To keep muscles safe and healthy, we need to realize that our clients’ bodies aren’t all built the same and everyone’s stretch reflexes are different. It is also essential that you have a good understanding of the body’s most basic underlying structural components, including Golgi tendon organs (GTOs) and muscle spindles, and how these two receptors work together within the body.
Muscle spindles and the GTOs are two important proprioceptors involved in flexibility, stretching and working together reflexively to regulate muscle stiffness. When the GTO is excited or activated, it causes the muscle to relax (which is opposite of a muscle spindle causing it to contract). When this relaxation happens, it causes a stoppage of the contraction called autogenic inhibition, which makes the same muscle relax.
Imagine muscle spindles being like a slinky wrapped around muscle fibers; as the muscle lengthens or stretches, so do the muscle spindles. This causes the muscle to contract (the slinky comes back together) and protects the muscle from being overstretched by contracting, which is referred to as the stretch reflex.
Muscle spindles cause two things to happen: 1) the muscle being stretched (agonist) contracts to prevent you from going too far too fast in that stretch; and 2) the opposing muscle force (antagonist) is inhibited (prevented from contracting) so that that muscle doesn’t pull that stretch further. The muscle spindle’s reflex contraction of its associated muscle simultaneously causes the antagonist muscle group to relax, which is called reciprocal inhibition. The muscle spindle serves as an alert to the muscles; they are located in the muscle belly and sense muscle stretch. These are important when it comes to body awareness (proprioception and kinesthetic), so you should be familiar with those terms, too.
Reciprocal inhibition can also be induced by having a muscle contract right before it is passively stretched through a type of stretching called proprioceptive neuromuscular facilitation (PNF). PNF stretching is a method of promoting the response of neuromuscular mechanisms through the stimulation of proprioceptors in an attempt to gain more stretch in a muscle. It is also referred to as a contract/relax method of stretching. This is a low-grade (50 percent) stretch of the antagonist (opposite) muscle for six to 15 seconds to reduce muscle spindle activity within the agonist (prime mover) muscle.
The GTOs sense muscular tension within muscles when they contract or are stretched. When the GTO is activated during contraction, it causes inhibition of the contraction (autogenic inhibition), which is an automatic reflex. When the GTO is activated during stretching, it inhibits muscle spindle activity within the working muscle (agonist) so a deeper stretch can be achieved. GTOs are sensitive to changes in tension and rate of tension and, because they are located in the musculotendinous junctions, they are responsible for sending information to the brain as soon as they sense an overload. Static stretching is one example of how muscle tension creates and activates a GTO response. So, when you hold a low-force stretch for more than seven seconds, the increase in muscle tension activates a GTO response, which temporarily inhibits muscle spindle activity and any tension in the muscle, and allows for further stretching.
The muscle spindles and GTOs go through this cycle to help you stretch safely and effectively. This is also the reasoning behind holding a stretch for seven to 10 seconds to allow the stretch to deepen. GTOs and muscle spindles work together through their reflexive actions to prevent injury.
For a scientific breakdown of GTOs and muscle spindles, check out: What’s the Difference between Autogenic and Reciprocal Inhibition?
If you are a visual learner, feel free to refer to outside credible sources, such as YouTube videos, to get further clarification on this topic.