Muscle Spindles and GTO
Your explosiveness is lagging, your jumps seem slow, your reaction time seems off and your lifts have no speed. Slowly, you feel like your body is deteriorating and you can’t seem to pinpoint why you feel so sluggish. You start to look into physiology and anatomy. Is there something that can help you conquer this weakness? Is there something that can be done to ignite some sort of speed and power gains?
There comes a point where it’s necessary to understand the control and monitoring of power output and muscular control. External information is utilized by the body to enhance coordination, improve awareness and raise the level of proprioception. This information is understood by the body and then contributes to overall execution and use of movement.
This control stems from peripheral receptors that lead to control over the reflex of muscle fibers and potentially even the inhibition of muscle fibers. Is there some sentient control from the actual human individual? Absolutely, this motor programming occurs within the premotor cortex but part of this control is also fed by the information provided by two very key receptors inside our muscle and inside our tendon complex.
Welcome to the Muscle Spindle
Within each muscle fiber, there are numerous receptors that are referred to as muscle spindles. These spindles provide feedback and information to the spinal cord or nervous system to understand the length and speed of change. These spindles exist within each muscle within the intrafusal fibers.
Understanding the makeup of muscle is incredibly important. Within a muscle, there are intrafusal and extrafusal fibers. The intrafusal fibers do not contract but instead have nerve fibers that detect stretch and positioning. They only contain actin and myosin at the end of the fibers. Because of the greater myofilament content of the extrafusal fiber, it can generate approximately 36 times more force than an intrafusal fiber.
When a muscle is stretched, the spindle which exists around the intrafusal fiber sends an afferent signal to the spinal cord. At the spinal cord, interneurons stimulate the muscle to cause stretch “reflex” which will have an inhibitory effect on the antagonistic muscle that is being stretched.
In short, the interneuron connects the afferent signal with the efferent signal which is then connected to the alpha motor neuron which is responsible for activating the extrafusal muscle fibers to contract. This means the greater the stretch, the greater the contraction comes from the muscle. This takes us into the golgi tendon organ.
Golgi Tendon WHAT??!?!?!
The Golgi Tendon Organ! This is a sensory receptor organ that exists in the musculotendinous junction. This is where the muscle connects to the tendon, the tendon connects to the bone. The purpose of the GTO is to detect tension and potentially send inhibitory information then to the motor neuron that innervates the muscle. This protective mechanism then inhibits the most powerful motor neurons, the alpha motor neuron.
When a muscle and the connective tissue that attaches to the muscle are stretched or lengthened during pulling of the muscle or through activation of the muscle, the GTO is pinched by collagen and this excites the afferent signal (which is sent to the spinal cord). This is how the GTO is activated which makes it seem to be a monitor of muscular force. The amount of force it takes to excite the tendon organ depends on the mode of activation.
The goal over time is to strengthen muscle fibers which in turn will lead to thicker tendons and a more conservative GTO. However, the GTO will not be overly conservative, relative to body and its capability at that moment. So what can we do?
We understand muscle spindles and the role they play, we understand the Golgi Tendon Organ and how that protects muscles and in most cases is responsible for the eccentric movement of various exercises. It is important to note that both of these receptors contribute tremendously to proprioception. Proprioception is the knowledge or position of awareness in space. This is incredibly helpful for being able to fire a necessary amount of muscle fibers while executing an explosive movement.
This is where training starts to come into play. If we know and understand that longer eccentric movements (both time and distance) lead to greater increases in tendon size and muscle size, we can then comprehend that slower eccentric movements will train our GTO and muscle spindles to be more liberal in their inhibition. Additionally, we also know that if we train plyometric movements consistently, the velocity of our reaction will be based on the firing of the muscle spindles.
Revert back to our knowledge of muscle spindles. When the spindle recognizes a rapid stretching, they communicate with the brain to induce a contraction to shorten the muscle and protect it from tearing. This brings us back to the stretch shortening cycle and utilizing plyometrics.
Train Slow/Train Fast and Recap
Understanding how our peripheral receptors work helps us understand some of the best ways to enhance speed based training. If we need to alter our GTO inhibition, we can work through slower eccentric movements and strengthen the tendon with more time under tension and more muscular hypertrophy. To train the ability of our muscle spindles, we need to utilize plyometrics and rapid movements like Olympic lifts to enhance the communication during a stretch shortening cycle. When the body improves its ability to detect rapid tension, it will in turn move faster and apply force at a quicker rate!
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