Essentials: Improve Flexibility with Research-Supported Stretching Protocols

**Andrew Huberman** (0:00)
Welcome to Huberman Lab Essentials, where we revisit past episodes for the most potent and actionable science-based tools for mental health, physical health and performance.
I'm Andrew Huberman, and I'm a professor of neurobiology and ophthalmology at Stanford School of Medicine. Today, we are going to discuss the science and practice of flexibility and stretching. The important thing that I'd like you to know is that flexibility and the process of stretching and getting more flexible involves three major components.
Neural, meaning of the nervous system, muscular, muscles and connective tissue. Connective tissue is the stuff that surrounds the neural stuff and the muscular stuff, although it's all kind of weaved together and braided together in complicated ways. So here's a key thing that everyone should know, whether or not you're talking about flexibility or not.
Your nervous system controls your muscles. It's what gets your muscles to contract.
So within your spinal cord, you have a category of neurons, nerve cells, that are called motor neurons. Those neurons release a chemical. That chemical is called acetylcholine. The release of acetylcholine from these nerve cells, these neurons, onto the muscles causes the muscles to contract. And when muscles contract, they are able to move limbs by way of changing the length of the muscle, adjusting the function of connective tissue, like tendons and ligaments. Now, within the muscles themselves, there are nerve connections. And these are nerve connections that arise from a different set of neurons in the spinal cord that we call sensory neurons. These spindle connections within the muscle that wrap around the muscle fibers, sense the stretch of those muscle fibers.
So now we have two parts to the system that I've described. You've got motor neurons that can cause muscles to contract and shorten. And we have these spindles within the muscles themselves that wrap around the muscle fibers. And that information is sent from the muscle back to the spinal cord. It's a form of sensing what's going on in the muscle. Now, why would that be useful? Well, what this does is it creates a situation where if a muscle is stretching too much because the range of motion of a limb is increased too much, then the muscle will contract to bring that limb range of motion into a safe range again. Okay, so just to clarify, this whole thing looks like a loop and the essential components of the loop are motor neurons contract muscles, sensory neurons that we call spindles are sensing stretch within the muscles. And if a given muscle is elongating because of the increased range of motion of a limb, those sensory neurons send an electrical signal into the spinal cord, such that there is an activation of the motor neuron, which by now should make perfect sense as to why that's useful. It then shortens up the muscle. It actually doesn't really shorten the muscle, but contracts the muscle that brings the limb back into a safe range of motion. So that's one basic mechanism that we want to hold in mind. This idea of a spindle that senses stretch and can activate contraction of the muscles and shorten the muscles. The next mechanism I want to describe, and once again, there are only two that you need to hold in mind for this episode, has to do with sensing loads.
So at the end of each muscles, you have tendons typically, and there are neurons that are closely associated with those tendons that are called Golgi tendon organs, right? These are neurons that are sensory neurons that sense how much load is on a given muscle, right? So if you're lifting up something very, very heavy, these neurons are going to fire, meaning they're going to send electrical activity into the spinal cord, and then those neurons have the ability to shut down, not activate, but shut down motor neurons and to prevent the contraction of a given muscle. So for instance, if you were to walk over and try and pick up a weight that is much too heavy for you, meaning you could not do it without injuring yourself, there are a number of reasons why you might not be able to lift it, but let's say you start to get it a little bit off the ground or you start to get some force generated that would allow it to move, but the force that you're generating could potentially rip your muscles or your tendons off of the bone, right? That it could disrupt the joints and it could tear ligaments. Well, you have a safety mechanism in place. It's these Golgi tendon organs, these GTOs as they're called, that get activated and shut down the motor neurons and make it impossible for those muscles to contract. There are also mechanisms that arrive to the neuromuscular system from higher up in the nervous system, from the brain.