Improve Flexibility with Research-Supported Stretching Protocols | Huberman Lab Essentials
In a Nutshell
Static stretching, especially low-intensity holds of 30 seconds performed at least 5 minutes per week total per muscle group, is the most effective protocol for long-term gains in flexibility and range of motion. The nervous system—via muscle spindles, Golgi tendon organs, and von Economo neurons in the insula—can override reflexive limits, allowing deliberate increases in limb range when discomfort is reframed as purposeful. Yoga practitioners show doubled pain tolerance and increased insular gray matter, demonstrating that stretching builds both physical flexibility and the neural capacity to manage interoceptive stress.
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Flexibility and the process of stretching and getting more flexible involves three major components: neural (nervous system), muscular (muscles), and connective tissue (the stuff that surrounds the neural stuff and the muscular stuff). These components are all woven and braided together in complicated ways.
The nervous system controls the muscles. Within the spinal cord there is a category of neurons called motor neurons. These 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 way of changing the length of the muscle and adjusting the function of connective tissue like tendons and ligaments.
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.
There are two parts to the system: motor neurons that can cause muscles to contract and shorten, and spindles within the muscles that wrap around the muscle fibers and send information from the muscle back to the spinal cord. This creates a loop where motor neurons contract muscles, and sensory neurons called spindles sense stretch within the muscles. 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 that activates the motor neuron, which then contracts the muscle and brings the limb back into a safe range of motion.
The second mechanism has to do with sensing loads. At the end of each muscle there are typically tendons, and there are neurons closely associated with those tendons called Golgi tendon organs (GTOs). These are sensory neurons that sense how much load is on a given muscle. If lifting something very heavy, these neurons fire and send electrical activity into the spinal cord, then shut down motor neurons to prevent contraction of the muscle. This is a safety mechanism that makes it impossible for muscles to contract if the force could rip muscles or tendons off the bone, disrupt joints, or tear ligaments.
There are also mechanisms that arrive to the neuromuscular system from higher up in the nervous system from the brain. These involve sensing things in the external world called exteroception and sensing things in the internal world within the body called interoception. Interoception includes the volume of food in the gut, organ pain or discomfort, or feeling good in the gut and organs.
The main brain area associated with interpreting what's going on in the body is the insula. The front of the insula is mainly concerned with things like smell and to some extent vision (approach good smells, avoid bad smells). The posterior insula is mainly concerned with somatic experience—how one feels internally. It batches information into "yum" (want to keep doing this, approach this thing) or "yuck" (need to get out, don't want more of this, it's painful or aversive or stressful).
In the posterior insula there is a population of very large neurons called von Economo neurons. These are exceptionally large neurons that seem uniquely enriched in humans. These neurons integrate knowledge about body movements, sense of pain and discomfort, and can drive motivational processes that allow leaning into discomfort and overcoming discomfort if the discomfort is determined to be good or directed toward a specific goal.
Von Economo neurons are connected to brain areas that can shift internal state from sympathetic activation (alertness, stress, panic) to parasympathetic activation (relaxation). They sit at the junction where they evaluate what's going on inside the body and allow access to neural circuitries to shift relative level of alertness or stress down, increase parasympathetic activation, and literally override some of the spindle mechanisms and GTO mechanisms, especially the spindle mechanisms at the neuromuscular and muscular spinal junction.
An example of this capacity is the monosynaptic stretch reflex. If stepping on a sharp object with a bare foot, the foot would automatically retract without needing to make the decision, provided a healthy nervous system. In the case of stepping on a sharp object like glass, a nail, or a tack, the hip flexor would be activated to lift up the foot as quickly as possible. That same neural circuit would activate a contralateral circuit to ensure the leg and foot not stepping on the sharp object would extend to make sure one doesn't fall over. All of this happens reflexively without thought or decision-making.
However, if life depended on walking across sharp objects or hot stones to get away from something to avoid, one could override that stretch reflex by way of a decision made with upper motor neurons, the insula, cognition, and almost certainly the von Economo neurons. These neurons would shuttle information to brain areas allowing override of the reflex and pushing through the pain, possibly even not experiencing the pain to the same degree or at all.
Von Economo neurons pay attention to what's going on in the body (pain, pleasure), including what's going on with limbs and limb range of motion. They also pay attention to and can control the amount of activation, alertness, or calmness in response to a given sensory experience. They seem uniquely enriched in humans and related to aspects of evolution allowing decisions about what to do with the body in ways other animals cannot.
There are four major categories of stretching: dynamic, ballistic, static, and PNF (proprioceptive neuromuscular facilitation). Dynamic and ballistic stretching both involve some degree of momentum and can be distinguished from static and PNF type stretching.
Dynamic stretching tends to be more controlled with less use of momentum, especially toward the end range of motion. Ballistic stretching tends to involve more swinging of the limb or use of momentum, especially at the end range of motion. Both involve movement and require generating some force to create that movement.
Static stretching involves holding the end range of motion and minimizing momentum. Static stretching can be further subdivided into active or passive. There is also passive static stretching involving more relaxation into a further range of motion.
PNF stretching involves proprioception, which includes knowledge and understanding of where limbs are in space and relative to the body, typically relative to the midline. An example protocol: put a strap around the ankle and pull the limb toward you to get it back over the head, then progressively relaxing into that or putting additional force to push the end range of motion, then relaxing it, then trying to stretch that same limb or increase limb range of motion without the strap. PNF protocols can be done by oneself with or without straps, with machines, with weights, or with training partners.
For increasing limb range of motion in the long term and truly becoming more flexible rather than transiently more flexible, static stretching which includes PNF appears to be the best route. To maintain, reestablish, or gain limb range of motion, static stretching holds of 30 seconds appear to be best.
A review paper titled "The Relation Between Stretching Typology and Stretching Duration: The Effects on Range of Motion" found that 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 when compared to ballistic or PNF protocols. Static stretching might even be superior not just to ballistic stretching but also to PNF protocols.
The authors state that 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 not 5 minutes per stretch, but at least 5 minutes per week total. This means doing anywhere from two to four sets of 30-second static hold stretches 5 days per week.
For improving hamstring flexibility and limb range of motion: do three sets of static stretching for the hamstring by holding the stretch for 30 seconds, resting some period of time, then doing it again, holding for 30 seconds, resting, then holding for 30 seconds. That would be one training session for the hamstrings. This would yield 90 seconds of stretching time per session. Ideally do this five times a week or more.
To avoid injury, it is a good idea to raise core body temperature a bit before doing static stretches. If already warm from running, weight training, or some other activity, doing the static stretching practice at the end of that session allows going immediately into the stretching session. Otherwise, raise core body temperature by doing 5 to 7, maybe even 10 minutes of easy cardiovascular exercise or calisthenic movements. The warm-ups don't have to be extremely extensive. Doing static stretching after resistance training or cardiovascular training seems to be most beneficial.
Static stretching prior to cardiovascular training and maybe even prior to resistance training can limit performance in running and resistance training. Doing static stretching after some other form of exercise, or after a brief warm-up to raise core body temperature, is the right way to go.
Getting repeated sessions even if they are short for an individual muscle group turns out to be important. Dedicated practice will offset age-related losses in flexibility.
The Anderson method 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. Rather than thinking about always being able to touch toes as the starting place, take the entirety of the system into account each day. Define the end range of motion as the place where the stretch can be felt in the relevant muscle groups. Feel the muscles as stretching them. Don't just go through the motions. Don't get so attached to being able to always achieve a stretch of a given distance within a given session. Finding the place where one can't get much further and holding the static stretch there may result in range of motion increasing considerably on the second and third set.
A paper titled "A comparison of two stretching modalities on lower limb range of motion measurements in recreational dancers" was a six-week intervention program that compared low-intensity stretching (called micro stretching) with moderate-intensity static stretching on active and passive ranges of motion. A six-week training program using very low-intensity stretching had a greater positive effect on lower limb range of motion than moderate-intensity static stretching.
The most interesting aspect was the greater increase in active range of motion compared to passive range of motion by the micro stretching group. Very low-intensity stretching (effort that feels not painful and might even feel easy or at least not straining) turns out to be not just as effective but more effective than moderate intensity stretching.
Low-intensity static stretching was defined as stretches completed at an intensity of 30 to 40% where 100% equals the point of pain. This induced a relaxed state within the individual and the specific muscle. The stretches were held for 1 minute, not 30 seconds.
The control group did the exact same overall protocol (daily stretching for 6 weeks, same exercises, holding each set for 60 seconds) but used an intensity of stretch of 80% where 100 represents the point of pain. Operating at 30 to 40% of intensity prior to reaching the pain threshold is more beneficial for increasing range of motion than operating at higher intensity. Lower intensity static stretching appears to be the most beneficial way to approach stretching, and injury risk is lower than pushing into the pain zone.
There are instances where an individual might want to do some static stretching to increase limb range of motion prior to
There are instances where an individual might want to do some static stretching to increase limb range of motion prior to doing weight training, even if it's going to reduce that person's ability to lift as much weight. This approach makes sense when somebody has a tightness or a limitation in their neuromuscular connective tissue system someplace in their body that prevents them from using proper form. Static stretching can overcome that limitation and allow proper form, which would be beneficial. There are instances where people are trying to overcome injuries, where they're trying to come back from a reparative surgery or something of that sort, or coming back from a layoff. In these cases, additional static stretching prior to cardiovascular, weight training, skill training, or sport of some kind is going to be useful because it's going to put the person in a position of greater safety, confidence, and performance overall, even if it's adjusting down speed or the total amount of loads that are used.
There are a lot of data points showing that doing some dynamic or even ballistic stretching prior to skill training, cardiovascular, or weight training can be beneficial. This approach helps warm up the relevant neural circuits, joints, connective tissue, and muscles. It may also improve range of motion or the ability to perform movements more accurately, with more stability, and therefore with more confidence.
Stretching protocols can be pursued for reasons beyond increasing limb flexibility and range of motion. These include the ability to relax and access deep relaxation quickly. The insular cortex (the insula) handles interpretations of the internal landscape, including pain and dedication to a practice. This brain region determines whether pain is being experienced because a practice is being done intentionally to improve oneself, or whether pain is arriving through externally imposed demands or situations.
A paper published in the journal Cerebral Cortex entitled "Insular Cortex Mediates Increased Pain Tolerance in Yoga Practitioners" explored the effects on brain structure volume in yoga practitioners. Subjects were pulled from backgrounds in Vinyasa yoga, Ashtanga yoga, Iyengar yoga, and Sivananda yoga. Some practitioners were new to these practices while others were experienced. The study did not explore brain structure in the context of yoga itself but instead looked at things like pain tolerance using thermal stimulation. They gave participants very hot or very cold stimuli and compared yoga practitioners of varying levels of experience to controls with no yoga experience.
The pain tolerance of yoga practitioners was double or more that of non-yoga practitioners. The study also found significant increases in insular gray matter volume. Gray matter refers to cell bodies, the location in neurons where the genome is housed and where housekeeping functions occur. White matter volume tends to be the axons, the wires, which appear white in MRIs and under the microscope because they are wrapped in myelin, a lipid. Increased gray matter volume of the insula is significant because it suggests that people doing yoga have increased volume in brain areas associated with interoceptive awareness and the ability to make judgments about pain and why one is experiencing pain. This allows practitioners not just to lean away from pain, but to utilize, leverage, or even overcome pain.
It appears that it's not just the performance of the yogic movements, but the overcoming 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 brain areas that help them cope with pain better than other individuals. This also helps them cope with other kinds of interoceptive challenges, including cold and discomfort of being in a particular position. The study emphasized the extent to which practitioners of yoga don't just learn movements—they learn how to control their nervous system in ways that really reshape their relationship to pain, to flexibility, and to the kinds of things that the neuromuscular system was designed to do.
Yoga is a useful practice because it not only increases flexibility and limb range of motion, but also allows one to cultivate improved mental functioning as it relates to pain tolerance and other features of stress management that extend into other areas of life.
Static stretching appears to be among the more useful forms of stretching. 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. This is best achieved by 5-day, 6-day, or even 7-day week protocols. These can be very short protocols limited to three sets of 30 seconds, maybe even 45 or 60 seconds of static hold, although 30 seconds seems to be a key threshold that can provide maximum benefit. Sessions should always begin with a warm-up or arrive at the stretching session already warm.
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