If you want to stretch your hamstrings please continue to do so.

I was informed of a recent post in The Huffington Post entitled "Stop stretching your hamstrings".  This style of article (where we lambast some exercise)  has been pretty popular for a number of years and I have certainly contributed my share.  This one in particular I could have written 20 years ago when I was pretty strongly against yoga and stretching. I'd go out of my way to find any research that supported my bias.

These types of opinions are certainly more in the movement pessimism camp versus the movement optimism camp where I've found myself lately.  

What I thought I would do would be to go through some of the main points of the article and look at the research around those ideas.  These posts (when you don't agree with them) are great to challenge your own biases and see if the reason you disagree with them can be substantiated.  

I will expect some debate and discussion to come of this so to make it simpler and to avoid a discussion that just leads to tangents I will try to break things down into simple claims and then try to just discuss that claim specifically.

The Good

Pointing out that hamstring stretching or any stretching is not very effective to treat or prevent pain or injury is certainly accurate.  We shouldn't be forcing people to think that they need a great deal of flexibility (with exceptions for certain tasks) for general health or to avoid most injuries.

There are certainly conditions where you would want to avoid hip and spine flexion repeatedly.  Sometimes back pain is flexion related and avoiding it in the short term can help.  Other conditions like a high hamstring tendinopathy where you think the compression between the upper hamstring tendon and the ischial tuberosity is sensitizing that tendon can also benefit from short term avoidance of hip flexion related stretches.  

The article also points out that being strong and "tight" can be healthy.  There certainly are some research papers correlating reduced hamstring flexibility with running economy -  but we don't have any evidence to suggest that training to increase flexibility in the long term will decrease that economy.

There is more good, I'm just trying to keep this post not incredible long :)

The More Contentious Points

Questionable assertion #1: "Stretching the hamstring muscles will not make them longer because the tension or contraction of a muscle is under control of the nervous system. You simply cannot make a muscle longer by pulling on it."

I understand this point because it was me 20 years ago.  Its based on the work of Magnusson who would have people stretch their hamstrings and there would be an increase in the ROM but no change in the stiffness curve (meaning the same force to pull the hamstring to 60 degrees was found after stretching - a decrease in stiffness would see the same force pulling the joint to more ROM).  

They concluded that increases in ROM are due to increases your ability to tolerate the discomfort of stretching rather than some structural change in the muscle.  It should be noted that the researchers would try to control for nervous system contraction/tension by measuring the muscle activity - no muscle activity was allowed.  So its not true that the nervous system is "resisting' the stretch via muscle tension its more whether you can dampen your response to the nociception and tolerate the discomfort.

 

The above curve shows what happens to the stress-strain curve if you can change the stiffness of a joint.  The curve below shows that you get more ROM but no actual change in the stiffness.  This is what many people think that stretching merely does.

But there are a number of other studies that challenge this.  So if just look at these papers you conclude that muscles and the connective tissue doesn't adapt to stretching.  The graph below suggests that other studies would argue that connective tissue does adapt.

Of interest is the Freitas (2015) study.  They actually showed that long term stretching causes fascicle growth.  This is kind of huge and certainly challenges the statement that just pulling on a muscle won't change it.  You can also look at an EXCELLENT open access paper by Blazevich et al (2014).  The authors found an increase in ankle range of motion of 19.9%. They performed simple calf stretches for three weeks and concluded the following:

"Thus there was no apparent change in neuromuscular activity at elongated muscle lengths that could have contributed to the increased ROM after the training period. In contrast, an increase in both fascicle and whole muscle lengthening during stretch was observed after the training, simultaneous with a reduction in tendon elongation, and an increase in muscle length (13%) at stretch termination (end ROM). These changes appeared to result from a decrease in muscle stiffness without a concurrent change in tendon stiffness, which in integrated terms was translated into a trend (P 0.07) for decreased MTU stiffness. Collectively, these data strongly suggest that changes in the passive elastic properties of human skeletal muscle can be induced by stretch training, but also indicate that the mechanical properties of the whole MTU may appear to remain unaltered despite significant changes being elicited in the mechanical properties of the muscle or tendon separately."

From just those simple papers we can see that it is not really true to say that muscle doesn't change with stretching.  You can read a little more here as well when I addressed this issue a few years ago.  We know that range of motion can increase and this increase in ROM is due to both increases in stretch tolerance AND changes in passive properties of muscle.

Questionable Assertion #2:  "Many people injure their spine and lower backs, tear or inflame hamstring tendon attachments and even rupture discs doing stretches such as seated and standing forward bends in yoga."

This is just unsupported anecdote.  You need large studies to investigate this.  What it might be based on is the influence of compression on tendinopathy.  High hamstring tendinopathy (its not "inflamed") can occur.  Tendinopathy occurs when we fail to adapt to the loads we place on our tendons and selves.  One idea is that tendons are better suited for tension rather than compression.  They are springs that we want to pull instead of squeezing.  When you flex your hips you compress the tendon against the ischial tuberosity.  So, its true that hamstring stretching might contribute to this sensitivity.  But, any type of hip flexion would contribute to this and stretching might the least of your worries.  Squats, deadlifts, running uphill all would create compression on the tendon against the sit bone.  Stretching would too but the loads would lower because  you aren't actively using that muscle like you would during a squat.  

With respect to the spine, that is a massive debate!  Spinal flexion under low loads (like stretching) has not been proven to be a risk factor for disc pathology. I've written a massive review of spinal flexion and injury risk here.  Bottom line, to say that spinal flexion is inherently injurious while stretching your hamstrings is just an opinion and really requires support that doesn't exist.  

Questionable Assertion #3There are dubious benefits in stretching the hamstrings as it does not contribute to strength in the butt or the back and in fact, the flat lumbar/sacral position created in forward folds may give you a C shape to your spine and a flat lower back and butt. This is because the ligaments that hold our natural lumbar curve and sacral nutation become lax from forward bends that target the hamstrings.

The good: yes, stretching would not create strength in the butt or back.  That is not its job.  It also won't make you smarter.  But who said it would?

The dubiuous: ...and this is a doozy.  I will rephrase the statement as a question to try to answer it.

Can hamstring stretching during forward fold flatten your lower back/butt in the long term via laxity of the ligaments of the spine and pelvis?

Short answer:  This is really unlikely.  Long answer, this is a huge and interesting topic. But lets go for it.

#1.  We suck at changing posture via mechanical changes in tissues without consciously thinking about changing your posture.  The above assertion says that stretching will create laxity in the ligaments.  In general stretching does not change posture.  Here is a review.  Here is a further look at on my website.  One big reason goes back to the stress-strain curve of tissue.  When you are standing in neutral your ligaments aren't really engaged.  They are in the "toe region".  This is the neutral zone where it is easy to move joint because there is little passive resistance.  Ligaments "kick in" as the joint deviates from neutral.  They are stiff ropes who start loose and get tight.  Thus they don't really "pull" you into position.  

We are probably better at looking at posture as a habit rather than something predetermined by muscle structure.  Further, posture is probably more driven by passive bony architecture.  For example, a scoliosis is really influenced by the wedged shape of your vertebrae.

#1a:  Don't you find it odd that the author is pretty adamant that you can't change muscle length with stretching but the same stretching will increase the length of a ligament?  We actually have no evidence that a ligament gets longer with stretching.  We see the same thing with long term stretching and tendons.  And tendons are similar to ligaments in how they respond to load.  Go back up and read that quote from Blazevich.  Stretching did not influence the length of the tendon.  When you look at tendon and ligament biomechanics research we see that ligaments are incredibly slow to adapt.  And that tensile load, if anything, makes them stiffer and stronger.  There is no other load that you can put on ligament.  If you pull it/tension it, it responds by getting stronger. (here, here and here.).  If you think that a tendon or ligament gets longer with load then you are probably viewing that connective tissue like Taffy.  We probably shouldn't.  Taffy stretches and then permanently deforms.  Connective tissue will stretch transiently and then resume its shape.  The tension applied to it will catalyze an adaptation via mechnotransduction to better resist that tension in the future.  But, this process is slow and is minimal.  

2. Even if you could increase the laxity of ligaments around the sacrum and pelvis they wouldn't create posterior pelvic tilt and decrease nutation.  Remember, nutation is the anterior tilt of the sacrum relative to the pelvis.  It is a tiny movement.  Pretty much imperceptible.  Look at Kibsgard findings  But anyways, back to nutation.  The sacrotuberous ligament is the ligament that connects the biceps femoris on up to the sacrum/pelvis.  Its role is to LIMIT nutation.  The long dorsal SI ligament's role is limit counter nutation.  So, if you could increase the laxity of the ligaments with stretching (which you probably can't) you would actually increase the ability of the nutation and increase lordosis because of the influence on the sacrotuberous.  But don't worry, you can't do any of that.

3. Thought experiment:  why would you (me too) even think that a tendon or ligament would get longer or more lax with loading/stretching?

What is really the only type of load and tendon/ligament can "feel"?  Tension.  You pull on it. As you are pulling on it it will lengthen.  In the short term it will creep and deform but over the long term it will go back to its resting state.  But, say you are a tendon/ligament and you regularly get pulled on?  How should you respond?  If you were a muscle doing strength training you get stronger and stiffer.  And so do tendons with use .  Tendons increase their material and morphological properties with use (see Bohm for a review).  Meaning they get thicker and stronger.  And what type of force does a tendon "see" with strength training?  Tension.  The same type of force as tendon/ligament sees with stretching.  The only difference is the magnitude.  So how should connective tissue respond to tension?  It responds by getting stronger and sometimes stiffer.  

The type of load on the tendon is the same and responds similarly.  Its the amount of load that is different

The type of load on the tendon is the same and responds similarly.  Its the amount of load that is different

But remember, tendons/ligaments respond slowly and very minimally.  So thinking that stretching the hamstrings will have a dramatic effect on the connective tissue is just unsupported.

In summary, we don't have clinical trials suggesting posture can change very readily and we don't have biological plausibility or evidence to suggest that connective tissue will respond in the manner that would lead to these proposed changes.  Conclusion, don't worry about this.

Dubious assertion #4: "Ligaments do not have a lot of sensory nerves and so we cannot feel when they are being compromised. It takes years for pathology in the hip joints to show up, but a continuous tugging on the SI (sacral/hip/ hamstring) area that occurs in these poses undermines the curving forces needed for shock absorption and hip stability"

Actually, we can argue that its the role of the ligaments to provide sensory information. The ligaments are what trigger muscular activity to "stabilize" joints.

Again, we see this same pessimistic attitude that continuous "tugging" will change the alignment of the body.  We saw earlier that that is not well supported.  We can also suggest that there is no evidence that these deviations in posture are related to pain or dysfunction.  Here is a great blog that goes into this flimsy assertion in detail.

Dubious assertion #5: Hamstrings are most likely not short and tight but long, tense and weak. Pulling on them will only give a few minutes of relief as the stretch reflex in the nervous system creates inhibitory signals to create length and keep tissues from tearing

Again, this is just an opinion and is pretty much untestable.  You might have weak hamstrings.  So go ahead and strengthen them.  But stretching them won't hurt them and won't compromise your strength gains.  As for the stretch reflex, this isn't really how it works.  We have no evidence that stretching your hamstrings will cause you to lose "inhibitory signals" to keep you from tearing your hamstrings.  Yes, stretching can decrease the force producing abilities of a muscle in the short term but we actually haven't related that to an increased injury risk.   While pre-activity stretching may not decrease your injury risk it has  not been shown to increase your injury risk.  One thing that stretching does is it changes the viscoelastic response of your tissues.  That is one reason why you feel looser and less stiff after acute stretching.  Because you are.  You are also looser and less stiff with any warm up and you aren't more likely to injure.  This is just unsupported.

And remember, your increase in length in the long term is primarily NOT because there are inhibitory signals that turn off the muscles.  It is not active muscular contraction that inhibits range of motion.  It is your tolerance to the stretch discomfort AND passive connective tissue properties.  We know that it isn't the active muscle contraction that is primarily limiting ROM.  If this was the mechanism behind long term ROM increases then we would probably see a pretty substantial shift in the stress-strain curve to the right.  Meaning, there would be less muscle activity during the lengthening procedure, thus the muscle would resist the movement less and we would have substantial decrease in stiffness.  We don't see this.

Caveats of doubt related to nervous system creating tension:

1. In the short term (and maybe the long term) this might happen but perhaps or EMG sensors are not sensitive enough to detect very minor muscle activation levels - remember, in the experiments they measure EMG to make sure the muscle is "off" during stretching measurements.  I'd be curious to know if the stiffness of the muscle/joint changes when placed under anesthesia.  No doubt ROM would increase but would stiffness decrease too?

2. Further, when there is pain or perhaps fear or perhaps a previous painful injury we might see muscle activity during the testing.  These studies just look at "healthy normals". Perhaps there is a difference in other cases.  Again, I'd still argue static stretching might be helpful in these cases.  It would work like a graded exposure to the movement and the muscle activity may decrease - perhaps.  I don't know.

Dubious assertion #6: "Most people sit a lot during the day and this weakens the back muscles and hamstrings. Breathing muscles are inhibited and the muscles of the back body are not working since the chair back works like a brace"

and


"In particular, the groin and hip flexor muscles shorten and when we stand up, they still retain a shortness that puts a strain on the extensor chain (muscles of the back body)"

Prolonged sitting and being sedentary would weaken EVERYTHING.  There is nothing special about the hamstrings or back.  We have no evidence that breathing muscles are inhibited.  Your diaphragm works just fine.  Your chair back is not a brace.  It just lets you sit more efficiently.

Again, we have no evidence that there is selective muscular shortening from periods of sitting.  If you aren't doing anything to increase your ROM then you will lose it.  Nothing special about sitting

Even if sitting causes hip flexor shortening why wouldn't sitting cause shortening in all the other muscles in a shortened position?  Your knees are flexed?  Shouldn't your hamstrings now be tight? Oh, wait this Huffington post article just told me they aren't tight?  Which is it?  Does sitting cause shortening or not?  There just isn't any consistency in this argument.  

Further, sitting sees you in slight posterior pelvic tilt so shouldn't our hip extensors (glutes, adductor magnus) now be long? And since the knee is flexed won't this "lengthen" the rectus femoris (a hip flexor) and combat the negative effects of sitting in hip flexion.  We aren't puppets, we just don't work this way.  Last, and I'm not tracking down the reference, if you sit in posterior pelvic tilt and you lean back against your chair you a really aren't in a lot of hip flexion anyway.  So again, the dubious assertion is unsupported and has a logical inconsistency.

Dubious Assertion #6: So please stop stretching your poor hamstrings and remember that the nervous system controls tension in the hamstrings and stretching them to make them ‘longer’ is an antiquated myth, not grounded in any anatomical reality.

We have seen that stretching can certainly influence muscle length and ROM.  While the nervous system could influence muscle tension and ROM this variable is controlled for in the studies that measure muscle length.  They measure muscle activity and try to minimize it.  Again, if muscle length during passive testing/stretching was greatly influenced by muscle activity we would see a substantial decrease in muscle/joint stiffness after long term stretching when ROM increases.  But this doesn't occur.  We get large increases in ROM with no change in muscle activity and a very minor or non-existent (depending on the study) change in joint stiffness.

  "Longer" muscles is not an antiquated myth and in fact much of the argument for this belief is not grounded anatomical or in the biomechanics literature.

Final Caveat

If the point of the Huffington Post article is merely to say that there might be better uses of your time than stretching your hamstrings then we can all be on board with that sentiment.  Or perhaps there are other reasons why you "feel" tight and there can be other things you should do to influence that.  Or there are times when you should probably avoid stretching.  All of those things are plausible and I've written about perceived tightness before...5 years ago so not really ground breaking.

We have the science in this area and we can also reason from biological plausibility and consistency.  I don't think the HuffPost article does that and it goes way too far in its assertions.