I’ve heard it time and time again like a record on repeat from patients, “My doctor told me I need to stretch my hamstrings because they’re causing my low back pain.” The idea that low back pain is caused by tight hamstrings and stretching them is the solution to fixing the injury is one held by many in the medical and physical therapy profession. When you glance quickly at the research, it actually makes sense. If you do a simple Google search, you could easily come up with a number of studies that link low back pain with ‘tight’ hamstrings.1-4
However, while ‘tight’ hamstrings may be a common finding in those with low back pain, I’ve come to find in my years of treating patients as a physical therapist that it is NOT a direct cause. For this reason, I believe the idea that we need to stretch the hamstrings of every single patient who walks in the door with low back pain is very misguided. Today, I want to walk you through exactly why that is.
How We’re Taught to Assess Hamstring ‘Tightness’
One of the most common assessments performed in doctor’s offices across the world to assess hamstring flexibility is a passive straight leg raise (SLR) test. Here’s how you do it.
While lying on your back, relax your muscles and have a friend raise one of your legs as far as possible without your knee bending. If you can raise your leg to 80 degrees or more without any pain, you are considered to have “normal” hamstring flexibility.12 However, if you are unable to lift your leg to 80 degrees without excessive tightness/pain in the back of your thigh the limitation is usually attributed to ‘tight’ or inelastic hamstrings.
Why Do We Stretch?
Stretching is used to improve flexibility. In physical therapy school we’re taught to stretch muscles that are either stiff or short in order to restore the muscle to normal flexibility. In the strength & conditioning and fitness world, stretches are used to promote better quality movement, increase performance and limit injury risk. Over the past few decades there has been a tremendous amount of research published demonstrating the effectiveness of stretching in both the medical and strength & conditioning fields.12-22 But how does stretching really work?
When a muscle or group of muscles like the hamstrings are stretched (like when you lie on your back and a friend pushes your leg to the sky) there are some short-term changes that occur. The most obvious effect is improved flexibility (meaning you have more range of motion and can lift your leg higher during the next stretch), however this effect doesn’t always last very long.
For example, one research study showed that the improved hamstring flexibility seen after 5 rounds of a stretching protocol only lasted for 6 minutes!34 At 32 minutes after the stretching had stopped, the muscles had completely returned to their pre-stretch suppleness. Other more extensive stretching protocols however have shown that the effects of stretching can last anywhere from 60 to 90 minutes!23 This means the amount of time spent stretching and the length a stretch is held both have an effect on how long the short-term benefits of improved flexibility are seen. Regardless of how long these changes last, the improved range of motion always returns back to ‘baseline.’
Theoretically, there are two ways we can improve flexibility. The first model is called the mechanical theory. This concept is based on the idea that if improved range of motion is seen after stretching, it can either be due to increased muscular length or decreased muscular stiffness.
Naturally muscles are fairly elastic, meaning they can stretch when a force is applied to them and then return back to their normal length thereafter when the force is removed (similar to a rubber band being stretched). We can measure the amount of elasticity two ways: extensibility and stiffness.
- Extensibilityis the ability of a muscle to elongate or lengthen. Usually extensibility is defined as the ability to extend the muscle to a certain end point (most often the point at which a person feels they can’t tolerate any more stretch without pain). We can measure this by looking at the angle at which a joint can be moved (as with the hip joint during the SLR test).
- Stiffness is the ratio of how much force is needed to during a stretch to create elongation of the muscles. This can be a tricky concept to define (and isn’t at all how most people use the word today) because “stiffness” can change based on the how fast a stretch is applied and the angle of the joint.
Most medical books on the topic of stretching would have you believe increases in flexibility are often due to the mechanical model and the muscle itself is actually lengthening when stretching. There are three theories under this model that try to justify the changes in flexibility.
- Viscoelastic Deformation: this is a very fancy way of saying your muscles have both an elastic component (like a rubber band) and a viscous or gooey component (similar to honey). As a stretch is applied to a muscle, the idea is that the force causes the muscle to relax and flexibility improves. However, research has been quick to disprove this idea as any changes seen due because of viscoelastic deformation are likely very short-lived. For example, one research study showed that a hamstring stretch held for 45 seconds had no significant affect on the next stretch performed 30 seconds later.35If the muscle really deformed and stayed that way, the researchers would have seen significant difference.
- Plastic Deformation: another popular theory states that the increased flexibility observed after a stretch is because the connective tissues that make up the muscle are stretched to a point where they become permanently lengthened and do not return back to their original length. Research to support this notion however has been very poor.36
- Increased Sarcomeres in Series: the last theory is based on the idea that the building blocks that make up the muscle (called sarcomeres) will increase in number in response to a stretch. Think of a sarcomere like a small LEGO block. A muscle is made up of thousands of muscle fibers, each which is composed of a long block chain of sarcomeres. Based on a few animal studies, researchers found that if they create a long held stretch (by splinting or immobilizing a joint in a cast for a few weeks and holding the muscle therefore in a lengthened position) the body will increase the amount of sarcomeres (more LEGOs in a row) by upwards of 20%.37 Basically, the current block chain inside each muscle fiber is stretched to such a degree that the body decides to add on more blocks to the chain to restore the prior “normal” balance. However, what most people don’t understand about this adaptive change is that it appears without an overall change in muscle length! While there may be more blocks to the chain of sarcomeres, the overall length of each one becomes smaller! The size of each block basically shrinks! Also, these adaptive changes do not last very long and the muscle eventually returns to it’s prior “normal” amount of sarcomeres within a few weeks.37,39 So while these few animal studies have shown there to be changes in the muscle architecture after four weeks of an immobilization stretch, it is a bit far fetched to think these same changes would also be seen after an intermittent stretching program like that used by most athletes.
The second and more probable theory for how flexibility is improved through stretching is by a change in sensation. The latest research has shown that much of the short-term increases in flexibility comes from the improving your stretch tolerance (or pain tolerance) and NOT by actually increasing your muscles length!24-26, 36 Basically after stretching your hamstrings for a few sets of 30 seconds, you have more flexibility because of your ability to tolerate a greater amount of stretch before you tap out!
But what about the affects of a long term stretching program?
Unfortunately, almost all of the studies published on the effects of stretching have only lasted between 3-8 weeks in length. Yet again, the changes we find in improved flexibility with stretching programs even up to 8 weeks in length are not because we are actually lengthening the muscle fibers but instead due to the change we create in how we perceive and tolerate the force of a stretch.26,36
This isn’t to say that those who participate in extremely rigorous stretching programs for years and years (such as those in gymnastics or ballet) couldn’t alter the extensibility of their muscles. It could very well be possible with enough time and repetition. However, research has yet to find the answers for this question.
The Back Pain & Tight Hamstring Connection
Let’s now switch gears and talk about why so many of us are quick to stretch the hamstrings, especially for those who have back pain. Here is a normal thought process that goes through the minds of many in the medical community when assessing someone who has back pain.
“When assessing the patient’s straight leg raise, they were unable to raise either leg past 70 degrees. We were taught in school that this means the person has ‘tight’ or short hamstrings. Short hamstrings have been shown in research to be associated with low back pain. This means we need to stretch the hamstrings as a part of the rehabilitation plan to decrease the back pain.”
Unfortunately, there’s a problem with this reasoning. For years we have been taught to stretch the ‘tight’ hamstrings of those who have low back pain based on this premise their limited flexibility was the cause of the pain. Here’s the kicker, this isn’t the case.
In 2000, a group of researchers from the Netherlands performed a study where they looked at a number of different factors amongst three different groups: those with flexible hamstrings, those with ‘tight’ hamstrings, and those with low back pain.29 Here’s what they found:
- Those with low back pain had the lowest hip range of motion via the SLR test (even worse than those with just ‘tight’ hamstrings). The researchers concluded that this decreased range of motion was only due to the hamstrings and did not have to do with any pelvic tilting or back mobility issues.
- The hamstrings in both the flexible group and ‘tight’ group showed a similar ‘defensive reaction’ as the they were stretched. The further their legs were stretched, the more electrical activity rose in all of the hamstring muscles. However, those who had low back pain had a very abnormal ‘defense reaction’ and did not have simultaneous electrical activity in all of their hamstrings. Also the electrical activity they did record did not increase at the same gradual pace as those who were pain free. This difference was attributed to a heightened arousal or sensitivity to movement that occurs in those with back pain.
- There was no difference in the muscle stiffness between those with ‘tight’ hamstrings and those in the low back pain group.
- The hamstring flexibility of those with low back pain was was worse than that of those with ‘tight’ hamstrings but this limited motion was not accompanied by an increase in muscular stiffness.
Here’s what this means: someone with low back pain doesn’t have limited flexibility for the same reasons as someone who just has ‘tight’ hamstrings. Their muscles are reacting and behaving differently and appearing to be tight/short because they have less ability to tolerate a stretch. This occurs because their nervous system is going haywire due to the presence of pain in the back!
The Treatment Plan
So now that we know the limited hamstring flexibility of those with low back pain is not due to just being ‘tight,’ the solution can’t be to stretch away. So, what should we do?
The three step process for fixing back pain:
- Wind down pain sensitivity by avoiding motions, postures and loads that contribute to pain. This will be different for each person and requires an efficient screening.
- Improve core stability
- Improve movement quality (learn to move from the hips and re-activate the glutes).9,10
The first is one that we have discussed briefly in prior blog articles. Those who have had a back injury for a long time are often very sensitive to paindue to an overreaction from our nervous system! The second and third factors are ones we will talk in-depth over very soon.
Viewing The Hamstrings in a New Light
So now that we know that the hamstrings of those with low back pain appear inflexible as a reaction to the injury and are not themselves a cause of the injury, does that change how we might look at the ‘tight’ hamstrings of healthy athletes? My answer is yes.
We must first discuss the difference between flexibility and mobility. We can test the flexibility or length of the hamstrings by performing the SLR test as we discussed earlier. Mobility is different. Mobility has a movement component and describes the body’s ability to use its flexibility, muscle tension/quality, and nervous system (aka motor control) to coordinate the body through a range.
Mobility should always be evaluated before flexibility. Let’s take the example of Jim the national level Olympic weightlifter. Jim has great hip mobility and coordination of his core stability and posterior chain muscles, and can show great looking technique during his barbell lifts of the clean and snatch. If you ask him to bend over and touch his toes he will perform what looks like a barbell RDL without any weight in his hands because his body has been programmed to move correctly about the hips over and over again in the gym.
However, if you took Jim and without looking at the quality of his movement and had him perform the SLR test, you may find he has relatively ‘tight’ hamstrings as he is unable to lift his free leg straight past 80 degrees. Does this mean we should automatically prescribe hamstring stretches for this athlete?
What if I were to tell you that the relatively “stiff” hamstrings of some athletes are an advantage and not always a problem that needs to be stretched away? Think of your muscles as springs. When they are loaded with tension they have the ability to explode with power and propel your body down a track or a barbell over your head with tremendous force.
Now 20+ years ago, long duration stretching was all the rage because researchers believed that these static stretches prior to a workout or athletic completion could help reduce the risk of a muscular strain injury.18 So if that same athlete came to a physical therapist back then, he would most definitely have been prescribed some long duration (1+ minute) static stretching for his hamstrings by lying on his back and pulling his foot to the sky.
However, nowadays the tides are turning in how we approach the body in this manner. There is a growing amount of research lately showing that this type of stretching before exercise can lead to a decrease in strength, power and speed, therefore limiting an athlete’s performance.19,28 Basically, long duration stretching deadens an athlete’s natural set of ‘muscular springs,’ which means Jim could potentially hinder his ability to lift big weights if he stretched like some would advise!
Now does this mean we should never stretch? Not at all. There are definitely some athletes who would benefit from a certain amount of stretching and mobility work in order to improve their flexibility, get into better technical positions, and move more efficiently during their training and competition. However, we need to evaluate each athlete from a mobility and movement perspective first.
For example, during the first pull portion of the barbell clean, the hamstrings are lengthening as the hips rise faster than the knees extend. This requires a certain amount of hamstring flexibility. If an athlete is unable to perform this portion of the clean due to excessive hamstrings tightness, we can then look into using different stretches to address the problem. For example, using a barbell RDL is a great way to drop into a ‘sport specific’ hamstring stretch.
When using stretches for an athlete in need of flexibility work, it then comes down to how you are performing each stretch that makes all the difference. Research shows that short duration stretches (less than 30 seconds) have NO harmful affect on muscular performance while still bringing out improvements in flexibility.19,31,32 It’s not until the stretch is held for more than 45 seconds that we see significant losses in strength, power and speed! In fact, research has shown that the ability of a muscle to produce force when subjected to these long duration stretches (which are still very common to see prescribed by fitness coaches around the world) is decreased for up to 30 minutes.30 This is in part a reason why the American College of Sports Medicine and European College of Sports Sciences have both come out recently condemning long duration static stretching as a part of a warm-up routine!20,21
Will there ever be any athletes who need long duration stretches? Sure. I wouldn’t completely discontinue them, but their use must be individualized to the athlete that needs to improve flexibility with the goal to improve movement quality. If you are giving a stretch just for the sake of stretching, you have already failed your client/athlete.
What may surprise you is that there is a way to improve your flexibility without any stretching at all! Yes, you read that correctly. Research has shown that we can decrease our muscles level of stiffness (and therefore increase elasticity and flexibility) through a dynamic warm up! By increasing the temperature of the muscle through activities like a light jog, a fast walk, skipping, lunging or bodyweight squats, we can allow our muscles to become more supple and improve our ability to freely move!25
What you’re feeling when you warm-up is a “thixotropic” affect. Thixotropy is the ability of a tissue (like muscle) to become more pliable or liquid after motion and return to a stiffer gel-like state when resting. Basically movement decreases muscular stiffness. This is why your low back or legs feel “stiff” after sitting all day in a chair but the if you get up and walk around for a few minutes you feel much better.Think about the last time you opened a yogurt. When you peal back the lid, the yogurt is sometimes clumped together. After sticking your spoon into the bowl and stirring it around however it becomes more gel-like. This is an example of a thixotropic effect and just like the spoon stirring up the yogurt, your body’s muscles become more less stiff and become more movable in response to movement.
A generalized warm up (where you elevate your heart rate and improve blood flow to your muscles) is recommended for all athletes before training or competition to promote this thixotropic effect. For the barbell athlete this can include walking lunges, leg or arm swings, or even a fast pace walk. Following this general warm up, a sport specific warm-up should follow that includes movements tailored to the athletes’ particular training/sport. For example, if an athlete was to have clean & jerks for their workout, they may start with an open bar and perform a number of shrugs and high pulls followed by some full depth front squats before adding any weight. The combination of the general and specific warm up are key for improving flexibility, mobility and movement quality to help an athlete perform at their best and be as safe as possible.
Every aspect of a training or rehab program has a purpose. At the end of the day you have to ask yourself “why” you are implementing every exercise, including stretches. For years we have been educated and taught that if a muscle is tight we need to stretch it to return it to what a textbook would tell us is “normal.”
I hope you now understand that those who are in pain do not have “normal” hamstring flexibility. Their body is reacting differently due to their injury that is creating the façade of being ‘tight.’ For this reason, you can’t just stretch away the limited hamstring flexibility of someone with back pain. The problem isn’t coming from the hamstrings but their stiffness instead is a reaction of the low back pain itself!6-8
Also, just because someone has ‘tight’ hamstrings does NOT mean they will end up with back pain.5,11 In fact, some athletes have short or stiff hamstrings for a reason. Remember “muscle length testing” was developed years ago based on the idea that there is a “normal” or ideal range of muscle length for optimal movement and safety. Blindly stretching away an athlete’s hamstrings stiffness with long duration stretching just because he or she appears “abnormal” on paper is not always a good idea. I’m going to clue you into a little secret: most elite athletes have very abnormal traits that give them the ability to do things most of us ‘normal’ people cannot.
Instead, I urge you to go about addressing what you find in these athletes in a different manner than we have in the past. For those with back pain, treat the ‘why.’ Understand that the ‘tight’ hamstrings are a reaction to the pain and we need to learn how to stabilize the low back, turn back on the inactive glutes and fix the movement pattern that led to the injury in the first place in order to find a lasting fix! For the athlete with ‘tight hamstrings,’ learn to incorporate a proper warm up that includes dynamic movements and short duration stretching (if warranted) that will improve mobility and movement quality without a decrease in performance.
I hope this article was able to help empower you to make more educated choices on when and if to include stretching in the future.
Until next time,
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