What Stretching DOES and DOES NOT Do

author

Dr Jarod Hall

Physical Therapist United States

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When it comes to programming safe and effective exercise in training and rehabilitation plans, stretching has a long history a being staple. Some rehab and fitness professionals advocate for stretching prior to activity, while differing camps claim stretching can only be beneficial after activity. Some camps advocate only dynamic stretching, while others champion the benefits of low load long duration static stretching. However, the literature defining what we know stretching to actually do, and not do, is often overlooked. This article will address what the evidence has to say about the actual mechanistic changes associated with stretching, whether or not stretching prevents injuries, if stretching can influence posture, if stretching can impact performance, different modes of stretching, general guidelines on how to best program and apply stretching, and if stretching should be programmed.

What are the Different Types of Stretching?

There are a variety of different types of stretching that can be used in an attempt to improve flexibility and movement capacity. However, for the purposes of this paper we will focus on the most common modes of stretching that have been implemented and researched for effectiveness. Static, dynamic, ballistic, and proprioceptive neuromuscular facilitation type stretching are the most commonly used and studied methods of stretching.

Static Stretching:

Static is the most common stretching technique. Static stretching is executed by extending the targeted muscle group to its maximal point and holding it for 30 seconds or more. In most research studies this is completed by holding a stretch 4 times for 30 seconds each30. There are two sub-types of static stretches which include active and passive static stretching. In active, added force is applied by the individual for a greater intensity stretch. In the case of passive static stretching, added force is applied by an external force (e.g., partner or assistive device) to increase the intensity of the stretch.

Dynamic Stretching:

Unlike static stretching, dynamic stretching requires the use of continuous movement patterns that mimic the exercise or sport to be performed. Generally speaking, the purpose of dynamic stretching is to improve flexibility for a given sport or activity by taking the body through purposeful movements to increase comfort with and ability to complete the movements with ease. An example of dynamic stretching would be a sprinter doing long, exaggerated strides or walking lunges to prepare for a race. Similarly, a basketball player may use side shuffles, vertical hops, and T-drills as a mode of dynamic warm up and stretching. Unlike static stretching, dynamic stretching increases full body circulation, heart rate, and respiratory rate by using active muscle contraction and motor control as part of improving tissue extensibility. Furthermore, dynamic stretching and warm ups have been shown to be superior to static stretching and warm ups for performance prior to competition31. This could due to a variety of variables such as improved neural control, increased tissue extensibility, increased blood flow, increased tissue temperature, or various other factors.

Ballistic Stretching:

Ballistic stretches force limbs into an extended range of motion when the muscle has not relaxed enough to enter it. It involves fast “bouncing” movements where a double bounce is performed at the end range of movement. This type of stretching is typically used for athletic drills and utilizes repeated bouncing movement to stretch the targeted muscle group. While these bouncing movements usually trigger the stretch reflex and may cause increased risk for injury, they can be safely performed if done from low-velocity to high-velocity and preceded by static stretching.

Proprioceptive Neuromuscular Facilitation:

This stretching technique capitalizes on the use of autogenic and reciprocal inhibition, and includes three types of techniques:

  • Hold-relax
    • Perform a passive 10-second pre-stretch.
    • Hold and resist force applied by a second individual, causing an isometric contraction in the target muscle group, for 5-8 seconds.
    • Relax the muscle group and allow a passive stretch; hold for 30 seconds to increase range of motion (ROM).
    • There should be a greater stretch during this final phase due to autogenic inhibition.
  • Contract-relax
    • Perform a passive 10-second pre-stretch.
    • A second individual applies resistance, counteracting the client’s force of concentric contraction of the target muscle group, without completely restricting the joint through its ROM.
    • Relax the muscle group and allow a passive stretch; hold for 30 seconds to increase ROM.
    • There should be a greater stretch during this final phase due to autogenic inhibition.
  • Hold-relax with agonist contraction
    • This technique is similar to the Hold-relax technique, but differs for the final stretch.
    • Relax the muscle group and allow a passive stretch. Concentrically contract the opposing muscle group of the target muscle group that is being stretched to actively move the limb through the target ROM; hold for 30 seconds to increase ROM.
    • There should be a greater stretch during this final phase due to reciprocal and autogenic inhibition.

What Does Stretching Do?

In simple terms, stretching improves flexibility and ROM, and does so quite effectively and reliably. However, what stretching actually does on the muscular and neural level is an area that is less well understood and still under significant debate. Various theories have been proposed to explain increases in muscle extensibility observed after intermittent stretching. Some research groups propose that increases in flexibility are solely due to neural mediated effects and increased stretch tolerance15,16. While other groups propose that stretching does in fact make at least minor changes on the histological level of the musculotendinous unit12-14.

When we look deeper into the literature it would appear that both camps are correct to some degree. The work of Weppler, Folpp, Law, and Magnusson proposes that the bulk of flexibility increases with stretching are attributed to improved tolerance to stretch and neural dampening effects24-28. The results of these studies demonstrate that stretching does not increase true muscle extensibility, but instead simply increase stretch tolerance, which contradicts anecdotal evidence and the results of some clinical trials indicating the effectiveness of stretch for increasing extensibility24-28.

The authors state that the explanation for changes in stretch tolerance is not known25. They do however, propose stretch tolerance may be influenced by nociceptive nerve endings, mechanoreceptors, and/or proprioceptors25. Alternatively, these papers suggest that stretch may change some other aspect of the sensory neural pathways that is currently unknown25. For example, afferent input from muscles and joints during a stretch maneuver may interfere with signals from nociceptive fibers (stretch discomfort), subsequently inhibiting an individual’s perception of pain25. This explanation is consistent with the gate control theory of pain25. However, we know from the progression of the research on pain that the gate control theory of pain is incomplete and has since been partially abandoned32. Alternatively, changes in stretch tolerance may be psychologically mediated. It is possible that participants anticipated the positive effects of stretch and, therefore, their perception of the stretch discomfort was dampened25.

The authors state that the explanation for changes in stretch tolerance is not known25. They do however, propose stretch tolerance may be influenced by nociceptive nerve endings, mechanoreceptors, and/or proprioceptors25. Alternatively, these papers suggest that stretch may change some other aspect of the sensory neural pathways that is currently unknown25. For example, afferent input from muscles and joints during a stretch maneuver may interfere with signals from nociceptive fibers (stretch discomfort), subsequently inhibiting an individual’s perception of pain25. This explanation is consistent with the gate control theory of pain25. However, we know from the progression of the research on pain that the gate control theory of pain is incomplete and has since been partially abandoned32. Alternatively, changes in stretch tolerance may be psychologically mediated. It is possible that participants anticipated the positive effects of stretch and, therefore, their perception of the stretch discomfort was dampened25.

In conclusion, it would appear that stretching can increase flexibility and modulate stretch tolerance through a variety of neural mechanisms in the short term with relatively less consistent and aggressive stretching. Conversely, small samples of evidence (Kubo 2001, Freitas 2015) do demonstrate that musculotendinous architecture may be affected with longer duration stretching of a greater intensity. This potential adaptation with increasing intensity of stimulus is theoretically consistent with other adaptative mechanisms examined throughout the rest of the human body, though this cannot be stated with any true confidence.

Can Stretching Change Posture?

The question of whether or not stretching can influence posture comes up frequently due to the long-standing hypothesis that posture, or lack of “good posture”, is closely related to development of pain and injury. This thought is usually grounded in the traditional hypothesis of upper and lower crossed syndromes proposed by Vladimir Janda8. Upper crossed syndrome is a frequently cited risk factor for pain and movement dysfunction that proposes and imbalance between the length and strength of shoulder girdle and cervical musculature such as the traps, rhomboids, pec major, pec minor, cervical extensors, and deep neck flexors8. This model proposes that “strong and tight” anterior musculature combined with “long and weak” posterior musculature work together to negatively affect posture by increasing shoulder protraction, altering scapular mechanics, and forcing forward head posture, which will subsequently lead to pain and injury. Lower crossed syndrome is the lower body correlate to upper crossed syndrome which proposes weak glutes and tight hip flexors lead to similar dysfunctions in the lower quarter8.

Fortunately, this hypothesis has been tested and has not shown to hold up to rigorous studies. The poor connection between the hypothesized upper crossed syndrome and pain has been demonstrated in several studies examining and finding no association between forward head posture, thoracic kyphosis, and rounded shoulder posture with injury or pain18, 19. Similarly, the connection between the proposed condition of lower crossed syndrome has been evaluated in detail. In a systematic review of the literature encompassing 54 studies, Christensen et al. found no strong evidence for an association between spinal curvature and pain or injury20.

Despite the fact that posture does not appear to be associated with pain, the question of whether or not stretching can influence posture remains. To answer this question for the lumbar spine and lower quarter, we can look at a study completed by Muyor et al in 2012. This study was conducted to determine the effect of a stretching program performed in the workplace on the hamstring muscle extensibility and sagittal spinal posture of adult women21. Fifty-eight adult women volunteers from a private fruit and vegetable company were randomly assigned to experimental (n=27) or control (n=31) groups21. The experimental group performed three exercises of hamstrings stretching of 20 seconds per exercise, three sessions a week for a period of 12 weeks21. The control group did not participate in any hamstring stretching program. Hamstring flexibility, thoracic and lumbar curvatures, and pelvic inclination were measured in relaxed standing and toe-touch test with a Spinal Mouse21. After the intervention period, significant improvements in hamstring flexibility were found, however there was no change in standing posture21.

We can see a similar result in the upper extremity and shoulder girdle if we look to the following study conducted by Williams et al. in 2013. This study was designed to compare the acute effects of two passive stretches on pectoralis minor length and scapular kinematics among a group of collegiate swimmers22. Pre‐ and post‐test linear pectoralis minor length, as well as scapular kinematics (upward/downward rotation, external/internal rotation, anterior/posterior tilt) were measured as dependent variables. Immediately after the stretches there was an immediate increase in measured pectoralis minor length, but there were no statistically significant differences for all three scapular kinematic variables found among any of the groups. From this we can see that stretching, while it can alter tissue extensibility and available ROM, does not appear to alter resting or dynamic posture. Furthermore, Wang et al. demonstrated that a combination of stretching and strengthening over the course of 6 weeks had no effect on resting scapular position23. It would appear that posture may be much more of a habit the body prefers to rest in than a permanent structural position it must adhere to.

Does Stretching Increase Performance?

In general, it is accepted by most coaches, therapists, and the general public that increased flexibility is a positive property for sport, preventing injury, and over-all well-being. However, we have already demonstrated above that stretching and improved flexibility may not be as effective for injury prevention as we once thought. Similarly, it’s impact on performance appears to fall into a grey area as well. As with all effectively designed programs, the addition of stretching and flexibly training should consider the goals of the athlete or patient, the activities they are required to complete, and their individual physical limitations.

It has been well established that applying a series of static stretches prior to activity leads to an acute loss of strength in the short term (roughly 1 hour) after the stretching has is completed1,2. This effect has been referred to as the stretch-induced strength loss and has been primarily examined in the knee flexors, knee extensors and plantar flexors. Decreased amplitude of the surface EMG signal has been demonstrated during maximal voluntary contractions after stretching and provides evidence that stretch-induced strength loss is very likely a neural effect1,2. Additional evidence has further demonstrated that stretch-induced strength loss is due to a neural effect as evidenced by decreased force output in contralateral non-stretched limbs3.

It is notable that stretch-induced decrements in performance measures are generally smaller than decrements in strength measures. For example, stretch-induced decrements in vertical jump performance averaged approximately 3–4% and decrements in sprint performance range from approximately 0% to 2% depending on the study and stretching protocol used. It is also quite important to note that there is a trend demonstrating longer durations of stretch with increased intensity of stretch appear to have a much larger influence on subsequent measures of strength and performance. This means that short duration and less aggressive stretches likely have very little impact on performance. Finally, it is important to understand that these changes have only been well documented in static stretching and it has been shown that there is no stretch-induced strength loss with dynamic stretching.

To further examine the long-term effects of stretching on performance, we must take into account that consistent stretching and changes in flexibility may alter the stiffness in the associated musculotendinous units. The concept of stiffness is founded on Hooke’s law, which asserts that the force required to deform an object is correlated to a proportionality constant (spring) and the extent that object is deformed33. Simply put, stiffness is the relationship between the deformation of an object in response to an applied force or as the passive resistance to a stretching force. From a practical perspective, for optimal running, jumping, and hopping performance, an appropriate level of lower extremity stiffness is required to absorb ground reaction forces (GRFs), as well as to store and reuse elastic energy. When examining the requirements of various sports, there may be situations in which an increased compliance of tissue is necessary to perform the required tasks of the sport or activity. This can be seen easily in sports that require extreme joint ranges of motion such as gymnastics or dance. However, these factors could in fact play a role in the changes that occur to the gleno-humeral capsule in overhead athletes which allows for increased ROM and subsequently increased throwing performance.

Lower extremity stiffness has been shown to enhance athletic performance through improvements in running, jumping, and hopping tasks, as well as reducing the incidence of soft tissue injuries34,35,36. However, there is evidence that too much stiffness can also induce injuries37. At this point, a direct correlation between lower extremity stiffness and lower-body injury has not definitively been established because of a lack of studies. However, extreme levels of lower extremity stiffness have been related to reduced joint motion and increased shock and peak forces in the lower extremity, whereas too low a level of stiffness has been associated with excessive joint motion. Thus, the compromise between gaining ample levels of lower extremity stiffness and also the ability to have a compliant range of motion when needed for the demands of the client/patient is not simple. Training will require a complex and multifaceted approach from the rehab and fitness professional designing the program. If excessive stiffness is present, measures should be taken to improve extensibility and compliance of tissues in order to decrease the risk for injury and improve performance. If adequate stiffness is not present, training methods to improve stiffness and compliance should be applied in an effort to improve tissue stiffness as well as the clients inter and intra-muscular coordination potentially reducing the chances of noncontact injuries.

In conclusion, lower extremity stiffness is considered to be a key attribute in the enhancement of running, jumping, and hopping activities, which are prevalent in most sports. An athlete who can appropriately use greater stiffness characteristics will potentially store more elastic energy at landing and generate more concentric force output at push-off, possibly reducing the onset of fatigue and increasing running speed. Consequently, if a strength and conditioning (S&C) coach is able to advance their athletes’ ability to act like a “stiff spring” across an array of sporting movement patterns, performance enhancement may occur. It does not appear that long term implementation of stretching has any solely negative or positive impact on performance based on the current status of the literature as long as aggressively programmed stretching isn’t utilized immediately prior to strength and power activities.

Does Stretching Prevent Injuries?

Stretching is commonly practiced before sports participation, in injury prevention programs, as well as in rehab clinics around the world. However, effects on subsequent performance and injury prevention are not very well understood. As established previously in this article, there is a wealth of literature demonstrating that a single bout of stretching acutely impairs muscle strength, with a lesser effect on power. The extent to which these effects are apparent when stretching is combined with other aspects of a pre-participation warm-up such as practice drills and low intensity dynamic exercises is not well understood at this point4. With respect to the effect of pre-participation stretching on injury prevention a small number of studies of variable quality have shown mixed results. Several studies have examined the association between pre-participation stretching and injury risk due to the fact that stretching is consistently practiced before participation in a wide range of physical activities4.

Little attention has been given to the question of why stretching theoretically could impact injury risk. Stretching before performance appears to possibly have an impact on some types of injuries but not influence other injuries. For example, there is a good rationale for why stretching could impact the risk of sustaining a muscle strain injury, but the effect of stretching on muscle strain injuries has yet to be adequately examined in sports with a high incidence of muscle strains4. A plausible theory is that stretching makes the muscle–tendon unit more compliant, and that the increased compliance shifts the angle–torque relationship to allow greater relative force production at longer muscle lengths. This could subsequently enhance the ability to resist excessive muscle elongation and may decrease the susceptibility to a muscle strain injury4. However, this is merely a theoretical rationale for why pre-participation muscle stretching might decrease the risk of subsequent muscle strain that has not been sufficiently addressed in the literature.

A counter hypothesis could be that enhanced contractile force production when a muscle is in a lengthened position could increase the likelihood of injury4. Importantly, this rationale does not apply to the risk of other injuries such as ligament injuries, fractures or overuse injuries, such as tendinopathies. In fact, a general consensus is that stretching in addition to warm-up does not affect the incidence of overuse injuries4. Yet, when looking to acute injuries, the most recent work of Behm and Blasevich concludes their 2016 systematic review with the statement “Considering the small-to-moderate changes immediately after stretching and the study limitations, stretching within a warm-up that includes additional post stretching dynamic activity is recommended for reducing muscle injuries and increasing joint ROM with inconsequential effects on subsequent athletic performance.”38

As stated above, static stretching has been shown to be very effective for flexibility both the short-term and long-term. However, research is conflicting that stretching and flexibility is an important a factor in either performance or injury prevention7-11,38. Instead, other methods of increasing flexibility such as gradually progressive strength training into end ranges are available and may confer much greater benefits than static stretching alone as they alter force production and improve strength of connective tissues. There is however, considerable evidence that progressive strength training programs, which typically include an eccentric component, reduce injury risk, pain and disability in a range of musculotendinous conditions, as well as hastening return to sport. Therefore, it is prudent to allocate much more programming attention to strength and skill training than to stretching alone if one is trying to prevent injuries.

Should You Stretch?

There is no simple answer to this question, as the state of the literature still leaves much to be desired. At this point in time, there does not appear to be much of a protective effect of stretching or increasing flexibility on injury reduction so long as the client has the required ROM to complete their required activities without entering into the end range of their tissue capacity. Conversely, it would also appear that there are no real detrimental effects to stretching unless performed aggressively immediately prior to explosive power events. Therefore, if a client or patient reports that stretching “feels good” and/or helps to improve their mental status as part of a training program or pre-competition preparation sequence, it would be in the fitness professional’s best interest to allow stretching to be implemented.

Key Points:

    • There are various modes of stretching including static, dynamic, ballistic, and PNF.
    • For improving flexibility, static stretching appears to be a very effective intervention both in the short and long term.
    • Dynamic stretching appears to improve performance and confer more pre-competition benefits than static stretching.
    • Aggressive static stretching can decrease strength and power output if performed immediately prior to competition.
    • Static stretching appears to improve flexibility by improving stretch tolerance, as well as possibly altering the histological structure when completed for long durations at relatively high intensities.
    • Stretching does not appear to affect resting posture.
    • Static stretching prior to competition does not appear to decrease the rate of sports related injuries.
    • Dynamic warm ups and strength training programs do appear to decrease the rates of sports related injuries.
    • Stretching does appear to increase stiffness of tendons through application of sustained tension loading, thus allowing for less energy to be lost during the stretch-shortening cycle subsequently allowing greater force output.
    • The stretching induced impact on tissue stiffness, which could be both negative and beneficial dependent upon the client and requirements of their activities. (power and strength output vs need to get into extreme joint ROMs)
    • Static stretching is commonly practiced, but allocated time may be better used on more beneficial training modes such as dynamic warm-up, plyometrics, skill acquisition, and strength training.
    • Stretching appears to be neither dramatically good nor bad, therefore programming of stretching should be dependent upon client preferences and specific activity demands.
    • Simply put, if a client or athlete values stretching, it feels “good” to them, it helps pre-performance psychology, or improves perceived recovery one should not stand in opposition to their clients’ use of stretching.

Want to learn more about stretching? I have just recorded a podcast on ‘Where does stretching fit?’. You can listen to it here:

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https://podcasts.apple.com/us/podcast/the-modern-pain-podcast/id1435013970?i=1000476900923

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This was originally posted on Jarod Hall’s website. You can click here to read more blogs from him.

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About the Author

author

Dr Jarod Hall

Physical Therapist United States

Jarod Hall is a physio, clinic director, adjunct assistant faculty member, and regular lecturer on managing complex patients with a simple approach rooted in the biopsychosocial model. He’s written extensively on manual therapy, pain science, simplifying clinical practice, and various other physical therapy topics.

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