Andrew Huberman explains the neuroscience and practical protocols behind improving flexibility through stretching
Andrew Huberman, neuroscientist and professor at Stanford School of Medicine, presents a solo episode on the science of flexibility and stretching.
Summary
Andrew Huberman breaks down the neuroscience underlying flexibility, covering the roles of motor neurons, sensory spindles, Golgi tendon organs, and a little-known class of neurons called von Economo neurons that sit at the intersection of interoception, pain tolerance, and voluntary override of reflex responses. He argues that static stretching — performed at low intensity, for 30-second holds, at least five days per week — is the most effective method for producing lasting improvements in range of motion. A key finding he highlights is that stretching at only 30–40% of pain threshold is more effective than stretching at 80% intensity, which has significant implications for how most people approach flexibility training. He also presents research showing that yoga practitioners develop measurably greater insular cortex gray matter volume and double the pain tolerance of non-practitioners, suggesting that flexibility training reshapes the brain as well as the body.
Key Takeaways
FULL TRANSCRIPT
Introduction to the Neuroscience of Flexibility
Andrew Huberman: Today we are going to discuss the science and practice of flexibility and stretching. The important thing 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, meaning muscles; and connective tissue. Connective tissue is the stuff that surrounds the neural and muscular components, although it's all kind of woven together and braided together in complicated ways.
Here's a key thing that everyone should know, whether or not you're talking about flexibility. Your nervous system controls your muscles — it's what gets your muscles to contract. Within your spinal cord you have a category of neurons, nerve cells, called motor neurons. Those neurons release a chemical called acetylcholine. The release of acetylcholine from these neurons onto the muscles causes the muscles to contract. When muscles contract, they are able to move limbs by changing the length of the muscle and adjusting the function of connective tissue like tendons and ligaments.
The Spindle System and the Stretch Reflex
Within the muscles themselves there are nerve connections that arise from a different set of neurons in the spinal cord called sensory neurons. These spindle connections within the muscle wrap around the muscle fibers and sense the stretch of those muscle fibers. So now we have two parts to the system: motor neurons that can cause muscles to contract and shorten, and spindles within the muscles themselves that wrap around the muscle fibers and send information back to the spinal cord — a form of sensing what's going on in the muscle.
Why would that be useful? What this does is create a situation where, if a muscle is stretching too much because the range of motion of a limb is increased too much, the muscle will contract to bring that limb range of motion back into a safe range. To clarify, this whole thing looks like a loop. The essential components of the loop are: motor neurons contract muscles; sensory neurons called spindles sense stretch within the muscles; and if a given muscle is elongating because of increased range of motion of a limb, those sensory neurons send an electrical signal into the spinal cord, activating the motor neuron, which then contracts the muscle and brings the limb back into a safe range of motion.
So that's one basic mechanism to hold in mind — the idea of a spindle that senses stretch and can activate contraction of the muscles.
Golgi Tendon Organs and Load Sensing
The next mechanism involves sensing loads. At the end of each muscle you typically have tendons, and there are neurons closely associated with those tendons called Golgi tendon organs, or GTOs. These are sensory neurons that sense how much load is on a given muscle. If you're lifting something very heavy, these neurons fire — they send electrical activity into the spinal cord — and those neurons have the ability to shut down, not activate, motor neurons and prevent the contraction of a given muscle.
For instance, if you were to try and pick up a weight that is much too heavy for you — one that could potentially rip your muscles or tendons off the bone, disrupt the joints, or tear ligaments — you have a safety mechanism in place. These GTOs get activated and shut down the motor neurons, making it impossible for those muscles to contract.
Von Economo Neurons and the Insula
There are also mechanisms that arrive at the neuromuscular system from higher up in the nervous system, from the brain. Those mechanisms involve a set of neurons that I'm guessing 99.9% of you have never heard of — including neuroscientists. These neurons seem uniquely enriched in humans and probably perform essential roles in our ability to regulate our physiology and emotional state.
Within the brain we have the ability to sense things in the external world — something called exteroception — and the ability to sense things in our internal world, within our body, called interoception. Interoception can be the volume of food in your gut, whether or not you're experiencing organ pain or discomfort, whether or not you feel good in your gut and organs. The main brain area associated with interpreting what's going on in our body is called the insula — I-N-S-U-L-A. It's a very interesting brain region with two major parts.
The front of the insula is mainly concerned with things like smell and to some extent vision — whether to approach or avoid something based on sensory input. The posterior insula, the back portion, has a very interesting and distinct set of functions. It is mainly concerned with your somatic experience — how you feel internally. It mainly batches information into either "I want to keep doing this, approach this, continue down this path," or "I need to get out of here, I don't want any more of this, this is painful or aversive or stressful."
In your posterior insula, you have a very interesting population of exceptionally large neurons called von Economo neurons — neurons that are, again, largely unknown even to most neuroscientists, and that seem uniquely enriched in humans. Why is that interesting? These von Economo neurons have the unique property of integrating our knowledge about our body movements, our sense of pain and discomfort, and can drive motivational processes that allow us to lean into discomfort and indeed to overcome any discomfort if we decide that the discomfort we are experiencing is good for us or directed toward a specific goal.
There's another really interesting aspect of these von Economo neurons: they are connected to a number of different brain areas that can shift our internal state from one of so-called sympathetic activation — a pattern of alertness and even stress, sometimes even panic — to one of so-called parasympathetic activation, one of relaxation.
Oftentimes you'll hear that stretching should be done by relaxing into the stretch. What does it actually mean to relax into the stretch? These von Economo neurons sit at this junction where they're able to evaluate what's going on inside our body and allow us to access neural circuitries by which we can shift our relative level of alertness or stress down a bit, thereby increasing parasympathetic activation and literally overriding some of those spindle mechanisms — especially the spindle mechanisms at the neuromuscular and muscular-spinal junction.
Overriding Reflexes Through Conscious Decision
A brief example of this is the monosynaptic stretch reflex — something every first-year neuroscience graduate student learns. If you were to step on a sharp object with a bare foot, you would not need to make the decision to retract your foot. You would automatically do that, provided you have a healthy nervous system. Mechanisms in place cause the retraction of that limb by ensuring that the proper muscles contract and other muscles fully relax. In the case of stepping on a piece of glass or a nail, you would essentially activate the hip flexor to lift your foot as quickly as possible. That same neural circuit would activate a contralateral — meaning opposite side of the body — circuit to ensure that the other foot would do exactly the opposite and extend, making sure you don't fall over. All of that happens reflexively, without any thought or decision-making.
However, if your life depended on walking across sharp objects — let's say you had to walk across very hot stones to get away from something — you could override that stretch reflex by way of a decision made with your upper motor neurons, your insula, and your cognition, and almost certainly those von Economo neurons. Those neurons would be screaming "Don't do this," but could shuttle that information to brain areas that would allow you to override the reflex and push through the pain — and perhaps even not experience the pain to the same degree, or even at all.
So these von Economo neurons sit at a very important junction within the brain. They pay attention to what's going on in your body — pain, pleasure, and what's going on with your limbs and limb range of motion. They also can control the amount of activation, the alertness or calmness, that you are able to create within your body in response to a given sensory experience. And as mentioned, they seem to be uniquely enriched in humans — related to aspects of our evolution that allow us to make decisions about what to do with our body in ways that other animals simply cannot.
The Four Types of Stretching
There are a number of different types of stretching. Broadly defined, we can describe these as dynamic, ballistic, static, and what's called PNF stretching. PNF stands for proprioceptive neuromuscular facilitation.
The first two — dynamic and ballistic stretching — both involve some degree of momentum and can be distinguished from static and PNF type stretching. To distinguish dynamic from ballistic, the key element is momentum. Both involve moving a limb through a given range of motion. In dynamic stretching, however, it tends to be more controlled, with less use of momentum especially toward the end range of motion. In ballistic stretching there tends to be more swinging of the limb or use of momentum — sometimes a lot more momentum, especially at the end range of motion.
Both of those are highly distinct from static stretching, which involves holding the end range of motion and minimizing the amount of momentum used. Static stretching can be further subdivided into active or passive. You can hear about the Anderson approach or the Janda approach. There's also passive static stretching, which is more of a relaxation into a further range of motion. Nevertheless, static stretching involves both those types of elements but is really about eliminating momentum.
Then there's PNF — proprioceptive neuromuscular facilitation. Proprioception involves both a knowledge and understanding of where our limbs are in space and relative to our body, typically relative to the midline. If your goal is to increase your hamstring flexibility, you might put a strap around your ankle and pull that limb toward you to try and get it back over your head, then progressively relax into that, or put some additional force to push the end range of motion, then relax, and then try to stretch that same limb without the strap. There's a huge range of PNF protocols. Those protocols can be done by oneself, with or without straps, with machines, with actual weights, or with training partners.
Static Stretching as the Superior Method for Long-Term Flexibility
In terms of increasing limb range of motion in the long term — truly becoming more flexible as opposed to transiently more flexible — static stretching, which includes PNF, appears to be the best route. Whether you want to maintain, reestablish, or gain limb range of motion, static stretching with holds of 30 seconds appears to be best.
The question is: how long, how many sets, and how many times a week? To answer those questions, I'll turn to what I think is a really spectacular review. The title of the paper is "The Relation Between Stretching Typology and Stretching Duration: The Effects on Range of Motion."
First, I quote: "All stretching typologies showed range of motion improvements over a long-term period. However, the static protocols showed significant gains — with a P value less than 0.05, meaning a probability that cannot be explained by chance alone — when compared to ballistic or PNF protocols."
So again, static stretching is the preferred mode for increasing limb range of motion, and here the authors make the additional point that static stretching may even be superior not just to ballistic stretching, but also to PNF protocols.
The authors go on to say: "Time spent stretching per week seems fundamental to elicit range of movement improvements when stretches are applied for at least, or more than, 5 minutes per week."
This is critical. This is not 5 minutes per stretch — remember, 30 seconds per static stretch — but at least 5 minutes per week. What this means is that we should probably be doing anywhere from two to four sets of 30-second static hold stretches, five days per week.
A Practical Stretching Protocol
What would an effective stretching protocol look like? Let's talk about hamstrings, though this could be applied to other muscle groups. If you want to improve hamstring flexibility and range of motion, you would do three sets of static stretching for the hamstring — holding the stretch for 30 seconds, resting some period of time, then doing it again for 30 seconds, resting, then holding for 30 seconds again. That would be one training session for the hamstrings. You would probably want to stretch other muscle groups in that same session as well. Three sets of 30 seconds each gives you 90 seconds, and you would do that ideally five times a week or more.
Warming Up Before Stretching
One thing that showed up in my exploration of the peer-reviewed research is the notion of warming up. In general, to avoid injury, it's a good idea to raise your core body temperature a bit before doing these kinds of stretches — even static stretches, which don't involve ballistic movement by definition.
The basic takeaway is that if you are already warm from running, weight training, or some other activity, doing the static stretching practice at the end of that session is ideal because you're already warm. Otherwise, raising your core body temperature by doing 5 to 7, maybe even 10 minutes of easy cardiovascular exercise or calisthenic movements — provided you can do those without getting injured — seems to be an ideal way to warm up the body for stretching.
Doing the static stretching after resistance training or cardiovascular training seems to be most beneficial. I was able to find a number of papers making the argument that static stretching prior to cardiovascular training, and maybe even prior to resistance training, can limit performance in running and resistance training. There are those who say it's immensely beneficial, those who say it inhibits performance, and those who say it depends on exactly how you perform the static stretching, which muscle groups, and how much time elapses between static stretching and performance. But to leave all that aside, doing static stretching after some other form of exercise — or, if not after exercise, after a brief warm-up to raise your core body temperature — definitely seems like the right way to go.
Frequency and the Anderson Method
I'm guessing that most people are not doing five days a week of dedicated static stretch range-of-motion training. But it does appear that that frequency across the week — getting those repeated sessions even if they are short for an individual muscle group — turns out to be important. They're going to offset age-related losses in flexibility if one is dedicated about these practices.
Some of you may be familiar with the Anderson method, which has been around for a long time. Anderson has an interesting principle threaded through a lot of his teachings that is very much in keeping with the study I'm about to describe. He emphasizes stretching to the end of the range of motion, but not focusing so much on where that range of motion happens to be that day. So, for instance, not thinking, "I can always touch my toes, and therefore that's the starting place for my flexibility training today." Rather, take the entirety of your system into account each day and understand that your range of motion might be adjusted that day by tension and stress or by ambient temperature in the room. Define the end range of motion as the place where you can feel the stretch in the relevant muscle groups.
What this means is: feel the muscles as you stretch them. Don't just go through the motions. Don't get so attached to always achieving a stretch of a given distance within a given session. You might actually find that by just finding the place where you can't get much further and holding the static stretch there, your range of motion on the second and third set will be increased considerably.
Low-Intensity Stretching Outperforms Moderate-Intensity Stretching
Along these lines, there's this more nebulous variable of how much effort to put into a stretch. Should you push into the stretch? Would you want to bounce a tiny bit? Would you want to reach into that endpoint and try to extend it within a given set? For that reason I was excited to find a paper entitled "A Comparison of Two Stretching Modalities on Lower Limb Range of Motion Measurements in Recreational Dancers." It's a six-week intervention program that compared low-intensity stretching — which they call micro stretching — with moderate-intensity static stretching on active and passive ranges of motion.
To be very clear, micro stretching in the context of this paper means low-intensity stretching. What they found was that a six-week training program using very low-intensity stretching had a greater positive effect on lower limb range of motion than did moderate-intensity static stretching.
Quoting them: "The most interesting aspect of the study was the greater increase in active range of motion compared to passive range of motion by the micro stretching group."
This relates to what we were just discussing about the Anderson method — that very low-intensity stretching, meaning effort that feels not painful and in fact might even feel easy or at least not straining to exceed a given range of motion, turns out to be not just as effective but more effective than moderate-intensity stretching.
What is low-intensity static stretching? They define this as stretches completed at an intensity of 30 to 40%, where 100% equals the point of pain. So 30 to 40% induced a relaxed state within the individual and the specific muscle. They were holding these static stretches for 1 minute, not 30 seconds.
The control group was doing the exact same overall protocol — daily stretching for 6 weeks, the same exercises, holding each set for 60 seconds — but using an intensity of stretch of 80%, where again 100% represents the point of pain.
I find these data incredibly interesting. If you're going to embark on a flexibility and stretching training program, you don't need to push to the point of pain. In fact, it seems that even just approaching the point of pain is going to be less effective than operating at 30 to 40% of intensity prior to reaching that pain threshold. This is pretty subjective, but all of us should be able to register within ourselves whether a given range of motion brings us to that threshold of pain or near pain. According to this study, operating at an intensity that's quite low and very relaxing turns out to be more beneficial for increasing range of motion than doing exercises at a higher intensity.
Lower-intensity static stretching appears to be the most beneficial way to approach stretching, and I think that's a relief to many of us because it also suggests that the injury risk is going to be lower than if one were pushing into the pain zone.
When to Use Static Stretching Before Training
I want to briefly return to the question of whether to do ballistic or static stretching before skill training, weight training, sport, or cardiovascular exercise like running. There are instances where an individual might want to do some static stretching to increase limb range of motion prior to weight training, even if it reduces their ability to lift as much weight. Why? For instance, if somebody has a tightness or limitation in their neuromuscular connective tissue system that prevents them from using proper form — and that can be overcome by doing some static stretching — that would be a great idea.
There are instances where people are trying to overcome injuries, coming back from reparative surgery, or returning from a layoff, where some additional static stretching prior to cardiovascular, weight, or skill training is going to be useful because it puts them in a position of greater safety and confidence and overall performance, even if it adjusts down their speed or the total loads used.
Similarly, there are a lot of data points suggesting that doing some dynamic or even ballistic stretching prior to skill training or cardiovascular weight training can be beneficial — in part to warm up the relevant neural circuits, joints, connective tissue, and muscles, and to perhaps improve range of motion or the ability to perform movements more accurately, with more stability, and therefore with more confidence.
Yoga, the Insula, and Pain Tolerance
Thus far we've been talking about stretching for the sake of increasing limb flexibility and range of motion, but there are other reasons to embark on a stretching protocol — including our ability to relax and access deep relaxation quickly.
I'd like to return to the insular cortex, the insula. As we discussed at the beginning of this episode, the von Economo neurons that Constantin von Economo, the Austrian scientist, discovered allow us to make and perform interpretations of our internal landscape — pain, our dedication to a practice, whether or not we are in pain because it's a practice we are doing intentionally and want to improve ourselves, or whether it's pain arriving through some externally imposed demand or situation. The insula is handling all of that.
There's a wonderful paper published in the journal Cerebral Cortex entitled "Insular Cortex Mediates Increased Pain Tolerance in Yoga Practitioners." This study explored the effects on brain structure and volume in yoga practitioners. They pulled subjects from backgrounds in Vinyasa yoga, Ashtanga yoga, Iyengar yoga, and Sivananda yoga — some people were new to these practices, some were experienced.
The important takeaway was that they took these yoga practitioners and did not explore their brain structure in the context of yoga itself. They looked at things like pain tolerance. They used thermal stimulation — basically putting people into conditions where they gave them very hot or very cold stimuli — and compared those yoga practitioners of varying levels of experience to those with no experience with yoga, so-called controls.
They found some really interesting things. The pain tolerance of yoga practitioners was double or more that of non-yoga practitioners. They also found significant increases in insular gray matter volume. Typically, when we talk about gray matter, we're talking about the cell bodies — the location in neurons where the genome is housed and where all the housekeeping functions occur. White matter volume tends to be the axons, the wires, because they're in sheets of myelin that appears white in MRIs and indeed is white — it's actually lipid.
Increased gray matter volume of the insula is a significant finding because it suggests that people doing yoga have an increased volume of these brain areas associated with interoceptive awareness and with being able to make judgments about pain and why one is experiencing pain — not just to lean away from pain, but to utilize, leverage, or even overcome it.
It appears that it's not just the performance of the yogic movements, but the overcoming of — or pushing into — the end ranges of motion and pushing through discomfort to some extent, that allows yoga practitioners to build up the structure and function of these brain areas. This enables them to cope with pain better than other individuals and to cope with other kinds of interoceptive challenges — not just pain, but cold, not just pain, but discomfort of being in a particular position.
Again, we wouldn't want people placing themselves into a compromised position that would harm them, especially given that earlier we heard that micro-stretching of the low-intensity, non-painful sort is actually going to be more effective for increasing end range of motion. But this study really emphasized the extent to which practitioners of yoga don't just learn movements — they learn how to control their nervous system in ways that genuinely reshape their relationship to pain, to flexibility, and to the kinds of things the neuromuscular system was designed to do.
So if ever there was a practice that would not only increase flexibility and limb range of motion but would also allow one to cultivate improved mental functioning as it relates to pain tolerance and stress management — features that no doubt carry over into other areas of life — yoga appears to be a quite useful practice.
Summary and Synthesis of Stretching Protocols
Of course, yoga isn't the only way to increase limb range of motion and flexibility. We've described a number of different ways to do that and arrived at some general themes and protocols. Let's revisit a couple of them in summary.
Static stretching appears to be among the more useful forms of stretching. It really does appear that getting at least 5 minutes per week total of stretching for a given muscle group is important for creating meaningful, lasting changes in limb range of motion. That is best achieved by five-day, six-day, or even seven-day-per-week protocols, but those can be very short — limited to, say, three sets of 30, maybe even 45 or 60 seconds of static hold, although 30 seconds seems to be a key threshold for maximum benefit. And of course, always warm up, or arrive at the stretching session already warm.