Has this ever happened to you? You are working out, let’s say you are doing biceps curls. You get to the last set, increase the weight and start. By the time you reach the end your muscle is very tired and feels like it’s burning. You try to do that last rep, but suddenly your muscle fails, it relaxes, your hand stretches and your dumbbell falls on the floor. If the answer is yes, than congratulations. You have experienced the autogenic inhibition reflex, or so called Golgi tendon reflex.
What is it?
Simply put, autogenic inhibition reflex is a way your body protects the muscles from damage due to increased tension within them. The main player of autogenic inhibition is Golgi tendon organ. As the name says Golgi tendon organ is located in the tendons of muscles, unlike muscle spindle, which is in the muscle itself. It is made of encapsulated tendon fibers, which are bound to muscle fibers. Sensory nerves transfer the signal from the Golgi tendon organ to the spinal cord, and the brain itself.
How does it work?
Let’s go back to our little story. During your sets of biceps curls the tension in the muscle and tendon grows. For your last set you’ve increased the weight. Now the tension in the muscle is becoming dangerous. It can lead to tendon snapping, muscle damage etc. This tension is detected by Golgi tendon organ. It sends signals to spinal cord, and inhibits the neuron in charge for the movement you are currently doing. After this, the muscle relaxes, tension drops rapidly, you drop your dumbbell, and there is no damage.
Another great effect of autogenic inhibition can be seen during static stretching. Golgi tendon organ can inhibit muscle spindle for a short period of time allowing for further stretching.
Since not all fibers in the same muscle experience the same level of tension, Golgi tendon organ has one more job. It equalizes the tension between fibers, so there wouldn’t be any damage.
The muscle spindle
As we mentioned the muscle spindle, it is only fair to explain what it is. It is a receptor located in the muscle itself. The muscle spindle detects the change in muscle length, not tension. When the muscle is stretched too much, the spindle sends signals to spinal cord, and causes strong contraction. It also causes the relaxation of antagonist muscle (the muscle with opposite function, e.g. triceps to biceps). This is called reciprocal inhibition. So if you stretch your biceps too much, the spindle will contract it, while relaxing the triceps. This mechanism is also protective, as it prevents muscle damage.
Just remember the differences. Golgi tendon organ is in the tendon, while the muscle spindle is located inside the muscle. Golgi tendon organ detects tension, and the muscle spindle detects the stretch. Autogenic inhibition inhibits one muscle to relieve the tension, while reciprocal inhibition activates the stretched muscle, and relaxes the antagonist.