Introduction to Muscle Spindles

Todays fitness blog will look at the role of Muscle spindles in postural control and flexibility.  Muscle spindles are sensory organs made up of intrafusal muscle fibres.  They monitor the rate and change of length in a muscle and are distributed throughout skeletal muscle (extrafusal muscle fibres) aligned to run in the direction of that muscle.

There are two types of muscle spindle:

• those that monitor posture – Type Ia
• and those that monitor dynamic activity such as a running gait – Type II

Both provide sensory feedback to the central nervous system (CNS) via the peripheral nervous system (PNS).  Their roles help the brain to understand the length of muscles, the rate of muscle length changes, as well as the positioning of our body, allowing it to monitor and control movement safely.

In the following sections, we will delve deeper into the mechanics of muscle spindles, exploring their response to muscle lengthening and shortening and how they contribute to the range of motion (ROM) in our muscles. We will also investigate how muscle spindles influence posture and the brain’s role in determining the potential dangers associated with changes in muscle length and rate of change. Additionally, we will examine the intriguing stretch reflex triggered by muscle spindles during rapid dynamic activities and its protective function.

The Function of Muscle Spindles

Muscle spindles serve several important functions in our bodies:

• Monitoring muscle length and rate of change
• Providing sensory feedback for proprioception
• Regulating muscle contraction and relaxation
• Contributing to reflex mechanisms like the stretch reflex
• Adapting to changes in muscle length through spindle adaptation
• Facilitating motor learning and skill acquisition

These functions collectively contribute to our overall movement, coordination, flexibility, and safety during physical activities.

Muscles Spindles and Range of Motion

Muscle spindles play a crucial role in monitoring and regulating muscle lengthening and shortening, thus influencing the range of motion (ROM) in our muscles. When a muscle undergoes lengthening or shortening, the muscle spindles’ nuclei, located in the center of the spindle, are stimulated through compression, squashing, or movement. This stimulation triggers a feedback mechanism to the central nervous system (CNS), providing information about the muscle’s current length and rate of change.

Based on the feedback received from the muscle spindles, the CNS determines whether the muscle can safely continue to lengthen or if it has reached the end of its range of movement. If the feedback indicates that further lengthening is permissible, the muscle can continue to stretch and extend its ROM. On the other hand, if the feedback suggests potential risks or dangers, it will inhibit further lengthening by activating nerves that innervate/activate muscle actions (the alpha motor neurons), increasing tension in the muscle and preventing further stretching.  

PNF (Proprioceptive Neuromuscular Facilitation) stretching utilises muscle spindles by combining muscle contraction and relaxation to enhance flexibility. The stretching phase activates the muscle spindles, providing feedback to the CNS. The subsequent isometric contraction stimulates the Golgi Tendon Organs and muscle spindles further, while the relaxation phase resets their sensitivity.

Muscles Spindles and Posture

Muscle spindles not only contribute to ROM but also play a significant role in monitoring posture and providing feedback to the brain. As we move our bodies, muscle spindles constantly relay information about our postural alignment to the CNS. This information allows the brain to have a clear understanding of our body’s positioning and make necessary adjustments to maintain balance and stability.

The brain, in response to the feedback received from muscle spindles, evaluates the potential dangers associated with changes in muscle length and rate of change. If the brain perceives a threat or instability, it activates specific nerves that innervate muscle actions, (the alpha motor neurons again). The activation of these nerves leads to muscle tension, which helps stabilise the body and mitigate potential risks.

Furthermore, in situations where the rate of change of muscle length is sufficiently fast/dynamic/volatile enough, the brain will activate more of the same nerves and cause a stretch reflex, also known as the stretch-shortening cycle (SSC).  This is a sudden, involuntary shortening of a muscle, with the classic example being the knee-tap reflex test. The test works by tapping on the patella tendon, which causes the quadriceps muscle to lengthen quickly. The muscle spindles sense the quick lengthening and to prevent further ‘injury’, trigger a quick shortening of the quadriceps, resulting in knee extension. The doctor performs this test to assess how well your PNS is communicating with your CNS.

You can also see this process being applied when kicking a ball. If you stand and kick your foot high in the air, your muscle spindles will monitor this dynamic activity and register that the muscle is lengthening at speed. They will immediately activate the safety response, or stretch reflex, and pull the leg back down again to protect the muscle from damage. In addition, it is the innervation of the SSC, that makes the performance of plyometrics so effective at increasing an individual’s power output.

Summary

Muscle spindles are sensory organs that monitor muscle length and rate of change, providing feedback to the central nervous system (CNS). They contribute to our range of motion (ROM) and help regulate muscle contraction and relaxation. PNF stretching utilises muscle spindles to enhance flexibility by combining contraction and relaxation. The stretch reflex, triggered by muscle spindles, supports balance and stability, as well as protects against potential injuries during rapid dynamic activities. However, if applied correctly, this process can assist with improving performance.

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