Flexibility's relationship to cardiovascular health

An article came out in August 2009 that suggests flexibility and cardiovascular health are related; that you can stretch yourself into vascular well being.

If it's true, that's a pretty huge discovery. No more cardio for the health-conscious. Too much joint trauma there. Even on the low-impact stuff. Plus it's too laborious. Instead, just stretch your hammies and you'll never have a heart attack.

If this is true, that's some seriously groundbreaking research. I'm just not really convinced that it is true, despite the fact that it was published in an extremely reputable journal.

If you feel like reading the actual article, it's here (reproduced with no permission whatsoever):


Okay, now that you didn't read any of it... you didn't even skim it... I'll sum it up for you:

Some researchers from Japan and Texas gathered up 526 subjects between the ages of twenty and eighty three, and had them stretch. The people who were pretty flexible were put on one side of the room. The people who weren't very flexible were put on the other (figurative room... they were actually put on opposite sides of a spreadsheet).

Now, with all the flexible people on one side (of the room), and the inflexible people on the other, the researchers divided them further, this time into three different age brackets (young, middle-aged, and old).

Then - to every single person in the room - they measured how stiff their arteries were (stiff arteries being a very bad thing as far as cardiovascular health is concerned).

The young group on the flexible side of the room was then pitted against the young group on the inflexible side. Lo and behold, the stiffest arteries ran alongside the stiffest muscles. The same result (though more pronounced) was found with the middle-aged and older groups.

Thus leads to this conclusion: muscle flexibility is "a determinant or predictor of arterial stiffness."

Okay, now that that's over with, can I tell you the problems I have with this study? I'm obviously going to. Don't feel you have to keep reading.

First off, the subjects.

The majority of the subjects were women. And women have menstrual cycles. At least the women in this study do. Most of them anyway. Some women don't, but those ones don't matter in the context of what we're talking about. Or in the continuation of the human species (as far as reproduction is concerned). That wasn't gender biased. The chromosomal counterpart (i.e. guys) hardly matter but for insemination. Either way, moving on:

The phases of a menstrual cycle impact elastic properties of the central arteries. I think there's probably very little chance that this matters to any practical degree in this study though. Possible. But improbable.

What's more probable is the age issue. The subjects are lumped into three different age categories: 20 to 39 year olds (young), 40 to 59 year olds (middle-aged), and 60 to 83 year olds (older).

This is ridiculous. Twenty years is a very long time. Maybe not relative to the duration of the Neolithic era, but in a study about flexibility and heart health? That's long.

I'm 29. A decade ago, I was pretty limber. In another decade, should my flexibility continue to decline at the rate it has been, I'll probably be hobbling around like Gollum... who was actually pretty limber. Bad example. If I rewrite this article, I'll make some reference to the local senior center. Either way, I would have started the descent of my flexibility in the same age bracket I ended it.

However, in the results, the authors say "in each age category, age, height, weight, and all metabolic risk factors did not differ between high-flexibility and poor-flexibility groups."

This is not believable. What they're suggesting is that, from 20 to 39, flexibility in the human body doesn't change. Then from 39 to 40 it changes (somewhat drastically). And then from 40 to 59 it doesn't anymore.

If this were truly the case, there would have to be some variable of statistical significance that I would regard as confounding were I able to identify it.

So in gender, there might be an issue. In age, there's probably an issue. And in the physical characteristics of the subjects themselves regardless of age and gender, conceivably.

What I'm conceiving is this: the subjects on the lower flexibility side of the room all had higher blood pressures than their age-semi-matched counterparts on the other side of the room.

And higher blood pressures often come in a package deal with excess adiposity.

And so if they're measuring flexibility with something a huge, distended gut could interfere with (i.e. the sit and reach test, which is exactly what they were using), that might interrupt the values. It's obviously much more cumbersome to sit and reach through a gut than to do so through air.

Granted, none of the subjects were regarded as obese (so says the methods). But all they're really doing is using a BMI of less than thirty to rule out obesity. That doesn't say much (or say anything for that matter) about waist circumference.

For all I know, every participant in the low-flexibility group could have a ton of abdominal adiposity perched above little pirate stilts for legs, like some American equivalent of Kwashiorkor......

All in all, I'm just not sure they controlled the subjects that well. But even so, that's not altogether damning of their ultimate validity.

It still appears as though flexibility is a really good predictor of arterial stiffening. The authors mentioned this in the discussion ("These findings statistically support the idea that flexibility is identified as a determinant or predictor of arterial stiffness")... They were a little overeager in the use of "determinant or", but, after reading the article, "predictor of arterial stiffness" seems completely reasonable.

The problem is "muscle flexibility is a predictor of arterial stiffness" doesn't really warrant an entire article.

I say this because number of gray hairs, or annual complaints about lumbar pain, or a bingo-appreciation-measurement via Likert scale would all yield a similar correlation to arterial stiffening. And I'd have a really hard time getting any of those published in any branch of the American Journal of Physiology.

Right at the start of the article, it says "higher levels of physical fitness, especially cardiorespiratory fitness, appear to delay an age-related arterial stiffening. Although flexibility is one of the components of physical fitness, the relationship between flexibility and arterial stiffness remains unclear."

I don't know if it really is all that unclear though. I could be wrong, but it strikes me as little more than specificity of adaptation. And when was Wolff's Law written? The 1800s? Granted, it's not the exact same thing, but it's certainly the same idea. And by the mid-90's, physiologists had exhausted all possible expressions of the idea. At least until this one, in late 2009.

Here's what's happening:

Achieving cardiorespiratory fitness means you're using your leg muscles a lot over long periods of time. This puts a very specific stress on both the musculature and your cardiovascular system. Both will then adapt to better accommodate the specificity of those stresses they're enduring.

In this case, the vasculature would respond with better arterial compliance and the involved musculature would respond with enhancements to functionality (on a lot of levels, flexibility obviously among those levels).

And if you look at the data, you'll find huge peak oxygen uptake differences between the flexible and inflexible groups of all ages. This reveals that the flexible people are routinely doing cardiovascular exercise (either that or they're blood doping... but probability suggests exercise) while the inflexible people aren't (they're neither exercising, nor blood doping).

And since the sit-and-reach test measures trunk and hammie flexibility, and just about every mode of cardiovascular exercise keeps those two regions limber above all else, hmm...

Again, I could be wrong... But I'm not sure there's all that much else going on here. And when it gets to the discussion, it feels like the authors are toying with the idea of admitting it: "In fact, a positive relationship between cardiorespiratory fitness and flexibility was observed in the present study. It is well known that cardiorespiratory fitness was inversely related to arterial stiffness (25)."

So that's my take on it. Specificity of adaptation. Some people exercise, some people don't. Those who do are rewarded with both muscle and vascular adaptations. Muscle adaptations include increased trunk and hamstring flexibility. Vascular adaptations include better arterial compliance.

Moreover, people who exercise tend to be the same people who stretch. There aren't a lot of stretching enthusiasts out there that ignore the other major facets of health. So behavioral patterns like this would just compound the effect.

As a pilot study, it's an interesting idea. But it was published as something other than a pilot study. That's the weird part. Especially weird that a reputable journal accepted it as such when far better studies are routinely rejected.

The token example obviously being Kary Mullis, the really weird surfer who invented PCR (i.e. polymerase chain reaction; it's a DNA-amplifying method that revolutionized genetic research). His was rejected by Nature and Science before he finally got it published (and thereafter won the Nobel Prize for it). Contrarily, if this research won a Nobel Prize, the honor would forever be tainted.

If they wanted to write something worth reading, they should have matched oxidative values while better controlling the subject variables and then measured flexibility through a bunch of angles (or at least throw something else like a shoulder into the mix). Then they could see if actual flexibility has an association with arterial stiffening.

And to interest me, they should have thrown a bunch of weight lifters into the mix, who are notorious for terrible flexibility and arterial stiffness both.

"How do you know there weren't any weight lifters involved?"

Because they measured leg extension power. And had the subject pool been flooded with lifters, the poor flexibility group would have fared a bit better in that measurement, being as cardiovascular exercise isn't going to increase power much in that setting (specificity of adaptation, remember?). But the poor flexibility groups had lower values here. So I can't very well chalk it up to a history of bodybuilding.

But there were a couple interesting points that made the article worth reading. It's just that they were discussing other articles during these moments.

The first was one was this: Francois B, De Paepe A, Matton MT, Clement D. Pulse wave velocity recordings in a family with ecchymotic Ehlers-Danlos syndrome. Int Angiol 5: 15, 1986.

The authors discussed Ehlers-Danlos syndrome (hyper-flexibility), concluding that those who had it were less likely to have arterial stiffness.

Interesting. Number two:

Cortez-Cooper MY, Anton MM, Devan AE, Neidre DB, Cook JN, Tanaka H. The effects of strength training on central arterial compliance in middle-aged and older adults. Eur J Cardiovasc Prev Rehabil 15: 149155, 2008.

And here's what they had to say about this one: "Recently, Cortez-Cooper et al. examined the effects of strength training on central arterial compliance in middle-aged and older adults. In this previous study, a stretching exercise group was included as a control group. An unexpected finding of the study was that a stretching program significantly increased carotid arterial compliance."

Again, interesting. But I'm not going to comment on them. Because I'm trying to be shorter-winded.

Feel free to send me your thoughts: courtney.jensen@huskymail.uconn.edu