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The Biomechanics of Breasts

By Stacy Hackner, on 24 February 2014

Stacy Hackner_ThumbnailBy Stacy Hackner

Have you ever wondered what biomechanics has ever done for you? Well, if you’re a runner, it can tell you a lot about your gait and efficiency. It tells us why people with long legs are good at running and people with long arms are good at swimming, and the forces they use per stride or stroke. It can teach us proper runner techniques. If you’re a female runner, you may have encountered a problem biomechanical researchers are actively working to solve: bouncing breasts.

I’ve only been a runner since I started my PhD. As you may remember from my last post, I learned from my research that it’s very important for your bone strength to practice weight-bearing activity (sorry, astronauts), which includes running. As professional running goes, for some reason marathon organizers decided to exclude women from participating until the mid 1980s, when just a few women snuck into the Boston and London marathons and achieved quite good times (see Heminsley’s book for an exciting run-down of the sneaking). Since then, women have been participating in most major sports, including (very recently) American football; 36.5% of 2012 London Marathon finishers were female (Brown et al 2013). But sports equipment for women is still catching up, and biomechanics – long applied to gait and stride, torso and head movements – is now being brought in to design a better bra. Most biomechanical studies of breasts involve attaching markers to women on treadmills in clothed and unclothed conditions and filming them with an infrared camera – a slightly awkward study for the volunteers, but it’s worth it for the results.

The breast in three dimensions. Zhou et al 2012.

The breast in three dimensions. Zhou et al 2012.

First, let’s look at a breast from an anatomical perspective. Breasts are composed of milk glands and ducts, fat, connective tissue, and Cooper’s ligaments. The latter are fibers that attach the breast to the underlying fascia and pectoral muscles; throughout life, and in vigorous exercise, they can stretch or break and cause sagging and breast pain. Imagine a laundry line with wet clothes hanging on it – now shake it: that’s what happens during intense exercise. Of course, these forces have been measured, which can be difficult as unlike bones and muscles, they squish and deform, and each of the above types of tissue reacts to running forces differently. During the beginning of a running stride, the breasts are found to accelerate at up to 3 G (where G is the force of gravity – at that point in time, it’s like the breast weighs 3 times as much). This is considerably more than the rest of the trunk, and puts strain on Cooper’s ligaments. You can then imagine that each breast goes through a cycle of acceleration, stasis, and deceleration for each stride, like your head when stopping and starting in a car. For each stride, the breasts move forward into the air and backward into the ribcage, in what is called anterior displacement.

The figure-8 movement of breasts in a digital reconstruction. Image via ShockAbsorber website.

The figure-8 movement of breasts
in a digital reconstruction.
Image via ShockAbsorber website.

Now it gets more serious. In addition to being displaced anteriorly, breasts move in three dimensions. When running, the goal is generally to move forward. In order to do this, you need to move up as well. And with each step, you also sway side to side. Breasts respond to this combination of forces by actually moving in a figure-8, experiencing additional vertical and horizontal displacement. Studies show that it’s these three directions of displacement rather than acceleration that cause breast discomfort and pain; the worst seems to be vertical displacement, which peaks at mid-flight. At this point, the Cooper’s ligaments are basically floating upwards and then being tugged back down during deceleration (not to mention the fat and glands moving about internally). This is important to know for bra design, as many sports bras take the approach of “flattening everything is best” – however, as we’ve now learned, flattening will only reduce anterior displacement! Flattening bras can also cause breast pain, so it’s lose-lose situation.

Now let’s discuss what a bra actually does. The everyday padded bra is an attempt to compromise comfort, sexuality, and stabilization, often emphasizing one to the detriment of the other two. The goal is to hold the breasts in an uplifted position so they appear firm and don’t jiggle around too much while walking or climbing stairs. Sports bras, on the other hand, prioritize stabilization, as they’re to be worn in high-impact activities. Many sports bras take the approach that flatter is better, which as I’ve shown is not quite the case, but they do prevent one kind of displacement. Regular, everyday support bras lift the breasts up, reducing strain on the Cooper’s ligaments, but in tests of treadmill running do little to prevent any kind of displacement. Running bare-chested causes the most displacement, and – in the case of marathons – can lead to a breast injury experienced by men as well, where the nipples chafe against the fabric of the shirt. (This is actually very common, and there are marathoner web forums devoted to sharing prevention tips.) A newer kind of sports bra attempts to encapsulate rather than flatten, and researchers from biomechanics and textile manufacturing have been collaborating on new design. This bra holds each breast separately and matches the figure-8 to the overall movement of the torso, reducing displacement in all three directions.

As more women get into sports (which is particularly important for the prevention of osteoporosis), making us comfortable and keeping us engaged should be a high priority for sports equipment manufacturers. Most runners can find shoes that fit, as shoes have been tested and re-designed for the last thirty years, and have a high profile in the press. Despite the increase in women running, 75% of female London marathoners still reported a problem with their sports bra, with the prevalence higher among larger-breasted women. However, proper fitting technical sports bras receive significantly lower press coverage than proper running shoes. Clearly, there is more work to be done!



Brown, N., J. White, A. Brasher, and J. Scurr. 2013. An investigation into breast support and sports bra use in female runners of the 2012 London Marathon. Journal of Sports Sciences 2013:1-9.

Heminsley, A. 2013. Running Like a Girl. London: Huntchinson.

Scurr, J., J. White, and W. Hedger. 2010. The effect of breast support on the kinematics of the breast during the running gait cycle.  Journal of Sports Sciences 28(10): 1103-1109.

Zhou, J., W. Yu, and S.P. Ng. 2012. Studies of three-dimensional trajectories of breast movement for better bra design. Textile Research Journal 82(3): 242-254.

Update: The post originally stated that Cooper’s ligaments connect breast glands to the clavicle; this was incorrect. 

15 Responses to “The Biomechanics of Breasts”

  • 1
    mark wrote on 24 February 2014:

    So interesting although I have personally not experienced these issues.

  • 2
    Stacy Hackner wrote on 24 February 2014:

    Tons of discussion on my facebook wall which I thought I’d relay here. Bruce asked: “I’ve been randomly thinking a lot recently about the fact that basically no human society alive today lives the way we apparently evolved to live (as hunter-gatherers in an open grassland) so I guess the bias I bring to this article is this: how do we square the existance of breasts with the fact that humans evolved to run? Do the problems that sports bras are still not 100% adept at solving only emerge at the high level of running that modern humans engage in for sport and not at the level of sprinting away from a lion that our ancestors had to worry about?”

    My response: There are lots of arguments regarding the evolution of human breasts, most of which are incomplete and rather biased (were at my talk years ago when we did those Snell-Hitchcok lectures?). I recommend Williams’ book “Breasts: A Natural and Unnatural History”.
    As for humans evolving to run – that’s slightly more straightforward. The evolutionary theory I subscribe to has humans evolving to be capable of long-distance running, but not everyone needing to run all the time. It’s not like we were /always/ getting chased or /always/ chasing. I bet there was a far greater amount of walking with brief running interludes: gathering plants, checking traps, sitting and waiting for slow animals to go by. Even something like your typical “big mammoth hunt” wouldn’t be anything close to extreme marathoning.
    Additionally, there’s the “man hunt! woman cook!” argument that I’m trying to break down because it’s so, so basic and ridiculous and sexist, but its principle is that women wouldn’t be running anyway. Which is wrong, but that ties in with some of the (flawed) arguments for breast development.
    The other thing to consider is that evolution isn’t always optimal. We tend to think of “survival of the fittest” meaning that everything that makes it must be perfect, but that’s far from true. Check out the arguments for human childbirth – female hips are suboptimal for both running /and/ giving birth, and yet we’re able to do both pretty well. So yeah, we can run with breasts bouncing. It’s just not that pleasant. Men can run without jockstraps, but I hear that’s unpleasant as well (or as some say, “freeing”). We do what we must.

    Marguerite commented: “This is really fascinating! The thing that comes to mind for some reason, though, is the articles that circulated a while back claiming that going without a bra was the best thing to prevent sagging. I didn’t look into it well enough to determine whether it was a legitimate thing or not, but I am curious whether/to what degree this is true in day-to-day life, as opposed to during exercise.

    The other thing I am curious about is whether Cooper’s ligaments can be repaired if broken, or somehow trained to better endure the strain from something like running.”

    My response: the article you mention, which got a lot of press attention, had a sample size of like 4, some of whom didn’t always conform to the study guidelines. Cf: any Discovery doc with images of topless tribal women. Sag happens. Bras reduce the weight on Cooper’s ligaments by transferring it to the shoulders, but still gravity takes its toll as does weight gain and loss, particularly if an individual chooses to brestfeed. I don’t know of any repair techniques for Cooper’s ligaments, but I’m sure it’s been investigated vis-a-vis breast reconstructive surgeries. Actually, a whole branch of breast biomechanics is trying to help rebuild post-mastectomy breasts to be more lifelike and accurate to breast shape before the surgery, including the invention of new materials that will better recreate natural fat and ligaments. (yes! We’re trying to make new fat!)

  • 3
    Shannon Love wrote on 12 April 2016:

    Actually, the very fact that humans presents large bodies of fat on the breast and hips makes as running exhausting predators.It’s the most ergonomic area to place fat when you have to run. Look at how military gear has shifted to the chest and hips in the few decades. In any case, human’s are are sexually dimorphic species so that means specialization of labor, always. Woman can run, men must. Since women spent the majority of their reproductive years, pregnant, nursing or carrying children its unlikely they actively participated in active hunts. Most definitively, patterns of broken bones and similar injuries in the arms and upper torso in male skeletons but not females tell us men closed with and fought with game while women in the main did not. There will always be exceptions of course, always outlier individuals but that is not what we are talking about.

    Going through intellectual contortions just because some self-appointed priest class has declared against all evidence and rules of natural selection and biology that by some mystic process males and females in a sexually dimorphic species are in fact utterly indentical in all behavioral respects is simply stupid.

    Just remember, these are the braniacs who 30 years ago, thought Freud, “scientific” especially the idea of infantile sexuality and in process covered up 50+ years for Pedophiles. When I was in as college in the early 80’s I argued that homosexuality was innate and not the result of choice and environmental factors and got called a fascist. Within a decade, those same people had turned on a dime and now call any who questions whether only the vast majority of homosexuals are born that way, fascist. Today, it’s 100% biological or nothing. Yes, I’m bitter.

    That’s the class of fad and market driven “intellects” who point out that women, as a population, under primitive conditions, especially when pregnant and nursing, do not run marathons to chase wonder game into the ground, “sexist.”

    They’re just bullies at best and whores for politicians at worse. Stick to the science. Stick to what can be measured and produce testable predictions. Doesn’t always work short term, but at least don’t have play 1984 every decade or so and throw 90% of what you ever said down the memory hole.

  • 4
    Z G wrote on 17 April 2016:

    Do you know what the current consensus is on bras and sagging? Do sports bras worn during exercise definitely help, while the jury is out on day-to-day bras?
    (I ask as a woman who doesn’t really like wearing bras but does so sometimes for work. I’d wear them more if I thought they were beneficial.)
    If bras are beneficial, do they really need to be as tight as recommended fits suggest, which means they leave deep marks on skin?

  • 5
    jasonmarkwebber wrote on 25 February 2014:

    The Biomechanics of Breasts http://t.co/jeeTGLAVg1

  • 6
    ResearchEngager wrote on 25 February 2014:

    The Biomechanics of Breasts by @stacytg http://t.co/Gc2dSAkkts

  • 7
    MsMakkah wrote on 26 February 2014:

    To all my lady runners out there, invest in a good sports bra because apparently breasts move in three dimensons: http://t.co/sH2fdzMoh4

  • 8
    Ivy Alexandra wrote on 28 February 2014:

    Thank you for this very interesting insight. Having recently gotten into running I’ve always wondered about this; if we had evolved to be efficient running machines, weren’t breasts be getting in the way of efficient running? Being hunter-gatherers, was running solely a man’s activity or did women partake in it too? I hypothesize that both men and women did run to escape the clutches of wild animals but when it came to hunting, I believe it was men who hunted and women who nurtured the children. Could the extent to which either sex ran have influenced the evolution of our structure and related biomechanics? Of course, nowadays, there are many accomplished women runners from around the world but I was just curious.

  • 9
    stacytg wrote on 8 March 2014:

    Hi Ivy – I talked a bit about this in the comments above, but I argue that evolution isn’t a “best of the best”, but more of a “meh, good enough”: you don’t have to be the fastest, you just have to be faster than the slowest to outrun the bear. Humans (and other animals, certainly) are a muddle of evolutionary compromise. Female hips are not the best suited for childbirth (it can often take hours, there are numerous complications, and we often need other humans to assist), nor are they the best suited for running (as the hips are wider than the knees, it makes running gait less efficient), but we manage to do both well enough to have 7 billion humans and a horde of marathoners. I don’t know if anyone has looked into whether faster female runners have a “more male” hip angle or vice versa, but it would be really interesting to find out.

  • 10
    Dr. D J Anthony wrote on 22 July 2015:

    Interesting article. Thank you. Please let me correct one thing. Cooper’s ligaments are not attached to the clavicle. They connect the skin of the breast with the underlying Pectoralis fascia. Also It is difficult to believe that anteroposterior compression will not prevent sideways movements. (In facts those bras has lateral support as well.

  • 11
    stacytg wrote on 20 October 2015:

    The article has been corrected, thanks!
    For lateral movement, it may depend on the bra. The body moves side-to-side enough in each running step that a even an anteriorly compressive bra will only limit that movement slightly. I know I’ve felt it!

  • 12
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    The Biomechanics of Breasts | Stacy Hackner wrote on 12 September 2016:

    […] published on Student Engagers on February 24, […]

  • 14
    The Būband enhances any bra – not just sports bra! – Būband wrote on 7 March 2017:

    […] move a lot during high-impact activity – up and down, side to side and in a figure 8 motion – even if you’re wearing a sports bra. Studies also show this movement is damaging – […]

  • 15
    Five years of research: a summary | UCL Researchers in Museums wrote on 3 July 2017:

    […] some of you have been following my research here, starting from astronauts and moving on to runners and foraging patterns. It all ties together, I promise. I recently gave a talk at the Engagers’ […]

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