Victor Forge
Muscle spindles are a type of sensory receptor located in skeletal muscles, which inform the central nervous system (CNS) about the stretch and contraction state of a muscle. They are a few millimetres long and rarely over 100 μm wide, and are surrounded by a spindle-like capsule of connective tissue. Muscle spindles play a crucial role in monitoring and regulating muscle lengthening and shortening, influencing the range of motion (ROM) in our muscles. To stimulate a muscle spindle, mechanical pressure can be applied near the spindle capsule, or muscle vibration can be used to rapidly increase cortical somatosensory representations of the vibrated body part.
| Characteristics | Values |
|---|---|
| Definition | A muscle spindle is a specialized sensory receptor consisting of fine muscle fibers that inform the central nervous system about the stretch and contraction state of a muscle. |
| Structure | A muscle spindle is a fusiform capsule that is 4–10 mm long and 80–200 µm in diameter. |
| Location | Muscle spindles are located in the periphery of a muscle. |
| Function | Muscle spindles play a crucial role in monitoring and regulating muscle lengthening and shortening, thus influencing the range of motion (ROM). They provide feedback to the CNS and contribute to maintaining posture and balance. |
| Stimulation methods | Percussion stimulation, vibration, mechanical pressure, electrical stimulation, and PNF stretching have all been used to stimulate muscle spindles in different studies and experiments. |
| Applications | Understanding muscle spindle function is important in the context of neuromuscular diseases, therapeutic strategies, and personal training to optimize training and prevent injuries. |
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What You'll Learn
Using percussion stimulators
Percussion stimulators, such as massage or physical therapy vibrators, can be used to stimulate muscle spindles. These devices send vibratory waves into a muscle, causing it to lengthen artificially without changing the angle of the joint it crosses. This creates a kinesthetic illusion of limb position, as the muscle spindles send length information to the CNS that does not match the actual joint angle.
To stimulate the biceps muscle, for example, a percussion stimulator can be applied to alter arm position information. Similarly, when applied to the triceps muscle, the test subject attempts to maintain their finger position during triceps muscle vibration. With the percussion stimulator set to the highest speed of vibration, the observer applies stimulation to the triceps muscle of the outstretched arm, creating the illusion of a change in limb position or standing posture.
Percussion stimulators like the Deep Muscle Stimulator (DMS) use percussion and mechanical vibrations to reach deep into the muscle tissue and stimulate proprioceptive functions. DMS provides deep muscle tissue with kinetic forms of percussion and concussion vibration, increasing muscle metabolism and the lactic acid cycle to relieve pain. DMS can be used to manage acute and chronic pain, including migraine headaches, sciatica, carpal tunnel syndrome, tendonitis, and frozen joints.
Percussion massage guns, such as the Hypervolt 2 Pro, are another example of percussion stimulators. These devices feature quiet glide technology and multiple speeds and interchangeable heads to help relieve sore muscles and stiffness. They are often used for muscle recovery and to relieve pain in various body parts, including the lower back, arm, shoulder, leg, and foot.
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Applying mechanical pressure
Muscle spindles are delicate sensory receptors that inform the central nervous system (CNS) about changes in the length of individual muscles and the speed of stretching. They are stimulated through compression, squashing, or movement. This stimulation triggers a feedback mechanism to the CNS, providing information about the muscle's current length and rate of change.
To apply mechanical pressure to stimulate muscle spindles, a handheld probe can be used to apply light-to-moderate pressure near the spindle capsule, which is located in the centre of the spindle. This pressure can be applied during movement to further stimulate the muscle spindles. The spindle capsule can also be vibrated, which creates an artificially incorrect message to the CNS that a muscle has lengthened, resulting in an illusion of limb position or standing posture change.
Additionally, PNF (Proprioceptive Neuromuscular Facilitation) stretching can be used to stimulate muscle spindles by combining muscle contraction and relaxation to enhance flexibility. The stretching phase activates the muscle spindles, and the subsequent isometric contraction further stimulates them, while the relaxation phase resets their sensitivity.
By understanding how to stimulate muscle spindles through mechanical pressure, we can gain insight into the role of these specialised fibres in controlling and refining muscle contraction, as well as their importance in proprioception and sensorimotor performance.
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Vibration
The mechanism through which vibration therapy works has been attributed to the tonic vibratory reflex (TVR). The TVR is stimulated by a sequence of rapid muscle stretching that occurs when vibration is applied, triggering muscle spindles and causing an involuntary production of strength. Tonic vibration reflex is a sustained contraction of a muscle subjected to vibration. This reflex is caused by vibratory activation of muscle spindles — muscle receptors sensitive to stretch. 30–100 Hz vibration activates receptors of the skin, tendons and, most importantly, muscle spindles. Muscle spindle discharges are sent to the spinal cord through afferent nerve fibers, where they activate polysynaptic reflex arcs, causing the muscle to contract.
Research finds that vibration therapy induces non-voluntary muscular contraction through the tonic vibration reflex (TVR) via activation of the proprioceptive sensory system; this system is based on the excitation of Ia afferent signals from the neuromuscular spindle, which leads to subsequent recruitment of muscle fibers that were previously inactive. Other possible mechanisms for vibration therapy on muscle include influencing the Golgi tendinous organs (GTO), which are sensitive to variations in muscle tension, and inhibition of the agonist-antagonist co-activation mediated by Ia-inhibitory neurons.
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Microneurography
In microneurography, the tungsten needle microelectrode is inserted through the skin into a nerve. The procedure causes minimal discomfort, and anaesthesia is typically not required. The microelectrodes have specific dimensions: a shaft diameter of 100-200 μm, a tip diameter of 1-5 μm, and they are insulated with epoxy resin. Nerve discharges are determined by voltage differences between the intra-neural electrode and a reference needle. The electrodes are connected to a differential amplifier, and signals are monitored and stored for offline analysis.
In a microneurography experiment investigating muscle spindle receptors, custom-built surface electrodes were used to record electromyography (EMG) from various muscles in the forearm. This study explored the goal-dependent tuning of muscle spindle receptors during movement preparation, demonstrating the role of spindle afferents in representing firing patterns of the underlying afferent population.
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PNF stretching
Proprioceptive Neuromuscular Facilitation (PNF) stretching is an advanced form of flexibility training that can improve your range of motion and overall flexibility. It involves both stretching and contracting the target muscle group to achieve maximum static flexibility. PNF stretching can be used on most muscles in the body and can be modified to be performed alone or with a partner.
There are three main PNF methods: contract-relax (CR), contract-relax-antagonist-contract (CRAC), and hold-relax-swing. The CR method involves lengthening the target muscle and holding it in that position while contracting. This can be done by starting with a passive stretch for 7-12 seconds, followed by an isometric contraction for 7-15 seconds. The CRAC method is similar but involves additional steps, including a passive stretch by the partner and a final passive stretch at the end. The hold-relax-swing method combines ballistic or dynamic stretches with static and isometric stretches and is only recommended for experienced individuals with a high level of control over their muscle stretch reflex.
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Frequently asked questions
Muscle spindles are specialised sensory receptors that inform the central nervous system (CNS) about the stretch and contraction state of a muscle. They are fusiform capsules that are 4-10mm long and 80-200 µm in diameter. Human muscles contain 35-500 muscle spindles per muscle.
Muscle spindles monitor the rate and change of length in a muscle. They are located in skeletal muscles and are excited by the stretching of their sensory endings. When a muscle undergoes lengthening or shortening, the muscle spindles' nuclei are stimulated through compression, squashing, or movement. This stimulation triggers a feedback mechanism to the CNS.
Muscle spindles play a crucial role in monitoring and regulating muscle lengthening and shortening, thus influencing the range of motion (ROM) in our muscles. They also contribute to maintaining posture and providing feedback to the brain.
Muscle spindles can be stimulated through mechanical pressure, vibration, or electrical stimulation. In some studies, percussion stimulators have been used to vibrate muscle spindles, causing an artificially incorrect message to the CNS about the length of a muscle.
