By Michael Edgar
We have all probably stretched at some point in our lives, but when asked why, we typically have a wide range of reasons or claims from "it feels good" to "injury prevention". Are these claims truly supported by the research or evidence? In this article, we will be looking at several claims and the evidence which supports or refutes the utility of stretching in the context provided.
What is stretching?
Stretching is a form of physical exercise involving a musculotendinous group. It is deliberate action to improve a muscle’s felt elasticity to achieve a comfortable muscle tone (1). One thing I would like to point out, is the fact that this definition explains stretching as a perceptual experience, as it is one’s sense of improved elasticity in the tissue (1).
What are the main forms of stretching?
We typically can categorize stretching into three main categories: static, dynamic and proprioceptive neuromuscular facilitation, or PNF for short (2-3). Static stretching involves reaching a certain range of motion (ROM) and holding the muscle (group) lengthened for a predetermined period of time (2-3). Dynamic stretching involves muscular activation through rhythmic movement with the subcategorization of ballistic stretching which involves the execution of fast, bouncing movements used to achieve a greater ROM (2-3). Finally, PNF involves static stretching with isometric contractions of the target muscle in a cyclical pattern (2-3).
PNF stretching can further be differentiated into contract-relax (CR) which involves contraction of the muscle being stretched at periodic intervals and secondly, contract-relax agonist contract (CRAC) which involves a secondary contraction of the agonist muscle, or muscle opposite to the stretched muscle, to increase the ROM reached during the PNF protocol (2-3). The benefits and efficacy of PNF have typically been believed to occur through autogenic inhibition, a muscular relaxation phenomenon, and reciprocal inhibition, a spinal reflex believed to relax an antagonistic muscle when its agonist is activated (2-3).
Now that we have a basic understanding of stretching and its sub-classifications, we can delve into the validity of several claims related to its utility.
Tendon Adaptations
Long-Term Mobility Adaptations
Proprioceptive Neuromuscular Facilitation – Reciprocal Inhibition
Soft Tissue Injury Prevention for Runners
Low Back Pain Relief
Hypertrophy
Tendon Adaptations
In order to better understand the topic of stretching and the physiological changes which occur with it, it is important to understand how tendons adapt. A review by Bohm et al. in 2015 looked into the drivers of adaptation for tendons by measuring mechanical stiffness, Young’s modulus (we can think of this as mechanical stiffness) and morphological cross-sectional area (CSA) of tendons during isometric, concentric-eccentric and purely eccentric activity (4). They found that devoid of the form of contraction, the primary driver of adaptation was the utility of high intensity protocols, which meant higher than 70% of one’s maximal voluntary contraction (MVC) (4). They also found that adaptations occurred through the quantity of the tendon material and not the morphological properties themselves. More so, these adaptations tended to occur in programs which were over 12 weeks in duration (4).
One thing I would like to point out is that due to the way muscles generate force, we can work at a higher intensity during eccentric movements, then isometric, and finally concentric. This has to do with the formation of myosin cross-bridges during muscular contraction.
Long-Term Mobility Adaptations
Many of us use stretching as a means to increase mobility and ROM, but the underlying reason for such adaptations has not been as clear cut (5). That being said, a review by Weppler et al. in 2010 looked into the theories regarding stretching and long-term adaptations. What did they find? They found that there are generally four major theories for these adaptations (5).
Firstly, the viscoelastic deformation theory, which was initially shown from immediate increases in muscle-tendon unit length from repeated cycling of static stretching (5). Although, an attractive theory, the transient nature of this phenomenon did not lend itself to a cumulative effect (5).
The second theory involved plastic deformation, which means that once the elastic limit of a tissue is passed, there is permanent deformation and it will not return to its original length (5). A good way to think of this is when one stretches a piece of gum, realizing it will not recoil back to its original shape. Although this would equate to a cumulative effect, this has not been found in human studies and mathematical models have shown the forces needed to reach plastic deformation thresholds are outside human physiological limits (6).
The third theory involved sarcomeres in series, as muscle fibres get longer over time (5). This was initially shown in animal studies, such as immobilized birds, where their wings were pinned in a stretched position for long periods of time (5). Although this supports the theory, when their wings were returned back to their normal position, the sarcomere adaptations regressed back to normal (5). A couple of other concerns with this includes the inherent issues comparing humans to birds, and the practical or ethical issues of immobilizing humans in a stretched position for prolonged periods of time.
Finally, this led to the sensory theory in which torque and angle curves were assessed (5). Although adaptations were seen, they only found changes in the end-range joint angles at increased torque (force) and not the curve pattern itself (5). This led to the belief that the subject’s perception of pain or inability to continue simply occurred later in the application of the stretch (5). So maybe, these are not purely physiological adaptations occurring, but instead our own mental toughness to handling increased stretching loads.
Proprioceptive Neuromuscular Facilitation (PNF) – Reciprocal Inhibition
Moving on from this, we can start to discuss the utility of PNF stretching and the theory of reciprocal inhibition. As previously stated, reciprocal inhibition is purported to occur through antagonistic relaxation of a muscle when its agonist is contracted. This is believed to be the reason for the increased range of motion during PNF stretching compared to traditional static or dynamic stretching. That being said, several EMG studies have shown conflicting results when relating to this theory. For example, Felicio et al. in 2019 performed squats with either an adductor squeeze or abductor squeeze to either maximally recruit the adductor muscles or abductor muscles (7). Based on the theory of reciprocal inhibition, one would expect that the gluteus medius (abductor) would be "turned off" with maximal adductor activation and vice-versa (7). They ended up finding that there was greater co-contraction of both muscle groups with abduction and adduction (7).
Furthermore, other studies have looked at the EMG patterns during static and PNF stretching and found that during PNF-CRAC protocols, the activation of the muscles and ROM were both increased which would directly contradict the theory of reciprocal inhibition (8-11). More so, these same studies found increased pain with the PNF-CRAC protocols compared to static stretching, which may further enforce the sensory theory of increased tolerance to pain or discomfort (8-11). There could also potentially be an increased pain threshold from exercise-induced hypoalgesia or the transient passive connective tissue properties involved (8-11).
A glaring issue here, is the fact that these studies have been performed since at least 1980, which means that we have had data to dispute the theory of reciprocal inhibition for approximately 40 years. This begs the question as to why the theory of reciprocal inhibition has endured all these years? On a final note, I do not want to say reciprocal inhibition does not occur, but instead that it may be more complicated than we traditionally thought.
Soft Tissue Injury Prevention for Runners
Our next claim involves the widely held belief that stretching prevents injuries. Most of us have either stretched before an activity or had friends do so, but is this actually supported by the evidence? A systematic review by Yeung et al. in 2011 looked at this exact topic, which involved military recruits and recreational runners (12). Before I go any further, I do want to point out the gross variability in study protocols as Pope et al. in 2000 only used 1 set of 20 seconds for each muscle group involved as the intervention compared to a general warmup, versus Hartig et al. in 1999 which involved a much more voluminous program consisting of 5 sets of 30 seconds, three times per day outside of training times (12). Based on this, I think it is important to have some healthy speculation when looking at their results.
Well, what did they find? They found that there was no difference if athletes stretched prior to exercise or outside of their exercise sessions, in relation to the number of people sustaining lower limb soft tissue injuries, as well as the rate of lower limb soft tissue injuries (12). That being said, based on the high variability between the protocols involved, we can't confidently conclude whether stretching has a benefit for injury prevention or not in runners. This also brings me to an important point, in that how we define stretching and what our stretching protocol entails may ultimately determine its efficacy.
Low Back Pain Relief
The fifth claim I wanted to explore is the idea of stretching to help ease low back pain. Pourahmadi et al. in 2019 explored this topic by performing a systematic review which compared typical physical therapy or sham interventions to slump stretching, involving a flossing motion (13). On a side note, nerve flossing or nerve gliding typically involves the coordinated movement of different joints, in order to mobilize nerves and muscles in the area of focus (13). So, what did they find? Well, they ended up finding that stretching for low back pain does help resolve low back pain and that it may be a viable option for individuals to use (13).
But, hold on a moment, let’s look at the data here:
As we can see here, something interesting seems to be going on with the study by Kirthika et al. in 2016 (13-14). If these results are correct, they may have stumbled onto the holy grail of low back pain interventions. This sparked my curiosity and I ended up retrieving the study. Some of the glaring flaws in this study involved the lack of sample size declared, lack of statistical analysis, no defined stretching protocol (yes, you heard this right, they did not even describe how they stretched), no blinding and no baseline characteristics given (14). For those of you who may not be familiar with research methods, this means it is a very low quality study.
Based on the lack of information given in this study, can we really take its results at face value? In my opinion, probably not. The good thing about the review by Pourahmadi et al. is the fact that they tried to control for this by removing poor studies from their analysis, such as the study by Kirthika et al. (13). The issue with this is that it left them with only three studies and a high variability for their final results (13). So, although, they still had statistical significance, they were not able to describe a minimal clinically important difference (MCID) based on these studies (13). Therefore, I would not feel confident in using this review for application in a clinical setting.
Hypertrophy
The final topic we’re going to delve into is the utility of stretching for muscle building and hypertrophy. This was initially theorized from animal studies done on birds which found increased sarcomeres in series and hyperplasia (muscle cell division) from prolonged immobilization in a stretched position (5). More so, many bodybuilders tend to believe that by stretching, one can stretch their fascia, to increase the potential ceiling for muscle building capacity. The latter theory has been debunked by research on fascia, which has found that one would need to generate close to 800-1000kg of force to simply distort fascia by 1% (6).
So, what do we know on the topic? Nunes et al. in 2020 aimed to investigate this topic by reviewing the literature on muscle hypertrophy through stretching in human subjects (15). Their review found studies with stretching protocols ranging from 3 to 24 weeks, with a total time under stretching (TTUS) ranging from 36 minutes to about 11 hours (15). Based on this, it can be seen once again that there is huge variability in the protocols used between studies for stretching (15).
That being said, what did they find? When they categorized studies based on similar TTUS, they found that only one of these studies found a true change in muscle tissue, while all the other studies found no change (15). The study which found a change was done by Simpson et al. in 2017, but why exactly did they see changes when every other study did not? On further analysis, things became much more interesting when studies were categorized as either self-performed, based on one’s own threshold versus aided, in which intensity could be increased through an external apparatus (15). It seemed that when devices or external loading was used to perform the stretching protocol, structural adaptations were seen in the involved muscles, through increased fascicle length and increased muscle thickness (15).
What I found very interesting about this is that most research on muscle hypertrophy has found that volume is the primary diver for muscle building. If you would like to read more on this, feel free to read my article titled – ‘Hip thrusters - king of the booty builders?’'. Based on this review, it seemed that protocol intensity and the use of an external loading aid was potentially more important (15). Coming full circle, this would line up nicely with our initial discussion on tendon adaptation by Bohm et al. which found that intensity was the primary driver for physiological changes (4).
Take Home Points
Intensity may play a larger role for tendon adaptations (greater than 70% MVC)
A minimum of 12 weeks may be needed to observe tendon adaptations
Improved mobility over time may predominantly be a matter of improved pain thresholds and sensation
Proprioceptive neuromuscular facilitation may not work through reciprocal inhibition or if it does, it may be more complex than originally thought
We cannot consider stretching a method to prevent lower limb soft tissue injuries in runners at the current time
Stretching for low back pain may benefit some individuals but it appears weak at this time
Loaded or assisted stretching may lead to muscular adaptations
Current literature needs improved standardization between stretching protocols so we can be more confident in our guidelines for its utility
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Pourahmadi M, Hesarikia H, Keshtkar A, Zamani H, Bagheri R, Ghanjal A, Shamsoddini A. Effectiveness of Slump Stretching on Low Back Pain: A Systematic Review and Meta-analysis. Pain Medicine. 2019 Feb 1;20(2):378-96.
Kirthika S, Rajalaxmi V, Sudhakar S, Bhuvaneshwaran T, Thaslim KF. Effect of combining slump stretching with conventional physiotherapy in the treatment of subacute non-radicular low back pain TJPRC. Int J Physiother Occup Ther 2016;2(2):9–16.
Nunes JP, Schoenfeld BJ, Nakamura M, Ribeiro AS, Cunha PM, Cyrino ES. Does stretch training induce muscle hypertrophy in humans? A review of the literature. Clinical Physiology and Functional Imaging. 2020 May;40(3):148-56.