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 #3: There 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.