Why stretching theories lean toward neural explanations, less mechanical

While stretching is often recommended by many fitness, sports, and rehab professionals, few could explain how it works and why. Traditional explanations often include “it lengthens your muscles” or “releases fascia.” However, research in the last 30-plus years has shown that stretching is less about mechanical properties and more about a combination of structural, neural, physiological, and even psychological factors.

First, there are several types of stretching where each has different effects on muscles and joints:

• Static: Moving a muscle to its end range of holding that position for about 15 to 60 seconds.
• Dynamic: Using controlled, repetitive movements that take a joint through its range of motion without holding a stretch.
• Passive: Using external force, such as a strap, gravity, or a partner, to move a joint with the stretched muscle relaxed.
• Active: Contracting the muscle that is opposite of the muscle that is stretched without external force.
• PNF (proprioceptive neuromuscular facilitation): Combining stretching and voluntary muscle contractions, often using contract–relax or hold–relax techniques to increase range of motion.
• Ballistic: Using rapid, bouncing or momentum-driven movements to move a joint beyond its normal range of motion.

While most studies use static stretching, decades of research shows that no single type of stretching is “best,” and the type you use depends on your goal.

Stretching properties

In a 2010 paper published Physical Therapy, independent researcher Dr. Cynthia Weppler and Dr. Stig Peter Magnusson from the University of Copenhagen highlighted four mechanical theories of muscle extensibility.

• Viscoelastic deformation
• Plastic deformation of connective tissues
• Increased sarcomeres
• Neuromuscular relaxation

First they defined muscle extensibility as “the ability of a muscle to extend to a predetermined endpoint.” This endpoint would depend on the study’s intent, such as the subjects’ stretch sensation, they wrote.

Viscoelastic deformation

Weppler and Magnusson wrote that muscles are considered to be “viscoelastic,” which means that they can resume their original length once tensile force that is causing the stretch is removed. “Yet, like liquids, they also behave viscously because their response to tensile force is rate and time dependent,” they wrote. “This increased length is a viscoelastic deformation because its magnitude and duration are limited by muscles’ inherent elasticity.”

They cited a 1990 rabbit study that was commonly used to support the viscoelastic deformation theory, but they pointed out that this was not supported in human studies. Research on hamstrings and ankle plantarflexors showed that any muscle extensibility is temporary. The degree and duration of these mechanical changes depend on how long and how the stretch was applied.

“With stretch application typical of that practiced in rehabilitation and sports, the biomechanical effect of viscoelastic deformation can be quite minimal and so short-lived that it may have no influence on subsequent stretches,” Weppler and Magnusson wrote.

They gave one example in a 2000 study where a single 45-second static stretch produced no measurable effect on the next stretch performed 30 seconds later. The subjects in the study performed three consecutive 45-second stretches with 30-second rest intervals. Their muscles showed about a 20% viscoelastic stress relaxation during each stretch, meaning resistance decreased while the stretch was being held. However, this relaxation disappeared by the time the next stretch began.

Plastic deformation of connective tissues

Plastic deformation refers to a high-intensity stretch that pushes tissues past its elastic limit so they remain permanently lengthened. However, in 10 studies that Weppler and Magnusson cited that supported plastic deformation had no evidence to support the theory. What little evidence cited was based on animal studies, such as a 1971 rat tail tendon study that was often treated as a foundational piece of evidence for plastic deformation. 

That study isolated rat tail tendons, not muscles, and was conducted under strict laboratory conditions. What the researchers observed was that, when low loads were applied for long periods, the tendon exhibited viscous flow: it slowly elongated while the load was maintained. 

Another publication that Weppler and Magnusson cited is from a 1981 physical therapy manual that described the properties of stretching and clinical applications. 

“Neither of these works recommended the classical model of plastic deformation, which requires high stretching loads, but instead suggested viscoelastic deformation [by] using lower stretching loads with prolonged stretch duration in order to facilitate ‘viscous flow’ within the connective tissue,” Weppler and Magnusson wrote.

Increased number of sarcomeres

Muscle extensibility is sometimes attributed to having more sarcomeres in muscles, but the evidence comes almost entirely from animal studies, according to Weppler and Magnusson. Sarcomeres are the basic units of muscle fibers that contribute to muscle contraction.

In these studies, animal subjects have their muscles held in extreme lengthened positions for a period of time. Researchers have observed that this increases the number of sarcomeres, but this does not actually increase total muscle length because the individual sarcomeres become shorter, Weppler and Magnusson wrote.

“These animal studies suggest that muscles adapt to new functional lengths by changing the number and length of sarcomeres in series in order to optimize force production at the new functional length,” they wrote. “Despite substantial differences between muscle immobilization and intermittent stretching, this research has been generalized to suggest that short-term (3- to 8-week) human stretching regimens cause similar increases in sarcomeres.”

Due to research ethical reasons, no human research has been done to replicate the animal research, they added. Therefore, there’s no solid evidence that the sarcomere increase could be applied to humans.

Neuromuscular relaxation

The neuromuscular relaxation theory proposes that muscles resist stretching because the stretch reflex causes involuntary contraction, Weppler and Magnusson wrote. They described slow, static stretching or techniques like PNF stretching reduce this reflex activity, leading to muscle “relaxation.” Over time, repeated stretching is thought to retrain these reflexes, allowing muscles to relax more easily and become more extensible.

However, Weppler and Magnusson wrote that research does not support this theory. They wrote that stretch reflexes mainly occur during very fast, brief stretches in mid-range positions, not during the slow, sustained stretches typically used in rehabilitation or fitness. 

“Most studies of subjects who were asymptomatic and whose muscles were subjected to a long, slow, passive stretch into end-range positions did not demonstrate significant activation of stretched muscles,” they wrote. “Even studies that simulated ballistic stretching demonstrated no evidence of significant stretch reflex activation of muscles both in human and animal models.”

Alternative to mechanical theories

Some researchers in the 1990s tested these mechanical theories of stretching and found that stretching increases stretch tolerance with little change to muscle extensibility and its structure. Weppler and Magnusson called this the “sensory theory” where changes to muscle extensibility are primarily caused by the nervous system.

In a 1994 study of 14 young adults in the Netherlands, half of the participants performed a daily, home stretching program for the hamstrings while the other half did no stretching (control). Both groups’ hamstring muscles were measured before and after the four-week study.

When the researchers compared the data between both groups, they found a “slight but significant increase” in muscle extensibility with a large improvement in the stretching group, but muscle elasticity remained the same.

“It is concluded that stretching exercises do not make short hamstrings any longer or less stiff, but only influence the stretch tolerance,” they concluded.

Weppler and Magnusson wrote that because the stretch endpoints were based on the sensation, the studies suggested that increases in muscle extensibility after immediate and short-term stretching programs “are due to an alteration of sensation only and  not to an increase in muscle length.”

They pointed out that psychological factors also play a role in muscle extensibility because of the subjects’ expectations of the results of stretching: “…a willingness of subjects to tolerate greater torque application,” they wrote.

By the 2020s, several systematic reviews and meta-analyses found similar yet mixed results. In a 2023 study of 19 trials with a total of 467 participants, researchers Panidi et al. found that fascicles—muscle fiber bundles—can lengthen under high stretching volumes and intensity, but in a “trivial” amount. They cited that studies that use low-intensity stretching do not induce such lengthening.

However, they noted that two limitations where most studies: 

• Used perceived discomfort at the endpoint of stretching, which is not an objective measurement.
• Examined mostly the ankle joint, which may not extrapolate to other joints.

Panidi et al. cited that the high-volume studies averaged around 25,000 seconds total (almost seven hours of stretching) compared with about 3,000 seconds in the low-volume group. This difference is mainly from much longer stretching bouts (often 30 to 300 seconds per muscle, averaging about 100 seconds), not from doing more exercises or more sessions per week.

They wrote that athletes usually stretch for 10 to 20 seconds per muscle, which is far below the doses associated with any structural change. Fascicle length increase typically involves daily stretching, sustained holds for more than 60 seconds per muscle, and stretching programs lasting 10 weeks or more. Shorter programs (around five weeks), lower intensity, or brief stretches showed no physical effects, Panidi et al. wrote.

In a 2025 systematic review and meta-analysis by Ingram et al., the researchers found that acute and chronic static stretching reduced overall stiffness and stretch tolerance only increased after chronic static stretching. The study included 65 studies with more than 1,400 adults.

Similar to the systematic review by Panidi et al., this study found neither acute or chronic stretching had significant changes to fascicle length. Instead, Ingram et al. reported that increases in muscle extensibility is mainly attributed to an increase to stretch tolerance and reduction of stiffness. Contrary to Panidi et al., their findings do not support changes to fascicle length because of the differences in how the research was done.

Ingram et al. wrote that Panidi et al. “stratified effects by region of the same muscle (i.e. distal, medial, low and high regions of the gastrocnemius medialis) or immediately adjacent muscles (i.e. gastrocnemius medialis, gastrocnemius lateralis and soleus) obtained from multiple single studies, and did not account for within-study clustering, which may have inflated the magnitude of their summary effects.”

They emphasized that their position doesn’t mean sarcomeres cannot grow in humans. Ingram told Massage & Fitness Journal that this is the first study that showed stretch tolerance accounted for most of the range of motion improvements by using a meta-regression method. He said that a meta-analysis gives a single answer, such as a yes-no answer or if the results are statistically significant or not, as previous studies have shown.

“A meta-regression takes this a step further and tries to explain the role of specific variables in explaining the result,” Ingram said. “A lot of the benefits that we see in terms of improved flexibility are largely from [becoming] comfortable with the discomfort associated with stretching towards our end of range. There is plausibility for a physical increase in muscle length. However, the duration and intensity of static stretching needed to do so would make it impractical in the real world.”

A more practical application to improving flexibility would be using eccentric stretching and loading stretching, such as in weight training, to change the mechanical properties of muscles and joints, he said.

“Static stretching absolutely is beneficial for improving flexibility, especially acutely,” Ingram said. “Hence why you might happily trade-off an arguably minuscule temporary drop in power and explosiveness for an arguably greater improvement in [range of motion] in sports and activities where flexibility is of great importance, such as gymnastics.”