Understanding the Golgi Tendon Organ
There are two primary proprioceptors responsible for maintaining proper muscle length and tone—the muscle spindle and the Golgi Tendon Organ (GTO). These two specialized cells work very efficiently together to control movement. This article focuses on the GTO and some common misconceptions about leveraging its function for manual therapy.
The GTO may also be known as the Golgi end organ or the Golgi tendon apparatus. Its primary function is to relay information back to the central nervous system about contraction levels in the muscle. The GTOs are located mostly near the musculotendinous junction. They respond to tensile (pulling) stress in this area that occurs when a muscle contracts and pulls on the tendon fibers.
The GTO is primarily responsible for helping maintain an appropriate level of contraction in a muscle. It does this job by responding to the amount of tension generated at the musculotendinous junction. When a muscle contracts it pulls on the tendon fibers which stimulates the GTO. The GTO perceives the amount of muscle contraction based on how much the fibers of the musculotendinous junction are pulled.
For example, if you go to lift an object off a table and it looks like it is heavy, but it isn’t, you may be quite surprised. You will quickly lift it, but suddenly your muscles adjust and the proper amount of muscular force is applied. The immediate adjustment occurs in part because the GTO senses the differing amount of tensile force exerted at the musculotendinous junction and makes a rapid adjustment.
Tensile forces are generated at the musculotendinous junction during active muscle contraction and also during passive stretch. However, the threshold for activating the GTO is not the same for active contraction as it is for passive stretching. The GTO has a low threshold of activation during muscular contraction. That means that the GTO is sensitive to relatively small amounts of muscle contraction. The threshold for activation for passive stretching is far higher. In fact, passive stretching does not always stimulate the GTO at all.
Originally the GTO was thought to play a primary role in preventing excess muscular contraction by “shutting off” the muscle activation if there was too much tensile load at the musculotendinous junction from excess contraction. However, recent research has indicated that free nerve endings are more involved in this protective process of limiting excess muscle contraction. The GTO function is more about regulating appropriate levels of muscle tension regardless of the load.
Understanding the GTO function is vital for evaluating specific massage techniques. There is a massage technique often advocated for relieving muscle tightness called approximation. The theory behind this technique is that the practitioner grasps the two ends of a muscle and pulls them closer together. Supposedly, pulling the two ends of a muscle closer would stimulate the GTO and therefore cause the muscle to stop its contraction. There are several problems with the theoretical model of this technique.
First, there is the assumption that we can apply tensile stress to the musculotendinous junction by pulling its two ends together and therefore stimulating GTO activity. We now know that GTO activity has a very high threshold of response to passive movement and may not fire at all from passive stretching. So it is unlikely that there is any significant response to this passive force from the GTOs.
Second, it isn’t possible to grasp the ends of a muscle and pull them closer together. A muscle doesn’t shorten unless there is a corresponding joint movement that physically decreases the overall muscle length. When you grasp the ends of the muscle and pull your hands toward each other, you are just sliding the skin over the very slippery underlying connective tissue, so the muscle isn’t shortening at all.
This technique may produce beneficial results in reducing muscle tightness or decreasing muscle cramps. However, the underlying mechanisms by which these results occur are far more likely to be neurological responses to cutaneous nerve stimulation in the skin than activation of GTO receptors. When we understand the function of these specialized cells, we can better determine how our treatment strategies work, and that helps us produce more effective results for our clients.