Key Point

• Understanding how muscle spindles and Golgi tendon organs contribute to proprioception can aid in providing effective care for clients. 

Proprioception, often referred to as the “sixth sense,” is the body’s ability to perceive its position, movement, balance, and force in space. Without this sense, even simple tasks would be impossible, as seen in babies who have yet to develop full proprioceptive abilities.

This complex sensory system relies on several key structures throughout the body to function effectively. The primary structures involved in proprioception are cells called proprioceptors. These cells are highly specialized sensory receptors in muscles, tendons, and joints. They are vital to perceiving and responding to touch, pressure, and movement. 

The key proprioceptors include muscle spindles, Golgi tendon organs (GTOs), and joint receptors. Muscle spindles are embedded within skeletal muscles and detect changes in muscle length and stretch. GTOs are located at the interface of muscles and tendons and sense changes in muscle tension. Joint receptors in joint capsules provide information about joint position and movement.

Together with other structures, they form a complex network that sends detailed information about movement to the central nervous system. The sensory information collected by these proprioceptors is transmitted to the central nervous system via large afferent nerve fibers. These signals travel through the peripheral nervous system to the spinal cord and then up to various brain regions, including the cerebellum, somatosensory cortex, and motor cortex. In the brain, this information is integrated with other sensory inputs to create a comprehensive understanding of the body’s position and movement in space.

This article focuses on muscle spindles and GTOs. The proprioceptors’ vast sensory input makes them key to the effectiveness of manual therapy. With a solid grasp of these receptors, you’ll be better equipped to assess and treat clients, especially when addressing pain, faulty movement patterns, and injury risk caused by proprioceptive dysfunction. 

Function Differences

Muscle spindles, Golgi tendon organs, and joint receptors all have distinct functions and locations.

Muscle Spindles

• Location—Embedded within skeletal muscles, coiled around specialized muscle fibers.

• Function—Their primary role is sensing muscle length and the rate of muscle stretch.

• Reflex Action—They initiate the myotatic (stretch) reflex, which contracts the muscle to prevent overstretching.

• Reciprocal Inhibition—Muscle spindles also contribute to relaxing the antagonist muscles during movement.

Golgi Tendon Organs

• Location—Positioned at the interface between
muscles and tendons.

• Function—GTOs monitor changes in muscle
tension and force.

• Reflex Action—When tension becomes excessive, it activates autogenic inhibition, prompting muscle relaxation to safeguard muscles and tendons from injury.

Joint Receptors

• Location—Found within the joint capsules.

• Function—They detect changes in joint angles, position, and movement, offering feedback on limb positioning (kinesthesia).

• Role in Reflexes—Unlike spindles and GTOs, joint receptors do not directly trigger muscle contraction or relaxation.

Together, these proprioceptors provide comprehensive sensory feedback, allowing for precise motor control, coordination, and protection against injury.

The Muscle Spindle

Our investigation of proprioceptors begins with the muscle spindle, which plays a critical role in the stretch reflex—a rapid, involuntary response to muscle stretching. The muscle spindle is a specialized cell embedded within the muscle belly (Image 1). Although it has some contractile capability, it does not generate the same contraction force as primary muscle fibers. Muscle spindles are found in the bellies of all muscles but are more highly concentrated in muscles responsible for fine motor control, such as those in the hands and fingers. These muscles require more spindle cells to facilitate precise movements. 

The primary function of the muscle spindle is to act as a stretch receptor, providing critical information about muscle length and the amount and speed of stretching, which is essential for precise movements, motor control, and injury prevention. The muscle spindle detects two key aspects of muscle stretch: the extent of the stretch (tonic response) and the speed of the stretch (phasic response). These responses are crucial for maintaining static positions and coordinating movements.

If a muscle stretches too far, there is a risk of strain. Likewise, if a muscle stretches too quickly, there is a potential for overstretching and injury. When the muscle spindle detects excessive or rapid stretching, it sends a strong signal to the central nervous system. This signal triggers the myotatic (also called stretch) reflex, causing the muscle to contract and counteract the perceived threat. This reactive contraction helps keep the muscle within a safe range of motion, preventing injury.

If you’ve studied different stretching methods, you may have heard warnings against ballistic (bouncing) stretching due to concerns that the rapid movement could trigger the stretch reflex and negate the benefits. However, extensive research shows this fear is largely unfounded. In Yoga Biomechanics, Jules Mitchell highlights that ballistic stretching can be highly beneficial when done correctly.¹ Many activities, especially in athletics, involve ballistic movements. Training muscles to handle these types of stretches safely is an effective strategy for improving movement and flexibility.

The muscle spindle also has a complex feedback system called the gamma efferent system. It is a secondary means of managing muscle contraction and regulates muscle tone, ensuring proper muscle function by coordinating gamma and alpha motor neurons. Essentially, the gamma efferent system fine-tunes the sensitivity of muscle spindles to stretch. Gamma motor (efferent) neurons specifically innervate the intrafusal muscle fibers within the muscle spindle, helping to adjust the sensitivity of the muscle spindle to changes in muscle length. When a muscle is stretched, the gamma motor neurons maintain tension within the spindle, allowing it to continue sensing even during contraction. This fine-tuning enables precise control of movements and postural stability. 

In contrast, alpha motor neurons innervate the larger, extrafusal muscle fibers responsible for generating force and actual muscle contraction. Together, the gamma and alpha motor neurons coordinate to regulate muscle activity, ensuring smooth and controlled movements while protecting against overstretching and injury.

Structure of the Golgi Tendon Organ

The GTO is a simpler structure compared to the muscle spindle and is located in the muscle-tendon junction (Image 2). The GTO’s primary function is to monitor muscle contraction force. While muscle fibers contract, tendons remain relatively inflexible. As the contracting muscle fibers pull on the tendon, the GTO senses the amount of pulling force the muscle fibers exert on the tendon. 

The GTO lies within interwoven collagen fibers, which are compressed during muscle contraction, allowing the GTO to measure the tension being applied. The degree of compression from the tensile load is what the GTO is reporting to the central nervous system. If the GTO detects excessive contraction, which could cause muscle damage (such as when attempting to lift a heavy weight), it signals the central nervous system to reduce the muscle’s contraction force. In this way, the GTO prevents injury by counteracting excessive force, acting as the opposite of the myotatic reflex, which stimulates contraction. 

The GTO also works in tandem with muscle spindles to fine-tune muscle contraction. For example, if you reach for a jar you expect to be full but it is actually empty, the GTO quickly adjusts the muscle force, helping you control the movement with appropriate tension.

Proprioceptors and Pain Management

Muscle spindle cells and GTOs play a key role in pain management. Nociceptors, the sensory cells that transmit information about mechanical, chemical, and thermal stimuli, send signals that the brain processes as pain. These nociceptive signals are carried by two types of nerve fibers: A-delta fibers, which are thinly myelinated and associated with acute pain, and C fibers, which are unmyelinated and linked to chronic pain.

In contrast, proprioceptive information from GTOs and muscle spindles is transmitted via thickly myelinated A-alpha fibers, which carry signals faster than A-delta and C fibers. As a result, proprioceptive signals can reach the central nervous system more quickly, potentially blocking some nociceptive input. This phenomenon, known as the Gate Theory of Pain, explains why moving a painful body part can help alleviate pain.

Proprioceptors also play a role in pain conditions if they send incorrect information. There are reasons why the proprioceptors get out of calibration. Impaired proprioception can lead to abnormal movement patterns, poor body mechanics, and decreased stability, increasing the risk of falls and injury, particularly in older adults. These issues can cause increased muscle tension or guarding as the body compensates for faulty positional information, potentially leading to chronic muscle pain.

Massage Therapy Techniques and Proprioceptors

Researchers are still exploring how massage influences proprioceptors, but current evidence suggests that massage and manual therapies can positively impact pain management and improve movement by affecting proprioceptive systems.² Dysfunctional biomechanics often contribute to pain conditions, and massage can help restore proper movement by enhancing proprioceptive awareness.

Active engagement techniques are among the most effective ways to boost proprioceptive awareness. These techniques involve applying massage while the client performs active or passive movements. As these movements generate proprioceptive signals, they increase body awareness, reduce pain, and improve muscular coordination. Additionally, these techniques activate the Gate Theory of Pain management, further amplifying their therapeutic effects. The powerful physiological benefits of these approaches should not be underestimated.

Understanding muscle spindle function is crucial when using stretching or range-of-motion techniques. Since muscle spindle activation increases muscle contraction, avoiding positions that trigger the myotatic reflex is essential. This reflex can occur before reaching what might be considered the full range of motion. For example, in a client with a severe cervical muscle injury, gentle stretching may still trigger a protective response, causing the myotatic reflex to engage. This reaction could lead to increased muscle contraction and pain.

Massage improves proprioception overall, but it is important not to misapply proprioceptive concepts. For instance, some claim that grasping and shortening a muscle manually stimulates the GTO, similar to muscle contraction, thereby reducing tension. However, this technique does not actually activate the GTO. GTO engagement requires an active muscle contraction that cannot be stimulated by a manual technique alone. 

Conclusion

It’s crucial for massage therapists to understand proprioceptors when seeking to improve client outcomes. A solid grasp of the structure and function of muscle spindles and GTOs forms the foundation for applying techniques with greater precision and effectiveness.

Recognizing the role of proprioceptors in motor control, pain perception, and injury prevention allows therapists to tailor their methods to the specific needs of their clients. For example, knowledge of the myotatic reflex helps therapists perform stretching techniques that respect the body’s protective mechanisms while promoting flexibility and range of motion.

Additionally, understanding the interaction between proprioceptors and pain pathways through the Gate Theory of Pain equips therapists with a valuable tool for pain management. This insight supports the creation of treatment strategies that incorporate movement and proprioceptive stimulation to modulate pain signals, offering relief to clients with chronic pain conditions.

By integrating this knowledge into their practice, massage therapists can improve assessment skills, refine treatment techniques, and deliver more effective care. Staying informed about the proprioceptive system enables massage therapists to provide evidence-based, advanced care that addresses the intricate relationship between sensation, movement, and pain. 

Notes

1. Jules Mitchell, Yoga Biomechanics, (Handspring Publishing, 2019).

2. Mal-Soon Shin and Yun-Hee-Sung, “Effects of Massage on Muscular Strength and Proprioception After Exercise-Induced Muscle Damage,” Journal of Strength and Conditioning Research 29, no. 8 (August 2015): 2255–60, https://pubmed.ncbi.nlm.nih.gov/25226328.