the more fibers that are stretched in this process, the greater the length developed by the stretched muscle.
In relation to the stretching process, it is also important to understand how the brain / neural components of the musculoskeletal system adapt to stretching.
When the muscle is stretched, so is the muscle spindle (a nerve control point located between groups of muscle fibers). The muscle spindle records the change in muscle length and how fast this change occurs.
It then sends signals to the spine, which then relays this information to the brain.
Now, if the force and abruptness of the stretch exceed the muscle’s ability to safely contract protection (for example, exceed its strength), another neural component, the Golgi tendon organ, kicks in and takes over. the muscle spindle.
When this tension exceeds a certain threshold, it triggers the lengthening reaction, which inhibits the muscle from contracting and instead causes it to relax and lengthen.
Think of the two systems as a “double insurance against failure” that ultimately helps reduce the risk of injury.
Why we estimate slowly, and for a long period of time
One reason to hold a stretch for an extended period of time is to allow the lengthening reaction to occur (caused by the Golgi tendon organ), helping the stretched muscles to relax.
Obviously, it is easier to stretch, or lengthen, a muscle when it is not trying to contract.
What current research says about static stretching
Countless studies have been conducted on stretching and its effect on overall athletic performance.
Most of the current research suggests that static stretching before activity can induce temporary weakness in the muscle, decrease the ability of the muscle receptor to compromise the ‘stretch reflex’, and may increase the risk of injury.
The literature notes that there is a loss of power / contractile force for up to 30 minutes after static stretching and as such the muscle cannot fully contract if necessary.
This is not new information; Most of the research is based on studies done from the 90’s and 00’s.
If you must stretch statically before activity (because your sport requires / is part of your personal routine), consider progressing to a gentle, dynamic warm-up before actual activity.
There are positive benefits of static stretching for general health, mobility, and flexibility.
As we mentioned earlier, stretching causes the muscle and tendon to lengthen, and these changes can be somewhat permanent and definitely beneficial for sports and well-being.
I will argue (and have some research to support me) that activity after statically stretching has great value, especially if you are trying to be more flexible, treat injuries, and improve your overall mobility.
In that sense, if you are an airbrush, acrobat, gymnast, dancer or yogi, it is essential to acquire skills and improve technique.
When you stretch after activity, your muscles are tired and well vascularized (think healthy and nutritious blood flow).
Fatigue allows you to take advantage of the lengthening reaction (discussed above), and the sarcomeres are less capable of contracting (so they “can’t fight” stretch as strongly).
Several articles also suggest that stretching (gently) after activity can decrease post-workout pain.
Finally, a good training will cause some micro trauma to the muscles and the subsequent adaptation (which makes us stronger) can occur with an increase in the length of the fiber when we stretch afterwards.
This means that as you gain strength, stretching after a workout can help prevent the loss of flexibility associated with muscle fiber growth.